9 Enzyme and Microbial Dynamics of Litter Decomposition Robert L. Sinsabaugh University of Toledo, Toledo, Ohio Margaret M. Carreiro University of Louisville, Louisville, Kentucky Sergio Alvarez Universidad Auto ´ noma de Madrid, Madrid, Spain I. INTRODUCTION The decomposition of plant litter may be the biosphere’s most complex ecological process in that it involves the interactions of a large number of taxa, spanning much of the range of biotic diversity. Because of the complexity, nearly all efforts to model plant litter de- composition have approached the problem from an ecosystem perspective: predicting mass loss (the emergent biotic process) from litter composition and physical conditions—the abiotic template (26,53). Technological developments of the past two decades have re- moved many impediments to the study of microbial dynamics in natural systems; however, many of these tools have not been applied extensively to the study of microdecomposer communities; this is beginning to change (22,75). Consequently, fundamental information on structural and functional patterns across systems, a requisite for development of general models, is lacking. The significance of this gap is apparent in the context of global change. The effects of atmospheric carbon enrichment, nitrogen deposition, climate alteration, and other anthropogenic processes on ecosystems cannot be predicted without decomposition models grounded in biotic process (51,52). The biotic process of decomposition spans three levels of organization: biochemical, organismal, and community. At the biochemical level, the topics of interest are the struc- ture of plant fiber and the enzymological characteristics of degradation. The enzymological features are complex. For the polysaccharides at least one enzyme is required for every combination of monomer, linkage, and secondary structure (69). For lignin and other aro- matic molecules, the process is principally oxidative; the enzymes have lower specificity, but the full range of enzymes and oxidants involved has not been defined (2,25). At the organismal level, the questions focus on the regulation of enzyme expression and the kinetics of growth. Studies of model organisms suggest a common pattern for the regula- Copyright © 2002 Marcel Dekker, Inc. Figure 1 The decomposition process presented as a successional loop. The diagram emphasizes the dynamic interactions among microdecomposers, extracellular enzymes, and substrate and high- lights the role of extracellular enzymes as the rate-controlling agents of decomposition. tion of extracellular enzyme expression that proceeds from environmental induction, to derepression of transcription, translational expression, and then transcriptional repression, if enzymatic reaction products exceed metabolic needs (70). This system optimizes the allocation of resources to the production of enzymes that once deployed outside the cell are subject only to environmental regulation (5). At the community level, the subjects include metabolism, structure, competition, succession, and diversity. Aside from patterns of biomass and respiration, and in some cases fungal succession (19), there is probably less known at this level than the others. The first step toward integrating microbial decomposition with traditional ecosystem perspectives is to integrate these three levels into a useful representation. This can be done by considering the decomposition process as a successional loop (Fig. 1). The substrate selects the microbial community, which produces extracellular enzymes that degrade and modify the substrate, which in turn, drives community succession. In this model, extracel- lular enzymes link substrate composition and microbial community metabolism. This cen- tral role, plus substrate specificity and ease of assay, make enzyme kinetics a powerful tool for investigating the functional diversity of decomposers and the mechanisms that link environmental disturbance to ecosystem responses (60,61,72). In this chapter, we review the literature on enzyme activities and decomposition, propose some metrics for comparative analyses, and present conceptual models for future research. Our review is limited to studies of natural systems that include quantitative data on the activities of one or more extracellular enzymes in relation to mass loss from a cohort of particulate plant detritus. Studies in which enzyme activities are measured in relation to microbial biomass, production, or respiration are also included if they are di- rectly related to plant litter decomposition. Information on the enzymatic capabilities of individual taxa is not presented. Assay methodologies are not described except when rele- vant for cross-study comparisons. II. ENZYMES OF INTEREST The enzymes of interest in decomposition studies are generally those that break down the principal components of plant fiber (cellulose, hemicellulose, pectin, lignin) into soluble units that either are directly assimilated by microorganisms or enter the dissolved organic Copyright © 2002 Marcel Dekker, Inc. matter pool. Because the main constituents of plant fiber do not contain nitrogen (N) or phosphorus (P), extracellular enzyme systems involved in the acquisition and recycling of N and P for microbial growth are also of interest. The most commonly measured classes of enzymes are cellulases, hemicellulases (xylanases, mannanases), pectinases, (poly) phenol oxidases, peroxidases, chitinases, peptidases, ureases, and phosphatases. Each of these functional classes includes multiple forms of the enzymes, whose structure, kinetics, and deployment may vary considerably across taxa. In general, a different enzyme is required for each type of linkage and each type of monomer; also enzymes that act on the interior linkages of polymers (endoenzymes) are usually distinct from those that attack free ends (exoenzymes). This complexity creates a hierarchy of synergistic interactions: individual enzymes are components of multienzyme systems that collectively degrade specific polymers, multiple systems of enzymes degrade the matrix of polymers that con- stitute plant cell walls, and diverse microbial taxa deploy enzyme systems that interact to effect decomposition. The biochemical characteristics of these enzyme systems have been reviewed extensively (3–5,9,14,20,21,25,28,34,41,47,56,58,69,83,84). A. Aquatic Systems The literature on enzyme dynamics in relation to litter decomposition is not extensive. The first study, by Sinsabaugh and associates (64), reported patterns of cellulase activity (β-1,4-exoglucanase, β-1,4-endoglucanase, β-glucosidase) for senescent Cornus florida (flowering dogwood), Acer rubrum (red maple), and Quercus prinus (chestnut oak) leaves decomposing in a woodland stream. They observed that each enzyme showed a distinct temporal pattern and that the ratio of endoglucanase to exoglucanase activities increased through time and with initial lignin content. Chamier and Dixon (8) found that pectinolytic enzymes produced by hyphomycetes, the principal fungal decomposers of plant litter in aquatic systems, were important components of the decomposition process. Tanaka (76,77) studied decomposition of senescent leaves from the reed Phragmites communis in a coastal saline lake. The microbial community, initially dominated by bacteria, became fungus-dominant after a few months. Both groups produced cellulolytic and xylanolytic enzymes whose activities were correlated with mass loss. Sinsabaugh and Linkins (68) collected particulate organic matter (POM) from depo- sitional areas of a boreal river and examined the distribution of enzyme activities in rela- tion to particle size and composition. This descriptive study was followed by two others (74,85) that included analyses of structural similarity in POM-associated microdecom- poser communities. These studies showed that POM generally becomes more recalcitrant with decreasing particle size and that carbohydrase activities tend to decrease while oxida- tive activities increase. Fungi become scarce as POM size decreases below 1 mm and microbial community diversity steadily increases as size approaches 0.1 mm. To determine how particle comminution and the shift from a fungus-dominated to a bacteria-dominated decomposer community affect decomposition, POM was collected from a woodland stream and sorted into three ranges (1–4, 0.25–1, 0.063–0.25 mm), which were dried, placed in litter bags, and returned to the stream (73). Mass loss and the activities of seven enzymes involved in lignocellulose and chitin degradation were followed. Natural POM accumulations were also collected, size-sorted, and assayed. When the enzyme activities were integrated over time and regressed against mass loss, it ap- peared that the decomposition of particles Ͻ1 mm-was less efficient (i.e., lower mass loss Copyright © 2002 Marcel Dekker, Inc. increment per unit of enzyme activity) than the decomposition of POM Ͼ 1 mm by factors of 1.5 to 7. In addition, enzyme activities associated with the POM confined in litter bags were generally lower than those associated with in situ POM, suggesting that the litter bag technique was underestimating in situ turnover rates. Estimates of in situ turnover rates, generated from regression models relating enzyme activities and mass loss, were up to twice that of confined POM. This approach was applied by Jackson et al. (29) to study the spatial and temporal dynamics of POM turnover in a Typha sp. (cattail) marsh. Three size ranges of POM were collected and placed in litter bags at two sites. In situ enzyme activities were monitored along transects across the marsh. Size-specific relation- ships between enzyme activity and mass loss generated from the litter bag data were used to estimate instantaneous mass loss rates across the spatial grid. Sinsabaugh and Findlay (65) collected four size ranges of POM from a Typha sp. wetland, a Trapa sp. (water chestnut) wetland, and two channel sites along the Hudson River estuary and assayed for lignocellulose and chitin-degrading enzyme activities; bacte- rial and fungal biomass and productivity were estimated. Bacterial biomass and productiv- ity increased as particle size declined, whereas fungal biomass decreased. Using estimates of POM turnover rate based on enzyme activities, they calculated that production effi- ciency, i.e., production rate/decomposition rate, ranged from 1% to 30%; thus most of the soluble products of decomposition were exported as dissolved organic matter (DOM) rather than metabolized in situ. Denward et al. (15) used microcosms containing Phragmites australis to assess the effects of solar radiation on decomposition. Compared to that of shaded controls, bacterial abundance increased relative to that of fungi. β-Glucosidase activity also increased, shift- ing the α-glucosidase/β-glucosidase ratio from Ͼ1toϽ1. Other studies in aquatic systems have taken a comparative ecosystem approach. Kok and Van der Velde (36) placed litter bags containing fragments of senescent water lily leaves (Nymphaea alba) in alkaline (pH ca. 8) and acidic (pH ca. 5) freshwater ponds. The contents of each bag were analyzed for mass loss, cellulase activity, and xylanase activity. In a parallel study, they followed the decomposition of Nymphaea alba leaf disks in six freshwater microcosms with pH values from 4.0 to 8.0. Litter from each microcosm was analyzed for mass loss and the activities of cellulase, xylanase, polygalacturonase (pectinase), and pectin lyase. Corroborating the work of Chamier and Dixon (8), they concluded that the pH dependence of pectinolytic activity was a critical factor underlying differences in mass loss rates with system pH. The decomposition of Liriodendron tulipifera (tulip poplar) wood was studied in a small mountain stream from which new litter inputs were excluded (78). The activities of phosphatase and five lignocellulose-degrading enzymes were followed along with fungal biomass and breakdown rates. Compared to those in a reference stream, fungal biomass, enzyme activities, and breakdown rates were higher in the litter-excluded stream. Differ- ences in the ratios of phosphatase to carbohydrase and phenol oxidase to carbohydrase between the systems suggested that the increased decomposition activity was the result of higher nitrogen and phosphorus availability, a finding confirmed by water chemical analyses. Raviraja et al. (54) looked at hyphomycete diversity in relation to enzyme activities and mass loss at organically polluted river sites, using two litter types: Ficus benghalensis and Eucalyptus globulus. Although diversity was strongly depressed compared to that of unpolluted sites, mass loss rates and enzyme (cellulase, amylase, xylanase, pectinase) activities did not differ. Copyright © 2002 Marcel Dekker, Inc. Alvarez et al. (2) examined POM turnover in two ephemeral wetlands. Toro pond was surrounded by a Pinus pinea forest and had a littoral belt of Juncus and Scirpus spp. Oro pond was surrounded by a eucalyptus plantation and had a disturbed littoral zone. Both ponds dried completely during the summer. Two size ranges of POM (Ͼ1 mm and 0.063–0.5 mm) were collected from each site and placed in litter bags. In using the ap- proach of Sinsabaugh et al. (73) and Jackson et al. (29), confined and in situ POM samples were assayed monthly for β-glucosidase, β-N-acetylglucosaminidase, β-xylosidase, phenol oxidase, and alkaline phosphatase. When regressions of integrated enzyme ac- tivity and mass loss were compared, it appeared that decomposition of coarse particles was about 20 times more efficient than that of fine particles at the Toro site and about 10 times more efficient at the Oro site. Differences between sites were attributed to dif- ferences in organic matter quality and to the lower pH of Oro pond. At both sites, enzyme activities measured on material confined in litter bags were lower than those measured in situ. B. Terrestrial Systems Like those in aquatic systems, the terrestrial studies can be roughly classified into those dealing with fine-scale questions, such as the relationship between enzyme activities and litter composition or microbial dynamics and those that take a larger-scale comparative ecosystem approach. Linkins et al. (42) followed the decomposition of senescent Cornus florida (flowering dogwood), Quercus prinus (white oak), and Acer rubrum (red maple) leaves in a deciduous woodland in southwest Virginia. This step was followed by a micro- cosm study using the same litter types (43). They found that activity levels varied with litter type, that cellulose disappearance and mass loss were correlated with cellulase activi- ties, and that cellulolytic activity declined sharply as the lignocellulose index (LCI) ap- proached 0.7. (LCI is the fraction of acid-insoluble material in the residual plant fiber: [lignin ϩ humus]/[lignocellulose ϩ humus]). Zak et al. (88) examined fungal diversity, lignocellulase activities, and mass loss rates on mesquite sticks incorporated into the middens of desert wood rats. The dominant fungal taxa and fungal diversity varied with moisture availability, but these structural changes did not correlate with enzyme activity patterns or mass loss in these arid systems. Litter decomposition in a suburban forest in relation to N deposition has been studied (6). Senescent leaves of Quercus rubra (red oak), Acer rubrum (red maple), and Cornus florida (flowering dogwood) were placed on forest floor plots that were sprayed monthly with distilled water or with NH 4 NO 3 solution at dose rates equivalent to 2 or 8 g N m Ϫ2 y Ϫ1 . Mass loss responses to N amendment varied with the lignin content of the litter. Dogwood, a fast-decomposing, low-lignin litter, decomposed up to 25% faster than did the control plots. Maple, intermediate in lignin content, decomposed slightly faster at the lower N deposition rate and slightly slower at the higher rate. Mass loss rates for heavily lignified oak litter declined by up to 25%. Fungal biomass increased for all litter types (40% maple, 32% dogwood, 15% oak). Cellulolytic activity, measured by assays for β-glucosidase, cellobiohydrolase, and endoglucanase, increased with N deposition for all litter types; ligninolytic activity, measured by assays for phenol oxidase and peroxidase, varied with the lignin content of the litter. With added N, oxidative activity increased on dogwood litter, decreased on oak litter, and stayed about the same on maple litter. For all litter types, phosphatase activity increased with N deposition, indicating higher P demand. For dogwood, the activities of peptidase and chitinase, enzymes involved in N acquisition, Copyright © 2002 Marcel Dekker, Inc. wererepressedbyaddedN;formapleandoak,theseactivitiesincreased.Theresults suggestedthatwhiterotfungi,whichproduceligninasesinresponsetolowNavailability, weredisplacedbysupplementalN,slowingthedecompositionofrecalcitrantlitter. HenriksenandBreland(27)alsofocusedontheroleofNinthedecomposition process.Usingamicrocosmsystemofwheatstrawandsoil,theyfoundthatcarbonminer- alization,fungalbiomass,andactivitiesofcellulolyticandhemicellulolyticenzymesde- creasedwithNavailability. Intheareaofcomparativeecosystemstudies,Sinsabaughetal.(62,63)followed massloss,NandPimmobilization,andactivityof11typesofextracellularenzymesfor birchsticks(Betulapapyfera)decomposingateightupland,riparian,andloticsitesover afirst-orderwatershed.Masslossratesamongsitesvariedbyafactorof5andwere correlatedwithlignocellulaseactivities.Incontrast,relationshipsbetweenmasslossand activitiesofacidphosphataseandβ-1,4-N-acetylglucosaminidasevariedwidelyamong sites.TheserelationshipsalongwithanalysesoftheNandPcontentofthestickssuggested thatdifferencesinmasslossratesamongsitesweretiedtodifferencesinnutrientavail- ability. Inanotherexperiment,litterbagscontainingsenescentleavesofAgeratumconi- zoidesandMallotusphilippinensiswereplacedonthefloorofayoungtropicalforestsite innortheastIndia(38).OtherlitterbagscontainingleavesofHolarrhenaantidysenterica andVitexglabratawereplacedatamaturetropicalforestsite.Athigher-elevationsubtrop- icalsites,litterbagscontainingPinuskesiyaandMyricaesculentaleaveswereplacedin ayoungforestandbagscontainingPinuskesiyaandAlnusnepalensisleaveswereplaced inamatureforest.Sampleswereanalyzedformassloss,bacterialandfungalnumbers, cellulosecontent,Ncontent,solublesugarcontent,andactivitiesofcellulase,amylase, andinvertase.Cellulaseandamylaseactivitieswerecorrelatedwithmicrobialnumbers. Invertaseactivitycorrelatedwithsolublesugarcontent.Enzymeactivitiesandmassloss rateswerehigheratthelowerelevationsitesbutwerenotrelatedtostandage.Inasimilar study,thedecompositionofPinuskesiyaandAlnusnepalensisatadisturbedroadside forestsitewascomparedwiththatatanundisturbedsite(30).Againcellulaseandamylase activitieswerecorrelatedwithmicrobialnumbers,whereasinvertaseactivitywaslinked tosolublesugars. DillyandMunch(18)studiedenzymeactivitiesandmicrobialrespirationforAlnus glutinosa(blackalder)leavesdecomposingatwetanddrysiteswithinafenforest.Mass lossratesweremorethantwiceasfastatthewetsite.Microbialbiomassandrespiration decreasedovertime(16to2.3µmolg Ϫ1 h Ϫ1 ),buttheefficiencyofCutilizationincreased. Thesetrendswereparalleledbydecreasingβ-glucosidaseactivityandincreasingprotease activity. III.COMPARATIVEANALYSES Inthecontextofthesuccessionalloopmodel(Fig.1),therearethreedimensionsfor comparing studies of enzymatic decomposition: enzyme activity and litter composition, enzyme activity and mass loss rate, enzyme activity and community composition. A fourth dimension is large-scale patterns in relation to ecosystem type or disturbance type. These comparisons are external to the model but integrate enzymatic decomposition into large- scale perspectives. At present, there are too few studies in any of these areas to support Copyright © 2002 Marcel Dekker, Inc. much beyond inference, and in any case, comparisons are generally complicated by meth- odological diversity (72). A. Enzyme Activity and Litter Composition It is clear that enzyme activities vary with litter composition. Patterns are most easily seen when different types of litter decompose in the same environment. The patterns arise from both biotic and abiotic processes. The biotic processes are substrate selection of microdecomposer populations and physiological regulation of enzyme secretion. In addi- tion, litter-specific activity patterns to some degree reflect physicochemical processes of adsorption and stabilization (5,66), which are functions of the architecture and composition of the litter. The relative contribution of biotic and abiotic processes to enzyme activity patterns across litter types is probably a function of the structural resistance of the enzyme to inhibition, denaturation, and proteolysis. If the turnover time for a particular enzyme activity is longer than that for microbial populations, sorption processes may contribute to litter-specific patterns; if it is lower, then organismal processes predominate. Enzymes like α-glucosidase and invertase that process soluble saccharides appear to be in the latter category. Their activities are correlated with the soluble saccharide content of the litter (38,30). Activities generally peak early in the decomposition process, then decline markedly; activities tend to be higher on fast-decomposing litter (Fig. 2). β- Figure 2 Hypothetical distribution of relative enzyme activities with time for a decomposing cohort of herbaceous plant litter. The patterns reflect general trends reported in the literature. (A), invertase, α-glucosidase; (B), β-1,4-exoglucanase (exocellulase), (C), β-1,4-endoglucanase (endo- cellulase), (D), (poly)phenol oxidase; (E), peroxidase. Copyright © 2002 Marcel Dekker, Inc. Glucosidaseactivityalsotendstobehighestduringtheearlystagesofdecomposition, butbecauseofitsroleincellulolysis,activityremainssignificantevenathighmassloss values(18,64).Theactivitiesoftheothercellulases,β-1,4-endoglucanaseandβ-1,4,-exog- lucanase,increasemoreslowlyandgenerallypeakaboutmidway(40–80%massloss) throughdecomposition;earlypeaksareassociatedwithheavilylignifiedlitterandlater peakswithlabilelitter(42,62).Asaccessiblecellulosedisappears,theratioofendogluca- nasetoexoglucanasetendstoincrease,atleastpartlyasaresultofdifferentialsorption (66).(Poly)phenoloxidaseactivitytendstoincreasewithlignin-humuscontent(68),but activitiescanalsoberelativelyhighearlyindecompositionforlittersthathavehightannin contents(6).Inheavilyhumifiedmaterialperoxidaseactivitiespredominate.Thesetrends appeartobegeneral:theyoccurinbothaquaticandterrestrialsystems;theyapplyto individuallittertypesdecomposingthroughtime,aswellastodifferencesamonglitters ofvaryinginitialcomposition;andtheycanbeobservedalonggradientsofdecreasing particlesize.Thegeneralitiessuggestthatenzymeactivitiesmaybeusedtomakeinfer- encesaboutorganicmatterqualityacrossenvironmentaltemplates.Ratiosofβ-1,4-endog- lucanasetoβ-1,4-exoglucanaseactivity(64)orcellulolytic:ligninolyticactivity(65)have beensuggestedforthispurpose.However,environmentalfluctuationsthataltertempera- ture,moisture,andnutrientavailabilityalsoalterenzymeactivitiesandmayobscureor overwhelmpatternslinkedtolitterquality. B.EnzymeActivitiesandMassLoss Correlatingenzymeactivitieswithlittermasslossprovidesinformationonthemechanics ofdecomposition.Theactivitiesofseveralenzymeshavebeencorrelatedwiththerate ofdisappearanceofspecificlitterconstituentsorwithmasslossingeneral.Thisinforma- tioncanbeusedinvariousways.Oneistomodelenzymaticdecompositioninrelation totemperatureandwaterpotential(48,72).Anotheristousestatisticalmodelstoestimate instantaneousmasslossratesfromenzymeactivities.Thissecondapproachhasbeenap- pliedtoprovideestimatesoforganicmatterturnoverinheterogeneoussystems(65)and toestimatetheturnoverrateoffineparticulateorganicmatter,whichappearstobeunder- estimatedbythetraditionallitterbagmethod(2,29).Thethirduseistodescribetheeffi- ciencyandfunctionaldiversityofenzymaticdecomposition.Forthesecomparisons,turn- overactivitiesareausefulmeasurement. Turnoveractivitiesarecalculatedfrommodelsofmasslossasafunctionofcumula- tiveenzymeactivity,analogoustotraditionalmodelsthatdescribelitterdecayovertime asafirst-orderfunctionofresidualmass(Fig.3).Cumulativeenzymeactivityisexpressed in units of activity-days, which are calculated by integrating the area under a curve of enzyme activity vs. time. A linear regression, LN (cumulative activity-days) vs. time, generates a first-order rate constant called the apparent enzymatic efficiency with units of activity-day Ϫ1 . Inverting this rate constant produces an estimate of litter turnover expressed in units of activity-days. Implicit in this model is the premise, supported by field data (72), that mass loss per unit of enzyme activity decreases through time because of the increasing recalcitrance of the residual material. Like the traditional mass loss constant, and its inverse turnover time, these turnover activities provide a basis for comparison across sites, treatments, and litter types. These comparisons convey a sense of the quantity and type of ‘‘work’’ a microbial community has to do to decompose a cohort of litter and how this work is linked to substrate heteroge- neity and enzyme synergisms. They also provide a way to ‘‘map’’ the functional diversity Copyright © 2002 Marcel Dekker, Inc. Figure 3 Comparison between turnover time and turnover activity for a litter cohort. In this exam- ple, β-glucosidase activity was assayed during the decomposition of Acer rubrum litter at a forest site. The upper graph shows a traditional first-order exponential decay curve with cumulative mass loss plotted as a function of time. The slope of the linear regression is a rate constant (k) with units of day Ϫ1 ;1/k is the turnover time for the litter in days. The lower graph shows mass loss as a function of cumulative β-glucosidase activity. Cumulative activity is calculated by integrating the area under the curve of β-glucosidase activity vs. time and is expressed in units of activity-days. The slope of the regression is a first-order rate constant (k) with units of activity-day Ϫ1 ;1/k is the β-glucosidase turnover activity for the litter. Copyright © 2002 Marcel Dekker, Inc. Figure 4 Functional profiles of Cornus florida (flowering dogwood), Acer rubrum (red maple), and Quercus borealis (red oak) leaf litter decomposition based on turnover activities. A comparison of relative turnover activities shows that the enzymatic decomposition of dogwood leaves was more efficient than that of maple and oak. Oak leaves required the most phenol oxidase activity, whereas maple required the most peroxidase activity. Phosphorus acquisition activity was highest for oak; nitrogen acquisition activity was highest for maple. G, β-glucosidase; CBH, cellobiohydrolase; EG, β-1,4-endoglucanase; PhOx, phenol oxidase; Perox, peroxidase; NAG, β-1,4-N-acetylglucosamini- dase; GAP, glycine amino peptidase; AP, acid phosphatase. (Data from Ref. 6b). of decomposition. One example (6) shows that the decomposition of flowering dogwood leaves was accomplished with much less enzyme activity than that needed to turn over red maple and red oak litter and that extensive phenol oxidase activity was needed to decompose oak leaves, which have a lot of lignin, whereas a lot of peroxidase activity was required for decomposing maple leaves, which have a lot of nonlignin phenols (Fig. 4). Other studies suggest that the apparent enzymatic efficiency of decomposition declines with particle size (2,29), coinciding with the transition from fungal to bacterial dominance but also with increasing humification. Turnover activities can also be calculated for enzymes that are not directly involved in the decomposition of major litter components. For enzymes such as phosphatase, urease, peptidase, and chitinase, turnover activities are measures of relative effort directed toward obtaining N and P from organic sources. Such comparisons have proved useful in under- standing differences in decomposition rates among systems (63,78). Even within the same system, the enzymatic effort directed toward the acquisition of organic N and P varies among litter types (Fig. 4). A major constraint on the value of turnover activities is that direct comparisons across studies cannot be made unless the same assay methodology was used. C. Enzyme Activity and Community Composition In some decomposition studies, microbial numbers, biomass, or respiration has been linked with extracellular enzyme activities, but there have been few attempts to link community Copyright © 2002 Marcel Dekker, Inc. [...]... functional diversity in soils C-acquiring enzymes (cellulase, xylanase, β-glucosidase) were the least affected, phosphatase and sulfatase the most affected; N-acquiring enzymes (urease) were intermediate Another study of heavy metal contamination in grassland soils ( 39) showed that reductions in microbial biomass and substrate-induced respiration paralleled 1 0- to 50-fold reductions in extracellular enzyme... 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