[Advances in Ecological Research] Litter Decomposition: A Guide to Carbon and Nutrient Turnover Volume 38 || Decomposers: Soil Microorganisms and Animals

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Decomposers: Soil Microorganismsand AnimalsI. IADVAN# 2006ntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .CES IN ECOLOGICAL RESEARCH VOL. 38 0065-250Elsevier Ltd. All rights reserved DOI: 10.1016/S0065-25044/06(05)373$35.08003-II. Communities of Soil Microorganisms and Animals . . . . . . . . . . . . . . . 75A. Soil Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75B. Soil Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77III. The Degradation of the Main Polymers in Plant Fibers . . . . . . . . . . . . 79A. Degradation of Cellulose. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79B. Degradation of Hemicelluloses . . . . . . . . . . . . . . . . . . . . . . . . . . . 82C. EVects of N, Mn, and C Sources on the Degradation of Lignin . . 83D. Degradation of Lignin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87IV. Degradation of Fibers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92A. Fungi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92B. Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93V. Microbial Communities and the Influence of Soil Animals. . . . . . . . . . 94A. Microbial Succession and Competition . . . . . . . . . . . . . . . . . . . . . 94B. EVects of Soil Animals on the Decomposition Process . . . . . . . . . 96I. INTRODUCTIONThere is an unfortunate tradition ascribing to soil animals a large role in thedecomposition of organic matter, leaving a minor role to the soil microor-ganisms. Since 1980, an increasing number of studies and calculations haveshown that the relative roles are reversed. Thus, it has been found that in, forexample, boreal forests, the soil microbial population transforms more than95% of the plant litter carbon, leaving a maximum of 5% to soil animals. Thedominating primary decomposers in boreal and temperate forest soil systemsare the microorganisms, encompassing both fungi and bacteria. Both thesemain groups of microorganisms can degrade cellulose, hemicelluloses, andvarious lignins (Textbox 3 in Chapter 2).In this chapter, we emphasize the functional roles of microorganisms (e.g.,cellulolytic and lignolytic) rather than their taxonomy. The concepts ofwhiterot, brownrot, and softrot and what they functionally stand for interms of degradation processes will be presented. We use these functionalconcepts as a basis to discuss the degradation of litter tissues. Although theterms originally referred to visually diVerent types of lignin degradation, itnow appears that the degradation of not only lignin but also cellulose and0274 BJORN BERG AND RYSZARD LASKOWSKIhe micellul ose is di Verent among the taxonom ic g roups of micr oorgan-ism s (Worr all et al. , 1997 ). The terms, howeve r, relate to the type of rotrather than to the group of organ isms, namel y, rots givi ng the wood a whiteor brown color. In the followin g text , we adop t the common use of theterm s and refer to fungi when using the terms white rot, bro wn rot, andso ft rot. Regar ding de gradat ion by ba cteria, it is descri bed a nd discus sedas such.Man y micr oorganis ms in nature degrad e cell ulose and hemic ellulo se.Thes e organ isms ha ve in common the prod uction of extra cellu lar hydrolyticen zymes that are eithe r bound onto the outsi de of the cell or relea sed into thesu rrounding environm ent. Thus , the first steps in their de gradation acti vitytake place outsi de the cells. Som e polyme r carbohydrat es may be de gradedby both aerob ic a nd anaerob ic micr oorgan isms, but a complet e degradat ionof lignin (white rot type) require s the acti on of aerobic organ isms (fungi and /or aerobic bacter ia). Par tial ligni n de gradation (brow n rot type) may beca rried out also by anaerobi c bacter ia but is mainly found among fungian d aerobic ba cteria.The specie s composi tion of the micro bial communi ty (as regards cellulo-lyti c and lignolyti c specie s) may vary with the general propert ies of the soil /lit ter subeco system, such as nutrien t stat us and pH. A spe cific fun ctionalpr operty that may discr iminate among soil syst ems in term s of their micro-bial commun ity struc ture is, for exampl e, di Verenti ated sensitiv ity of sp eciesto concen trations of nitro gen in litter and hum us, which may be e ithersti mulating or suppress ing for pa rticular sp ecies. Such a supp ressing e Vectof nitro gen is not gen eral, but is common in specie s of bot h whi te rot andbr own rot organis ms as regards their lignin degradat ion.By traditi on, soil anima ls have been consider ed impor tant for litterde composi tion; such groups as sp ringtails, mites, and earthw orms, amongoth ers, have been ascri bed di Veren t roles in de composi tion, althoug h thero les are not always clear and not always proven. The decomposi tionby free livin g microo rganisms has also be en consider ed impor tant but therelat ive influenc es of the two main groups, namel y, soil anima ls and soilmicr oorganis ms, have not been apparent . It has become increa singly clear,howeve r, that for so me syst ems, at least boreal and tempe rate con iferousone s, the micro bial componen t is of absolute dominance , with more than95 % of the energy going through the micr obial communi ty. The impl icationsof such a finding and of such a propo rtion are consider able. As the bookfocu ses on boreal and tempe rate systems, with an evident dom inanceof micr oorganis ms in the deco mposi tion proc ess, we ha ve given specialatte ntion to microb ial communi ties (Sect ion II.A.) and the enzymat ic de gra-da tion mech anisms (Sect ion III. ) for the polyme r c arbohydrat es and lignin.This chapter thus presents basic properties of microorganisms, as regardsdegradation of cellulose, hemicellulose, and lignin. Although presented onDECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 75the basis of studies carried out in boreal and temperate forest systems, thesedecay mechanisms should be similar across ecosystems and climatic zones.What may diVer among systems and climates is the relative interactionbetween microorganisms and litter chemical composition and the influenceof microorganisms versus soil animals.For those microorganisms that decompose plant litter structures, the termdecomposer is sometimes used. The structure and development of decom-poser communities can influence the pattern of decay. Also, structuralchanges in the community and its function during the decay process willbe addressed. The eVects of moisture and temperature on the activity of themicrobiological decomposition are presented later, in Chapter 7.II. COMMUNITIES OF SOIL MICROORGANISMSAND ANIMALSA. Soil MicroorganismsThe two main systematic groups of litter decomposers are bacteria andfungi. Both groups include some of the same basic physiological propertieswhen it comes to degradation of the fresh litter polymers. Generally, thefungi are considered the more important group, which means that we knowmore about their litterdegrading properties and enzyme systems. Eachof these two groups may be subdivided into functional subgroups withdiVerent properties and the ability to degrade the main groups of chemicalcomponents. We will discuss them shortly.The systematics of both fungi and bacteria encompass numerous generaand subgroups, the description of which is beyond the scope of this book.The bacterial group also includes both aerobic and anaerobic organisms,which makes them diVer from the exclusively aerobic fungi. Further, amongbacteria belongs an important group of lignin degraders, namely the fila-mentous bacteria that earlier were called Actinomycetes. Both fungi andbacteria include organisms able to degrade all the main plant litter polymers:lignins, cellulose, and hemicelluloses. There are also organisms able todegrade woody tissue containing all the components combined into fibers.Still, a complete degradation of lignin appears to be carried out only by someof the fungi and some of the filamentous, aerobic bacteria. Some mainproperties are collected in Table 1.Bacteria may be immobile or mobile, with one or more flagella, a whiplikestructure. Fungal mycelia are mobile in another way since they simply growin one direction and thus move their protoplasm, leaving an empty cellwallstructure behind.Table 1 Some general properties of the main groups of bacteria and fungiProperty Bacteria FungiMobility Sporeforming ability Can degrade cellulose/hemicellulose Can degrade lignin completely Can degrade lignin anaerobicallya Can degrade intact fiber walls Species with N repression of the ligninase system ? Species without N repression of the ligninase system ? aIncomplete degradation to be compared to the brownrot type. With kind permission ofSpringer Science and Business Media.76 BJORN BERG AND RYSZARD LASKOWSKIThe diameter of most bacteria range from 0.1 to 2 mm, and filamentousfungi from approximately 1 to 20 mm. Whereas the lengths of rodshapedbacteria, in general, are less than, say, 20 mm, those of the fungal mycelia aremore undetermined. The size of a large part of the microorganisms isgenerally on the level of 1 mm in diameter, which gives them access todiVerent parts of the fibers and tissues.The numbers of soil microorganisms and the general biological diversityof the soil microbial community can be considered very high. We may see thepotential species diversity just by using crude numbers of identifiable specieswithin, for example, one square meter. The number of fungal species for anatural and unpolluted soil may be estimated to approximately 100 domi-nant species, and for bacteria, the number may be more than 5000.The high density of microorganisms in an organic soil creates a highpotential for invading new substrates, such as newly shed litter. Estimatesof 109 bacterial cells per gram organic soil, either active or in a resting stage,for example, as spores, are common when made by direct light microscopycounting. However, there are numerous bacteria that are simply too thin tobe seen in a light microscope and have to be counted using electron micros-copy. This figure is, thus, rather conservative. In similar soils, total myceliallengths have been estimated to reach approximately 2000 km per liter ofhumus, of which perhaps 10% would be live.Only those microorganisms for which the environmental conditions aresuitable for growth are active whereas the others remain in some kind ofdormant stage. Further, fungal spores are easily transported by wind andanimals, and this means that they may be transplanted among ecosystems.These two factors mean that an ecosystem may have a passive species bank,with microorganisms able to be revived when the conditions allow andto attack a variety of litter types, including those containing chemicalcomponents that are unknown in a particular environment.DECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 77Mycorrhizal fungi have been found to turn into aggressive decomposersunder certain circumstances and may decompose humus that has beenconsidered stabilized, and such a degradation can take place at a high rate.This phenomenon may be related to nutrient stress of the growing trees. Therole of mycorrhiza in decomposition is still under dispute and we set forwardobservations without taking part in that dispute (Section VI.G.) Chapter 3focuses on what may be called primary litter decomposers, namely, thosethat attack and degrade, at least in part, the polymer structures to carbondioxide and/or small, only partly degraded molecules.B. Soil AnimalsDetailed descriptions of soil fauna communities exceed the scope of thisbook, and separate handbooks are devoted solely to this topic. A goodoverview of soil organisms, including microorganisms, and their ecologycan be found in Soil Ecology by P. Lavelle and A. V. Spain (2001).Soil, being the most complicated subecosystem on earth, oVers extremelydiversified environments to organisms: it is rich in diVerent food resources,both as dead organic matter and as numerous live microorganisms andinvertebrates; it normally consists of microenvironments of very diversifiedhumidity and diVerent chemical properties. Further, soil pores can actuallyrepresent a freshwater environment rather than a terrestrial one. Due to thisdiversity, most of the invertebrate taxonomic groups can be found insoil. The soil system is also probably the environment richest in diVerentecological groups of animals: hydrobionts are actually aquatic organismswhich occupy the smallest soil pores, more or less permanently filled withwater; hygrobionts still require high moisture and, frequently, availablefree water, but are typical terrestrial animals; the driest parts of soil systemsare occupied by xerobiontsanimals preferring dry conditions.Traditionally, soil fauna is divided into three major classes, dependingsimply on the size criterion: the microfauna covers a size range up to 0.2 mm,approximately 100 to 2000 times larger than the main groups of bacteria;larger animals up to 10 mm belong to mesofauna, and still larger onescomprise the last group of macrofauna (Lavalle and Spain, 2001). Someauthors adopt slightly diVerent size criteria and also recognize yet anothergroup of megafauna for animals such as the largest earthworms, slugs, andsnails as well as all soilliving vertebrates (Gorny, 1975). The general classi-fication of major groups of soil fauna is presented in Fig. 1. Althoughthis might seem like a very artificial grouping, there is some deeper sensebehind the size classes recognized. The microfauna representatives livemainly in the waterfilled and small soil pores and belong chiefly to hydro-bionts. Due to their small size, their eVect on soil structure is very limited orFigure 1 Sizeclassified major groups of soil invertebrates.78 BJORN BERG AND RYSZARD LASKOWSKInone. Mesofauna inhabits larger soil pores with no free water but filled withwater vaporthey generally belong to hygrobionts. Through deposition offecal pellets and limited possibilities to burrow in soil, they may aVect soilstructure to some extent. In contrast to microfauna, generally, they are notable to decompose organic matter by themselves. Finally, macrofauna is thegroup of freemoving animals, large enough to actively burrow in soil andmix organic and mineral layers. Their eVect on soil structure is, by far, thelargest among all soilliving organisms. As they represent a huge variabilityof taxonomic groups and ecological niches, one may find in this group bothhygrobionts and xerobionts. In spite of their decisive eVect on soil structure,their capabilities for direct primary decomposition of dead organic matter islimited or nil. Their eVect on organic matter decomposition may be throughmixing organic matter with mineral soil (see Section VI. G.).Yet another classification of soil fauna, introduced by Van der Drift(1951), is based on an association of a species with specific compartmentsof soil environment. Thus, euedaphic species live in deeper soil layers. Mostmicrofauna and some mesofauna belong here. Surface layers of soil, such ashumus and litter, are inhabited by hemiedaphic species; most meso andmacrofauna can be classified as such. Animals that generally live on thelitter surface but temporarily may live in the litter layer, such as numerousbeetles, spiders, snails, or slugs, form a third group of epedaphic species.Finally, some species can be found on the soil or litter surface, although theyare in no way connected to the soil environment such species have beenclassified as atmobionts.Obviously, no single classification is perfect. Many animals spend onlypart of their life cycle in soil or litter, and later have no connection with it.For example, a number of insects, such as butterflies or dipterans, spendtheir larval and/or pupal stages in soil, but adults can hardly be named soilDECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 79animals. As can be seen from Fig. 1, sizebased classification is also farfrom perfect since a number of taxonomic groups spread over a few ordersof magnitude in size. Moreover, animals do grow and, during that process,even a single species can pass from one size class to another. Still, classifica-tion is helpful and, as we have indicated, usually there is some biological orecological meaning behind even the simplest grouping system.III. THE DEGRADATION OF THE MAIN POLYMERSIN PLANT FIBERSIn Chapter 2, we described the main polymer litter chemical components,namely, cellulose, hemicellulose(s), and lignins, the latter represented byspruce lignin. In this chapter, we focus on the main groups of organismsdegrading these polymers.A. Degradation of CelluloseCellulose is degraded by numerous species of both fungi and bacteria. Theseorganisms rely on extracellular enzymes that either are located on the cellsurface or secreted into the organisms immediate surroundings. A commonproperty of all cellulosedegrading organisms is that they produce extracel-lular hydrolytic enzymes that attack the cellulose structure. Due to the fibersize, the main part of the degradation of cellulose must take place outsidethe microbial cell (Fig. 2). Part of the cellulose in the plant fiber is arrangedin a way that makes it harder for enzymes to degradeit has a crystallineform and not all of the cellulolytic organisms have the necessary complete setof enzymes to degrade that structure. Several microorganisms, on the otherhand, are able to degrade the kind of cellulose that is arranged in a moreamorphous way (see, e.g., Eriksson et al., 1990). In the first steps of degra-dation, the insoluble macromolecules are degraded stepwise to oligomers(chains of diVerent lengths) and finally to the dimer cellobiose with just twoglucose units (Fig. 3), which is taken up by the cell and metabolized.The most studied group of cellulosedegrading organisms is the fungi. Nofewer than 74 species (Eriksson et al., 1990) have been studied in detail andclear diVerences have been observed among species.Probably the best studied wood decay fungus is the whiterot basidiomy-cete Phanerochaete chrysosporium Burdsall (previously called Sporotrichumpulvurolentum Novabranova). Much of our knowledge about the decay ofcellulose and lignin in nature is based on studies of this fungus (Eriksson et al.,1990) and we may use it as an example. Three main enzymes are involved incellulose degradation: one type of enzyme (endo1,4bglucanase) covers theFigure 2 Electron microscopic photo of the cellulose degrading bacterium Cellvibriofulvus growing on a fiber, in this case, of pure cellulose. Note the close contactbetween the bacterium and the cellulose. From Berg et al. (1972).80 BJORN BERG AND RYSZARD LASKOWSKIcellulose chain and splits the glucosidic linkages in a random way (Fig. 3). Inthis case, randomly means that oligosaccharide units of diVerent lengthsare formed in this first degradation step, although they may still be attachedto the microfibril structure. Another enzyme, an exo1,4bglucanase, splitsoV either glucose or cellobiose from the nonreducing end of the cellulosechain. Finally, a 1,4bglucanase hydrolyzes cellobiose and other watersoluble oligosaccharides, such as triose and tetraose, to glucose. This latterenzyme is located in the cell in contrast to the two cellulases (endo and exo)that are located on the outside of the cell wall. One important aspect of thisenzyme system is that the two cellulases with diVerent specificities (the endoand exoglucanases) exert a synergistic action that enables them to degradeboth crystalline and amorphous cellulose.The softrot fungi, as a group, generally appear to have a cellulosedegrading enzyme system similar to that of the whiterots. On the otherhand, in contrast to whiterot and softrot fungi, brownrots have not beenfound to have the cellulases with the synergistic eVects that are found inwhiterots and they appear not to have the exocellulase previously men-tioned. However, Highley (1988) found several species of brownrots thatwere able to solubilize microcrystalline cellulose. Thus, the generally heldconclusion that brownrot fungi seem merely to depolymerize cellulosewithout producing soluble glucose of cellobiose may not be entirely correct.Still, no other enzyme has been found to substitute for the missing exocellu-lase that splits oV soluble units, such as glucose or cellobiose (cf. Fig. 3). Thishas led Eriksson et al. (1990) to conclude that there may be a nonenzymaticmechanism involved in the brownrot degradation of cellulose.Figure 3 Part of a cellulose microfibril is attacked by an endo1,4bglucanase, alsocalled endocellulase, splitting oV oligosaccharides in a random manner, thusproducing chains of diVerent lengths. An exo1,4bglucanase, also called exocellu-lase, splits oV cellobiose units from the nonreducing end of the carbohydrate chains.The letter G symbolizes a glucose unit.DECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 81An observation that hemicelluloses are virtually absent in wood decayedby brownrots suggests that brownrot fungi may degrade hemicelluloses.Although the mechanisms for degradation of cellulose are far from clear,work on a basidiomycete (Wolter et al., 1980) suggests that, at least for somebrownrot species, a less specific or multifunctional enzyme that can degradeseveral diVerent polysaccharides was active.Also, in many bacteria, we can find the ability to degrade crystallinecellulose. Detailed studies on the anaerobic Clostridium cellulolyticum showthat the organism produces at least six diVerent cellulases, each with slightlydiVerent structural and catalytic properties. The cellulases and the xylanasesare held together in a large structure, the cellulosome, by a scaVolding82 BJORN BERG AND RYSZARD LASKOWSKIprotein (see Belaich et al., 1997), largely as was predicted by Eriksson et al.(1990). Already in the very early work of Viljoen et al. (1926) on theanaerobic bacterium Clostridium thermocellum, a multicomponent complexof cellulolytic enzymes was named cellulosome. Close contact between thecellulose substrate and the organism often appears to be necessary. Suchcontact may be illustrated by an electron microscopic picture (Fig. 2) ofbacteria growing in contact with a cellulose fiber.The degradation of cellulose by bacteria has been suggested to be carriedout by hydrolytic enzymes; still, the mechanisms seem to be diVerent fromthose of the investigated fungi. For bacteria, the cellulolytic enzymes arearranged in clusters and act in a combined way, as has been described. Thisproperty seems today to be widely recognized (Wiegel and Dykstra, 1984).The few groups of cellulolytic bacteria that have been studied include Cyto-phaga, Cellulomonas, Pseudomonas, Cellvibrio, and Clostridium. It appearsthat these have their cellulolytic enzymes bound to the cell wall and thereforea close contact is needed between the cell and the substrate (Berg et al., 1972;Eriksson et al., 1990; see Fig. 2). Actinomycetes, in contrast to some otherbacterial groups, appear to degrade cellulose in a manner similar to that offungi and can also degrade the crystalline form. Several strains even have theability to degrade the lignocellulose complex. The fungal model for enzy-matic degradation of the cellulose molecule, namely that an endo and anexocellulase act synergistically, appears to be valid also for Actinomycetes,supporting their similarity to whiterot and softrot fungi in this respect.We know that the synthesis of cellulases is induced by cellulose, cellobiose,sophorose, and lactose. As cellulose is a large and nonsoluble molecule, itcannot be transported into the microbial cell and exert a direct inducingeVect. However, the presence of cellulose appears to be the best inductionagent and just the presence of the cellulose outside the cell appears to cause aninduction. Today, the accepted theory is that the microorganisms have aconstant, basic level of cellulase on their surface. Upon contact with cellulose,low amounts of inducing substances are released from the cellulose, enter themicrobial cell, and induce the formation of cellulase. It is likely that both thetype of a compound, for example, cellobiose or cellotriose, and a low concen-tration of these compounds influence the synthesis of cellulase. There are alsotheories that transfer products of glucose, for example, glucosyl, are active,one of these being the sugar species sophorose (cf. Eriksson et al., 1990). Onthe other hand, the cultivation of bacteria and fungi using glucose as the solecarbon seems to repress the synthesis of the cellulase system.B. Degradation of HemicellulosesWe mentioned in Chapter 2 that the hemicelluloses are composed of bothlinear and branched heteropolymers of Dxylose, Larabinose, Dmannose,DECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 83Dglucose, Dgalactose, and Dglucuronic acid (heteropolymers meaningthat the chains are built up of diVerent species of simple sugars). Theindividual sugars may be methylated or acetylated, and most hemicellulosechains contain between two and six diVerent kinds of sugars. This structur-al complexity means that the degradation of hemicelluloses requires morecomplex enzyme systems than are needed for the hydrolysis of cellulose.We may illustrate this with the possible structure of such a xylandominated hemicellulose with both 1,4blinkages and branched hetero-polysaccharides, which require a complex set of enzymes for degradation(see Dekker, 1985) (Fig. 4). The xylan backbone is made up of bothacetylated and nonacetylated sugar units. On the branches, there are unitsof methylated glucose and arabinose. The degradation of such a complexmolecule requires the concerted action of several diVerent hydrolyticenzymes (Eriksson et al., 1990).C. EVects of N, Mn, and C Sources on the Degradationof Lignin1. EVect of N Starvation on Lignin MetabolismLignin degradation may be repressed by high N levels in the substrate, aneVect seen mainly in whiterot fungi but also in brownrots and softrots. Ashas been mentioned, Kirk (1980) described Nregulated eVects on lignindegradation for P. chrysosporium and that lignin degrading enzymes weresynthesized under conditions of N starvation. In the first experiments on thiseVect, Keyser et al. (1978) found a drastic eVect of N on lignin degradationrate when the N concentration in the culture medium was increased from 2.6to 5.6 mM. The lignolytic activity (measured as transformation of 14Cligninto14CO2) was repressed by 83% at the higher concentration. This propertyhas been described for several fungal species in laboratory experimentswith pure cultures, although the levels of N and the magnitude of theeVect vary. For three species, Phlebia brevispora, Coriolus versicolor, andPholiota mutabilis, significant decrease in lignin degradation rate was foundat 7.8 and 34 mM N in the culture, but not at 2.6 mM N. The magnitudeof the eVect caused by 20 mM N varied from an almost complete repres-sion in P. chrysosporium to about a 50% repression in P. mutabilis whenusing14Clabeled lignin from red maple wood. Table 2 lists a number ofspecies investigated for this property.There are also several fungi that are not sensitive to N. For example, awhiterot strain isolated from an Nrich environment (cattle dung) showedno sensitivity to raised N concentrations. We may speculate about theecological significance of that. It may be so that in Nrich environmentsFigure 4 Degradation of part of a xylan molecule. The main enzyme attacking theunbranched part of the chain would be an endo1,4bxylanase, producingoligosaccharides of diVerent lengths, such as dimers and trimers. Part of these mayhave a short side chain with, for example, a uronic acid or an arabinofuranolsyl unit.To split oV the side chains, other enzymes are necessary as well as for splitting oV, forexample, the acetyl substituent which may occur in a xylose unit. bxylosidases splitthe oligomers into simple xylose units. From Eriksson et al. (1990). With kindpermission of Springer Science and Business Media.84 BJORN BERG AND RYSZARD LASKOWSKITable 2 Some fungal species for which raised N concentrations have, or alterna-tively, have not elicited a repressing eVect on lignin degradationSpecies Comments Literature referenceSensitive to NPhanerochaetechrysosporiumIsolated from wood Keyser et al., 1978Eriksson et al., 1990Phlebia brevispora Leatham and Kirk, 1983Coriolus versicolor Leatham and Kirk, 1983Heterobasidion annosum Some repression Bono et al., 1984Not sensitive to NPleurotus ostreatus Freer and Detroy, 1982Lentinus edodes Leatham and Kirk, 1983NRRL 6464 Not identified Isolated from cattle dung Freer and Detroy, 1982With kind permission of Springer Science and Business Media.DECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 85there is a dominance of whiterot fungi that are not sensitive to high litter Nconcentrations as regards lignin degradation.The results until today suggest that N repression of lignin degradation iscommon but not always the rule. The addition of N to fungal cultures may,in certain cases, even increase their eYciency to utilize lignin. We wouldexpect that such fungi whose lignin degradation is stimulated by N, andNtolerant fungi in general, would be found in environments with high Nconcentrations, as in the example previously given with cattle dung, whereasmost whiterot fungi that grow in and on wood are adapted to low Nconcentrations. Many of the fungi that have been studied were isolatedfrom wood, and the low N content in wood (with CtoN ratios in therange from 350 to 500) may explain the generally strong influence of N.2. EVect of ManganeseManganese is essential for the activity of Mnperoxidase, a lignindegradingenzyme with Mn as part of the functioning enzyme, and high Mn levelsenhance its production (Perez and JeVries, 1992). Manganeseperoxidasebelongs to the group of enzymes that are classified as phenoloxidases,enzymes that oxidize and open aromatic ring structures in lignin. Althoughnot much was published on this enzyme before 1983, Lindeberg (1944)discovered in the 1930s that Marasmius spp. in culture were dependent onthe Mn concentration for their growth and that a low level of Mn in asubstrate may hamper the degradation of lignin. This finding was notpursued and not until the 1980s did additional detailed studies follow.The role of Mnperoxidase in lignin degradation is not clear, although oneof its roles may be to form H2O2 (see Textbox 1). The enzyme itself shows noaYnity to nonphenolic compounds, which, on the other hand, are readilyTextbox 1 Manganese peroxidase, an enzyme in the lignindegradingenzyme systemManganese peroxidase was discovered in 1985 as an enzyme in the lignolyticenzyme system. The enzyme is dependent onMn2 as a component, a socalledcoenzyme. The Mn is essential for the activity of the enzyme. Mnperoxidase isthe most common ligninmodifying peroxidase produced by almost all woodcolonizing basidiomycetes causing whiterot and various soil litterdecomposing fungi. Multiple forms of this enzyme are secreted byligninolytic fungi into their microenvironment, where the enzyme candissolve parts of the lignin in wood to be released in soluble form. Theenzyme is not only active against diVerent lignin species but can alsoparticipate in the degradation of, for example, humic acids.When degrading a substrate, the Mnperoxidase preferentially oxidizesmanganese(II) ions (Mn2), which always are present in wood and soil, intothe highly reactive Mn3 ion, which is stabilized by, among other substances,oxalic acid, and sometimes precipitated. Such oxalicacid chelated Mn3,which has a low molecular weight and is diVusible, acts, in its turn, as aredoxmediator that attacks phenolic lignin structures, resulting in theformation of unstable free radicals. Mnperoxidase is capable of oxidizingand depolymerizing natural and synthetic lignins as well as entirelignocelluloses, for example, in milled straw or wood in cellfree systems.Depolymerization is enhanced in the presence of cooxidants such asunsaturated fatty acids.86 BJORN BERG AND RYSZARD LASKOWSKIattacked by ligninase. It has been found that MnO2 stabilizes lignin peroxi-dase and may accumulate in wood attacked by whiterots (Blanchette et al.,1984). Manganese is also involved in the regulation of other lignolyticenzymes, including laccase (Archibald and Roy, 1992) and lignin peroxidase(Perez and JeVries, 1992).3. EVect of the C Source on Lignin DegradationIt appears that the presence of a carbon source other than lignin stimu-lates the lignin degradation for several whiterot species, includingP. chrysosporium, Coriolus versicolor, Coriolus hirsutus, Polyporus spp.,and Lentinus edodes. It has been also found that cellulose has a strongerstimulating eVect on lignin degradation than, for example, glucose, anDECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 87observat ion that was ascri bed to its low er availabil ity, thu s an influen ce ofcatabol ite repres sion cou ld be expecte d (cf. Se ction C.1.). The major org aniccompou nds in litter are normally the insol uble ones, such as lignin, cell ulose,and hemic elluloses, an d the latter ones normal ly sup ply the lignin degrad ingorganis ms with alterna tive C sources .A large group of the white rots may degrad e ligni n prefer entially tocellulose (Hat akka, 2001). Alth ough almos t all white rot fungi produceMn peroxida se, this enz yme ap pears to be the mo st impor tant lignol yticenzyme for those fungi that prefer lignin to cellu lose.D. Degradation of LigninLignin de gradation is a pro cess that di Vers amo ng three gen eral groups ofdecompo sers called white rot, soft rot, an d br own rot. Alth ough the na mesare old an d refer to characteris tics easily seen by the eye, there are alsofunction al diV erences in the enzymat ic degradat ion mechani sms, whichmotivate continued use of the term inology . The names often are used inconnecti on with fungi, althoug h bacter ia also can degrade lignin andbe classified accordi ng to this terminolog y. Some charact eristics for thelignin degradat ion of each of these groups are given in the followi ng text ,starting with whi te rots, whi ch probably are the best investiga ted lignindegrader s known.1. Lign in Degr adation by White Rot Fun giThere is a large numb er of di V erent en zymatic mechani sms for lignin degra-dation in whi te rots, but only one is well descri bed so far, namely that forPhaneroch aete chryso spori um. Wh ite rot organis ms possess the ability tocomplet ely miner alize lignin to CO2 and H 2O. The atta ck on the ligninstructure ha s long been consider ed to start with the remova l of the methox ylgroup ( Fig. 5A,B ). More recent resear ch has shown that a first step is acombinat ion of demeth ylation and hy droxylation, resulting in adjacent OHgroups on the aromatic ring, creating a starting point for the next step,which is an oxidative attack on the aromatic ring (Eriksson et al., 1990),resulting in ring cleavage and the creation of carboxyl groups. This cleavageof the aromatic ring (Fig. 5) is an oxygendemanding step and experiments inan atmosphere of ambient air and pure oxygen (Table 3) illustrate a highermass loss from decomposing lignin in the pure O2. In the following steps,parts of the former aromatic ring are broken oV and larger units are alsobroken oV from the main lignin structure.88 BJORN BERG AND RYSZARD LASKOWSKIThe lignolytic enzyme system of our example fungus (P. chrysosporium) issynthesized as part of several physiological events that appear to be triggeredby N starvation, as described by Kirk (1980) (see following text). Compar-isons of the lignolytic system of P. chrysosporium to those of other whiterotfungi indicate that several diVerent lignolytic enzyme systems exist. It haseven been suggested that the lignolytic systems could be speciesspecific,which would mean that, for example, each fungal species would have itsown lignolytic enzyme system and be the basis for a special ecological niche(Hatakka, 2001). A good example of such a relation to ecological niche isthat of the whiterot Ganoderma lucidum, which produces Mn peroxidase ina medium with poplar wood but not in one with pine wood (DSouza et al.,1999). This observation may help to explain why whiterot fungi are morecommonly found on angiosperm than on gymnosperm wood (Gilbertson,1980).Figure 5 (continued )Figure 5 Part of a lignin molecule of spruce during degradation. (A) In thedegradation by whiterots (from Kirk, 1984), a demethoxylation and hydroxylationare followed by an oxidative step leading to ring cleavage. (B) The same moleculeunder attack by brownrot fungi. In this case, only methoxyl groups are removed bythe enzyme.DECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 892. Lignin Degradation by SoftRot FungiToday, it has been well confirmed that softrot fungi do degrade lignin and,in laboratory experiments using pure cultures and whole wood, up to 44% ofthe lignin was degraded at a wood mass loss of 77% (Nilsson, 1989). Ingeneral, soft rots are considered to degrade lignin, at least to some extentless than whiterots but clearly more than brownrots. An observation madeTable 3 Degradation of aspen wood lignin by diVerent whiterot fungi in thepresence of air or pure oxygenaFungal species14CO2evolution [%]Klason ligninloss [%]Air O2 Air O2Phanaerochaete 10.8 35.2 13 40chrysosporiumCoriolus versicolor 14.6 35.5 24 46Gloeoporus dichrous 9.7 18.1 22 24Polyporus brumalis 16.6 33.0 19 33Merulius tremellosus 14.0 22.3 30 40Pychnoporus 13.6 22.6 18 37cinnabarinusLentinus edodes 9.7 18.0 18 41Bondarzewia berkeleyi 9.0 13.8 25 27Pleorotus ostreatus 11.7 11.6 17 17Grifola frondoza 9.2 10.6 8 15aDeterminations were made as 14CO2 evolution and as Klason lignin. From Reid and Seifert(1982). With kind permission of Springer Science and Business Media.Textbox 2 Syringyl and guaiacyl units versus lignin degradationWe may speculate that since softwood lignin has a high level of guaiacyl units(see following text), at least softrots have less potential to degrade lignin fromconifers. In contrast, the syringyl units of deciduous species have beenobserved to be more readily oxidized by softrots.This might be of importance for the fungal populations of diVerent ecosystems,and could be an important factor for a diVerence in lignin (and litter)degradation between coniferous and deciduous forest floors.90 BJORN BERG AND RYSZARD LASKOWSKIon the fungus Daldinia concentrica may explain why these fungi prefer todegrade lignin of hardwood species to that of softwoods. This fungusdegraded birch wood eYciently but not that of pine (Nilsson, 1989) and anexplanation can be that the lignolytic peroxdidases of softrot fungi have lesspotential to oxidize the softwood lignin with a high level of guaiacyl units. Incontrast, the syringyl lignin in hardwoods is readily oxidized by softrotfungi (Nilsson et al., 1989) (Textbox 2).DECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 913. Lignin Degradation by BrownRot FungiBrownrot fungi decompose mainly the cellulose and hemicellulose compo-nents in wood and, although they have the ability to significantly modify thelignin molecule, they are not able to completely mineralize lignin. They candegrade cellulose and hemicellulose in a fiber with a relatively small loss oflignin.Brownrot fungi are considered to have some similarities in their degrada-tion mechanisms to those of whiterot fungi. In both cases, the formation ofhydroxyl radicals (see Textbox 3) that attack wood components is importantand high oxygen tensions support the degradation (Hatakka, 2001). Theradicals formed by brownrot fungi can remove methoxyl groups fromlignin, produce methanol, and leave residues of modified lignin (Erikssonet al., 1990). It was assumed earlier that all brownrot fungi use the samemechanism for wood decay. However, newer research has indicated thatsimilarly to whiterots, brownrot fungi have a number of diVerent mechan-isms. The initiation of the degradation of both lignin and cellulose appearsto be by diVusible small molecules that can penetrate the fiber cell wall. Incontrast to whiterots, only one brownrot fungus has been found to produceMnperoxidase.Relative to native lignin, brownrotted lignins are structurally modifiedand the aromatic rings have decreased numbers of methoxyl groups andincreased numbers of hydroxyl groups (Fig. 5) (Crawford, 1981). It has beenobserved that also carbonyl and carboxyl groups are formed (Jin et al.,Textbox 3 A hydroxyl radical participates in the degradation of ligninPart of the degradation of lignin is carried out through nonenzymaticprocesses. In one of these, the socalled hydroxyl radical plays an importantpart. Although not all steps in lignin degradation are understood, we mentionthe concept here.When oxygen is reduced, hydrogen peroxide is formed, which in its turn is splitin a reaction. Below we have given a general chemical reaction. So far it is notknown how fungi carry out the reaction.Fe2 H2O2 ! Fe3 OH OHIt seems clear, though, that the highly mobile radical (OH) is produced byfungal enzymes, among others, a cellobiase oxidase and laccase. Hydroxylradicals may cause an oxidation of lignin to quinines.92 BJORN BERG AND RYSZARD LASKOWSKI1990). Brownrotted lignin is more reactive than native lignin due to theincreased content of phenolic hydroxyl groups.IV. DEGRADATION OF FIBERSPreviously, we have described the degradation of the single compoundsthat build up the fibers and how the compounds are arranged. Still, whenlignin, cellulose, and hemicelluloses are combined into a fiber structure(see Chapter 2, Fig. 7), new eVects appear due to the increased complexityof the substrate and so diVerent decomposer groups follow diVerent organicmatter decomposition pathways.A. FungiWhiterot fungi carry out two diVerent types of fiber degradation, namely,simultaneous rot and selective lignin degradation. Some species can carryout both types (Blanchette, 1991). In simultaneous rot, both lignin and thecarbohydrates are degraded simultaneously. The fungi erode the cell walladjacent to the hyphae, creating erosion channels, or they generally erodethe lumen surface, resulting in an overall thinning of the cell wall. Inaddition, the hyphae move from cell to cell through pits or by boringthrough the wall. The other type of degradation, selective delignification,often results in cell separation as well as overall thinning of the cell walls.Whiterots sometimes seem to have a delay or lag time, with relatively slowmass loss before a period of mass loss that is more rapid. Blanchette et al.(1997) used a novel biotechnological approach to demonstrate why this mightoccur. They incubated loblolly pine wood with a whiterot fungus, Ceripor-iopsis subvermispora. They then placed the wood, in various stages of decay,into solutions containing proteins of known sizes. Using immunocytochemi-cal techniques, they were able to show that proteins of the size of cellulasesand lignindegrading enzymes could not freely pass through the wood untillater stages of decay. After cell walls had been thinned enough to increasetheir porosity, it was possible for extracellular enzymes to move freely fromlumen to lumen, thus initiating the stage with a higher rate of mass loss.Softrots generally develop and grow under conditions that are not favor-able for Basidiomycetes. However, a key for good growth of softrots is highavailability of nutrients. It is also generally held that softrots require moistconditions, though this requirement may not be diVerent from that ofBasidiomycetes (Worrall et al., 1991).Two forms of softrots are identified based on the morphology of thedegradation they cause (Blanchette, 1995). Type I causes the formation ofDECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 93cavities in the secon dary wall and is most commonl y found in conife rs,where ligninli ke mate rials accumul ate on the edge of the ca vities. Type IIcauses cell wall erosi on, but unlike white rot, soft rot does not degrad e themiddle lame lla (Fig. 7, Chapter 2). It is possibl e that the middl e lamella isresistant to this group of fun gi because its lignin contai ns more guiacyl propan e units . Type II is more common in angiospe rms.Brown rot fungi have the ability to de grade hol ocellulose in plan t cellwalls without first remov ing ligni n and ap parently be gin their atta ck onfibers by degradi ng the hemic ellulose matr ix. A support for this theory isthat xylans begin to disappea r before cellu lose (Hi ghley, 1987). The first stepis a rapid decreas e in the degree of polyme rization of the holo cellulosepolyme rs. In wood, the resul t is a rapid loss of fibe r stre ngth when thelarge polyme rs are fract ured. Thes e two factors suggest that agents smal lerthan enzymes are invo lved (Green and Highl ey, 1997). Thi s initial de grada-tion step is general ly acco mpanie d by a relat ively low mass loss.In fiber de gradat ion, brown ro t fungi appear first to atta ck the S2 layer ,leaving the S3 layer until later (see Fig. 7, Chapt er 2; Highl ey et al. , 1985).Althoug h the reason for this is not known, a pr oposed mechani sm thatagrees with observation s was given by Hirano et al. (1997). They suggestthat the bro wn rot fun gus grow s into the cell lumen an d relea ses a lowmolec ular mass substa nce (mol ecular weight 100 05000) that probably di-Vuses into the S2 layer. Fe(III) is then reduced to Fe(II) and becomes chelatedby this substance. The newly formed complex with the Fe(II) catalyzes a redoxreaction that produces hydroxyl radicals. These hydroxyl radicals are ableto cut canals through the S3 layer large enough for cellulases to penetrate(Textbox 3). Of course, more work is needed to vali date this mechani sm andto identify the unknown substances required for its operation.B. BacteriaThough bacteria have long been known to be involved in decomposition oflitter, they have received far less attention and have been less studied than fungiin regards to the enzymatic mechanisms. In most cases, bacteria coexist withfungi, and their presence has been shown to increase and even double the rate offungal growth on wood and increase the overall rate of decay (Blanchette andShaw, 1978). Although bacteria once were considered not capable of degradinglignified cell walls without some type of pretreatment, a variety of fiberdegrad-ing bacteria has now been identified. Three types of bacterial degradation havebeen recognized, the types based on the manner in which they degrade the cellwalls of the substrate, namely tunnelling, erosion, and cavitation (Blanchette,1995). Bacterial decomposition seems to be more common in situations wherefungi are under stress, whichmeans that they live under suboptimal conditions.94 BJORN BERG AND RYSZARD LASKOWSKIBacteria have also been found to degrade substrates, especially wood, thatresistant to fungal decay (Singh et al., 1987).V. MICROBIAL COMMUNITIES AND THEINFLUENCE OF SOIL ANIMALSA. Microbial Succession and CompetitionThe composition of the microbial community that invades newly shed litterand litter in late decomposition stages depends on the initial properties of thelitter and the changes in litter properties over time. Decomposer commu-nities undergo many of the same processes as do communities of primaryproducers. These processes include succession and competition, and thepathway of plant litter decay may be influenced by modifications in theseprocesses.The change in microbial communities composition over time (microbialsuccession) is related to the change in quality of the decomposing substrate,but it also occurs because diVerent organisms invade substrates at diVerentrates. An example is taken from a study on the fungal community oncommon ash, common oak, and European beech twigs, where the successionof species was followed (GriYth and Boddy, 1990). The primary colonizersincluded endophytes, that is, fungal species that were present on the twigsalready while they were still alive. Secondary invaders did not show up inappreciable numbers until about 11 months after twig death. This groupdid not include endophytic species. GriYth and Boddy (1990) identifieda third type of colonizer, which they called the superficial, which appearedon the surface rather early when decay had started. Still, these specieswere not present on the living twig. It is probable that this pattern is similarfor all litter types, though, of course, the species and the timing may diVer.As an example, spruce needles normally persist on twigs for some timeafter death but decomposition can begin when needles ultimately fall ontothe forest floor and the changing environmental conditions and the avail-ability of a rich variety of inocula result in a change in the microbialcommunity.In addition to the microbial succession that occurs along with decomposi-tion, there are seasonal changes in the microbial community reflecting theseasonal changes in temperature and moisture. For example, Kayang (2001)followed fungi, bacteria, and selected enzyme activities in newly shed leaveson Nepalese alder in India under a climate that was described as subtropicalmonsoon. Frosts occur there during December and January, and the dryseason lasts from November through March. The fungal and bacterialpropagule numbers varied by a factor of nearly five between winter andFigure 6 The three main enzymes in the cellulolytic system appear in a sequence inthe substrate being decomposed exocellulase, endocellulase, cellobiose dehydroge-nase. General pattern based on data from Linkins et al. (1990).DECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 95summer. Activity of diVerent enzymes such as invertase, cellulase, andamylase reached their peaks, in that order, before microbial numbers,namely between April and June, and then fell slowly. The sequence ofpeaks shows a succession of enzyme activities reflecting a succession ofmicroorganisms.When investigating the activity of cellulases and cellobiose dehydrogenaseon leaf litter in laboratory microcosms, Linkins et al. (1990) observed similarpatterns for three diVerent litter species. The litter originating from redmaple, flowering dogwood, and chestnut oak diVered in decay rates and inconcentrations of lignin. However, all three species exhibited an increase incellulase activity that reached a peak at the same time that cellulose disap-pearance rate was at its maximum. When cellulase activity began to decline,the cellobiose dehydrogenase activity started to increase (Fig. 6).As fungal communities are changing, so are the enzyme activities. Osonoand Takeda (2001) investigated the fungal populations on Japanese beechleaves as they decomposed in a cool temperate deciduous forest. Both totaland living fungal biomass increased during the first year of decay and thenfluctuated for the remainder of the study period. The percentage of fungibelonging to Basidiomycetes increased for the first 21 months of the study,and reached a maximum of 25 to 35% of the total living fungal biomass. Theauthors noted that the relative abundance of Basidiomycetes was linearlyand negatively related to the lignocellulose index (cf. Section IV.B.2, anindex of litter quality equal to the fraction of holocellulose in the lignocellu-lose complex. As part of their study, they identified over 100 fungal taxa on96 BJORN BERG AND RYSZARD LASKOWSKIthe beech leaves and distinguished three groups: (i) an earlyappearinggroup, (ii) a lateappearing group, and (iii) a group of species constantlypresent. The earlyappearing fungi were present during the period of netnutrient immobilization and the lateappearing fungi increased in number asthe litter moved into the phase of net mineralization (see Chapter 5).Decomposer populations may work synergistically or in competition.Competition is visible in, for example, decaying logs where clear and discretezones of decay caused by diVerent organisms can be easily distinguished.There are examples where the organisms define their boundaries with blackzonelines, which make them very clear. The interactions may change asdecomposition proceeds. For example, Bengtsson (1992) found a synergismwith no evidence of competition between fungi and bacteria on leaves ofcommon beech during their first year of decay in stream microcosms. Incomparison, Miller et al. (1999) found clear evidence of competition betweenfungi and bacteria on oneyearold beech leaf litter, and also in a microcosmstudy. This diVerence may relate to the litter age, and hence the state ofdecomposition, the litter quality, and the combination of species.As decomposition proceeds, the microorganisms themselves can becomeimportant substrates for the microbial community. Some fungi, includingwooddecaying fungi, are able to use the cell walls of other fungi or bacteria,presumably as an N source, and some bacteria are able to degrade hyphalcell walls (Tsuneda and Thorn, 1995).There are many interactions among the microorganisms involved in de-composition of litter and these interactions change over time. These dynamicsystems are complex and not easily described. However, this natural com-plexity does have implications for the interpretation of pure culture andmicrocosm studies. Such studies are often the only way to control variabilityenough to test the hypotheses about litter decomposition precisely. On theother hand, the behavior of a single, isolated species or of a simple commu-nity in a mesocosm may not reflect its behavior in the more complex naturalenvironment.B. EVects of Soil Animals on the Decomposition ProcessAlthough for tropical forests, some authors report litter decomposition bysoil animals to be twice as high as that performed by microorganisms (Swiftet al., 1981), in the light of newer findings, it is very doubtful that animals areable to decompose the polymer organic compounds in litter, in the strictmeaning of the term. Complex organic polymers, such as lignin, can bedegraded solely by microorganisms. Invertebrates able to digest such poly-mers do so through symbiotic microorganisms inhabiting their digestivetracts; also, in such cases, there are the microorganisms that are ultimatelyDECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 97responsible for organic matter degradation. This, by no means, should beunderstood as neglecting animals role in organic matter decay. Even if thebiochemical/enzymatic degradation is performed by microorganisms, soilfauna plays an important role in many ecosystems, even in the temperateclimate and boreal zones. In general, we may consider their role to be suchthat they increase litter palatability to microorganisms through mechanicaltransformation of freshly shed litter, for example, by comminution of leavesor needles and thus opening new surfaces for microbial attack.Soil animals, by grazing either directly on microorganisms (e.g., fungifeeding springtails) or on dead organic matter inhabited by bacteria andfungi, can also spread their populations and increase the turnover rate, thusenlarging microbial productivity and, in consequence, the amount of organicmatter transformed. Mixing organic matter with mineral soil and diggingactivity improves soil aeration and creates more favorable conditions foraerobic microorganisms, such as lignindegrading fungi. Thus, even if adirect participation of soil animals in organic matter decomposition isminor, their overall influence cannot be underestimated, especially inwarmer climates. For example, in Mediterranean forests, just one speciesof millipede, Glomeris marginata, can consume as much as 8 to 11% of theannual leaf litter fall (Bertrand et al., 1987). A population of anotherdiplopod, Cylindroiulus nitidus, in an oak forest in southern France wasestimated to consume as much as 10 to 14 g litter per square meter yearly,also a substantial amount (David, 1987). Couteaux et al. (2002) studied theeVect of temperature and presence of G. marginata on litter decompositionrate. Although the studies were carried out over a broad range of tempera-tures (4, 8, 15, 23, and 30C), a significant increase in decomposition rateattributable to the presence of G. marginata was detected only at 15 and23C (Table 4). More detailed studies allowed the authors to hypothesizethat the animals aVected litter decomposition by increasing the decomposi-tion asymptotic limit value (cf. Section IV.F, Chapter 4) rather than increas-ing the decomposition rate itself (Table 4; Couteaux et al., 2002).Other groups of soil animals besides arthropods, which are important fordecomposition through their eVect on fungal and bacterial populations, areprotozoans, nematodes, enchytraeids, and earthworms. The first twogroups, inhabiting the smallest soil cavities and pores and living in the thinwater film covering soil aggregates, graze on bacteria and fungi, causing arelease of soluble nutrients and aVecting microbial populations growth rates(Bamforth, 1988). In an experiment with metalpolluted soil, adding enchy-traeids and microarthropods to soil columns increased leaching of dissolvedcarbon and nutrients by 20 to 30% (Bengtsson et al., 1988). Komulainen andMikola (1995) found a significant increase in mineral nitrogen releasefrom microcosms containing the enchytraeid Cognetia sphagnetorum in acomparison between an enchytraeidmicroorganism system and one withTable 4 Remaining mass, out of original 5 grams, after 198 days incubation ofAleppo pine litter at diVerent temperatures in the absence or presence of Glomerismarginata (averages standard deviations) and calculated decomposition limitvaluesasymptotes for carbon mineralization estimated with asymptotic regressionmodel for CO2 release from litter during experimental incubation (Couteaux et al.,2002)Without G. marginata With G. marginataTemperature(C)Remainingmass(mg dry mass)Limitvalue (mg Creleased)Remainingmass(mg dry mass)Limitvalue (mg Creleased)4 3.47 0.01 1719 3.45 0.06 17208 3.29 0.01 1293 3.26 0.03 137015 3.14 0.08 1098 2.98 0.08 128323 3.10 0.04 1002 2.93 0.12 113130 2.99 0.04 1126 2.83 0.03 120498 BJORN BERG AND RYSZARD LASKOWSKImicroorganisms only. Raised CO2 evolution and mineralization of nitrogenand phosphorus from litter and organic soil as the eVect of the presence ofsoil fauna was also found by Huhta et al. (1991). As microorganisms may belimited by nutrient availability in litter and humus, at least in some ecosys-tems and decomposition stages, any activity increasing nutrient accessibilitywould promote microbial population growth and, in consequence, decom-position rate. As discussed previously, this is definitely one of the results offaunal activity in soil. Teuben and Verhoef (1992) calculated that Collem-bola alone increase NO3 availability by 2.4 times through its production offeces.Although a number of studies, such as those already cited, indicate theimportance of faunal activity for mineralization rates, for microbial activity,and for biomass development, the present state of knowledge is not clearenough to take a general influence of soil fauna for granted (see also theintroductory part of this chapter). In some studies, the overall eVect of soilinvertebrates on organic matter mineralization was found to be small or evennegligible. For example, Kandeler et al. (1994) did not find any influence ofmesofauna on microbial biomass under field conditions. Further, they foundthat activities of extractable enzymes in soil (xylanase, cellulase, and betaglucosidase) were not aVected by exclusion of meso and macrofauna,indicating that the fauna did not influence the microbial population.The presence of soil fauna may also exert diVerent influences on microbialbiomass and CO2 release rate, as in the study by Forster et al. (1995). Theseauthors, studying interactions between microorganisms and enchytraeids ingrassland soil and litter, found that the worms did not aVect microbialDECOMPOSERS: SOIL MICROORGANISMS AND ANIMALS 99biomass but increased soil respiration rate. In amore recent study on eVect ofspecies richness and density of soil mesofauna on nutrient mineralization inan Italian ryegrass field, Cole et al. (2004) found, in turn, that soil respirationdecreased with increasing density of microarthropods, while the biomass ofmicroorganisms was not aVected. Despite that, concentrations of total nitro-gen and NO3N in soil leachate increased with increasing faunal density,indicating an enhancing eVect of microarthropod abundance on nutrientrelease rate. Species richness had, however, the opposite eVect in regard tothe respiration rate and nitrogen concentration in leachate. Such resultsindicate an indirect influence of faunal activity, probably by stimulatingmicrobial population turnover rates.We have seen several studies in the literature involving adding diVerentbiocides to soil with the intention to eliminate part of the fauna. We haveavoided presenting the results of such studies since they are diYcult tointerpret. It is known that biocides may aVect microbial communities direct-ly, which means that a selective eVect is not achieved. Furthermore, some-times biocides may even serve as a carbon source for microorganisms,confusing the results.Yet another way in which mesofauna may aVect litter decomposition rateand nutrient turnover in ecosystems was described by Chapman et al. (2003),who studied eVects of arthropod herbivores on litter quality in a semiaridforest of pinyon pine. Although these eVects are obviously secondary and donot even relate to soil fauna, they are certainly worth mentioning whendiscussing the role of fauna on litter decomposition. The authors foundthat both species of herbivores studied significantly increased N concentra-tion and decreased the lignin: N ratios of aboveground litter. Also, litterphosphorus concentration and annual needle litterfall mass increased due toherbivory. Thus, herbivory produced litter that was richer in nutrients anddecomposed more rapidly. Chapman et al. conclude that herbivory mayincrease nutrient cycling rates in this system by altering the chemical qualityof litter.As we have mentioned, the eVect of faunal activity on litter decompositionseems larger in tropical ecosystems than in more northern, that is, borealones. However, even this diVerence is not that straightforward. For example,Gonzalez and Seastedt (2001) found higher faunal eVects on litter decompo-sition in tropical wet forests than in subalpine forests, but also in tropical dryforests, eVects of fauna on decomposition was lower than in the wet tropicalforests. As a result, no general diVerence in eVect of fauna on annual decayrates between tropical and subalpine forests was found. Although theseresults may seem contradictory at first glance, we may recall that litterdecomposition rates are strongly dependent on both temperature and soil/litter moisture. Gonzalez and Seastedt (2001) found that the total density ofsoil fauna was highest in wet tropical forests, followed by the subalpine100 BJORN BERG AND RYSZARD LASKOWSKIforests, and the lowest densities were found in dry tropical forest. Theysummarize their finding by stating that soil fauna has a disproportionatelylarge eVect on litterdecay rate in tropical wet forests as compared to thetropical dry forest or a subalpine forest.Besides climatic eVects on soil fauna activity, the eVect of forest floor type(humus type: mull, moder, or mor) is another obvious line of inquiry. Theresults, however, are not as clear as might be expected. Bocock et al. (1960)incubated European ash and durmast oak leaf litter in nets with 1 cm meshon mull and moder sites. Oak litter decay rates were independent of theforest floor type, but ash leaves disappeared much more rapidly on mullsites. It is important to note that there was significant earthworm (Lumbricusterrestris L.) activity on the mull site and that disappearance may be greaterthan actual decomposition because material could be easily moved out of thecoarse mesh nets.As can be seen from the examples presented, there is no general agreementabout the role of soil animals in litter decomposition. Advances in this areaof soil research are hampered by a number of technical complications. Forexample, allowing access of soil invertebrates, especially meso and macro-fauna, to litterbags or field micro/mesocosms makes it impossible to distin-guish any actual eVect on litter disappearance due to mechanical removal ofthe material. Similarly, distinguishing direct faunal decomposition of organ-ic matter from that due to activities of symbiotic microorganisms inhabitingdigestive tracts of many soil invertebrates is next to impossible at the presentstage of knowledge. We may state that eVects of soil fauna on litter decom-position, and soil structure in particular, are manifold and comprise suchprocesses as mechanical shredding of litter material, mixing organic matterwith mineral soil, distributing soil microorganisms and grazing on them, andincreasing palatability of dead organic matter and nutrient availability tobacteria and fungi. Further, soil fauna may structure soil through diggingactivity and deposition of fecal pellets as well as having a more directparticipation in decomposition either through their own digestive systemsor due to activity of symbiotic microorganisms. Thus, even if direct litterdecomposition through soil fauna might be negligible, the overall eVecton organic matter fate and soil properties may be significant. The primeexample is formation of mulltype soils, whose properties are largely deter-mined by eVective mixing of dead organic matter with mineral soilaprocess performed almost exclusively by soil meso and macrofauna. Inthe absence of these two groups of soil fauna, a completely diVerent soiltype is formed, with separate, thick layers of less decomposed organic matter(mortype soils).Decomposers: Soil Microorganisms and AnimalsIntroductionCommunities of Soil Microorganisms and AnimalsSoil MicroorganismsSoil AnimalsThe Degradation of the Main Polymers in Plant FibersDegradation of CelluloseDegradation of HemicellulosesEffects of N, Mn, and C Sources on the Degradation of LigninEffect of N Starvation on Lignin MetabolismEffect of ManganeseEffect of the C Source on Lignin DegradationDegradation of LigninLignin Degradation by White-Rot FungiLignin Degradation by Soft-Rot FungiLignin Degradation by Brown-Rot FungiDegradation of FibersFungiBacteriaMicrobial Communities and the Influence of Soil AnimalsMicrobial Succession and CompetitionEffects of Soil Animals on the Decomposition Process