Extracellular enzymes mediate the degradation, transformation and mineralization of soil organic matter. The activity of cellulases, phosphatases and other hydrolases has received extensive study and in many cases stoichiometric relationships and responses to disturbances are well established. In contrast, phenol oxidase and peroxidase activities, which are often uncorrelated with hydrolase activities, have been measured in only a small subset of soil enzyme studies. These enzymes are expressed for a variety of purposes including ontogeny, defense and the acquisition of carbon and nitrogen. Through excretion or lysis, these enzymes enter the environment where their aggegrate activity mediates key ecosystem functions of lignin degradation, humification, carbon mineralization and dissolved organic carbon export. Phenol oxidases and peroxidases are less stable in the environment than extracellular hydrolases, especially when associated with organic particles. Activities are also affected, positively and negatively, by interaction with mineral surfaces. High spatiotemporal variation obscures their relationships with environmental variables and ecological process. Across ecosystems, phenol oxidase and peroxidase activities generally increase with soil pH, a finding not predicted from the pH optima of purified enzymes. Activities associated with plant litter and particulate organic matter often correlate with decomposition rates and potential activities generally increase with the lignin and secondary compound content of the material. At the ecosystem scale, nitrogen amendment alters the expression of phenol oxidase and peroxidase enzymes more broadly than culture studies imply and these responses correlate with positive and negative changes in litter decomposition rates and soil organic matter content. At the global scale, N amendment of basidiomycete-dominated soils of temperate and boreal forest ecoystems often leads to losses of oxidative enzyme activity, while activities in grassland soils dominated by glomeromycota and ascomycetes show little net response. Land use that leads to loss of soil organic matter tends to increase oxidative activities. Across ecosystems, soil organic matter content is not correlated with mean potential phenol oxidase and peroxidase activities. A multiple regression model that includes soil pH, mean annual temperature, mean annual precipitation and potential phenol oxidase activity accounts for 37% of the variation in soil organic matter (SOM) content across ecosystems (n = 63); a similar model for peroxidase activity describes 32% of SOM variance (n = 43). Analysis of residual variation suggest that suites of interacting factors create both positive and negative feedbacks on soil organic matter storage. Soils with high oxygen availability, pH and mineral activity tend to be substrate limited: high in situ oxidative activities limit soil organic matter accumulation. Soils with opposing characteristics are activity limited: low in situ oxidative activities promote soil organic matter storage.

Phenol oxidase, peroxidase and organic matter dynamics of soil

Soil microbes produce extracellular enzymes that mineralize organic matter and release carbon and nutrients in forms that can be assimilated. Economic theories of microbial metabolism predict that enzyme production should increase when simple nutrients are scarce and complex nutrients are abundant; however, resource limitation could also constrain enzyme production. We tested these hypotheses by monitoring enzyme activities and nutrient pools in soil incubations with added simple and complex nutrient compounds. Over 28 days of incubation, we found that an enzyme’s activity increased when its target nutrient was present in complex but not simple form, and carbon and nitrogen were available. b-Glucosidase and acid phosphatase activities also increased in treatments where only carbon and nitrogen were added. Glycine aminopeptidase and acid phosphatase activities declined in response to ammonium and phosphate additions, respectively. In some cases, mineralization responses paralleled changes in enzyme activity—for example, b-glucosidase activity increased and respiration was 5-fold greater in soil incubations with added cellulose, ammonium, and phosphate. However, a doubling of acid phosphatase activity in response to collagen addition was not associated with any changes in phosphorus mineralization. Our results indicate that microbes produce enzymes according to ‘economic rules’, but a substantial pool of mineral stabilized or constitutive enzymes mediates this response. Enzyme allocation patterns reflect microbial nutrient demands and may allow microbes to acquire limiting nutrients from complex substrates available in the soil. q 2004 Elsevier Ltd. All rights reserved.

Responses of extracellular enzymes to simple and complex nutrient inputs

Soil microorganisms and plants produce enzymes thatmineralize organically bound nutrients When nutrientavailability is low, the biota may be able to increase production ofthese enzymes to enhance the supply of inorganicnitrogen (N) and phosphorus (P) Regulation of enzyme productionmay be a point where N and P cyclesinteract We measured acid phosphatase and chitinase(N-acetyl s-D-glucosaminide) activity in soilacross a chronosequence in Hawaii where N and Pavailability varies substantially among sites and longterm fertilizer plots had been maintained for over 4years Phosphatase activity was high at all sites Chitinaseactivity decreased significantly as age and Navailability increased across the chronosequencePhosphorus addition suppressed phosphatase activity atall sites, while N addition increased phosphataseactivity at the young, N-limited site In contrast,N addition repressed chitinase activity only at the Nlimited young site, and P additions had no effect onchitinase activity These results suggest that theregulatory relationship between nutrient supply andnutrient mineralization are asymmetric for N and P,and that the differences could help to explaindifferences observed in patterns of N and Pavailability

Regulation of soil phosphatase and chitinase activity by N and P availability

Exotic plant species are increasingly becoming the focus of research and have been identified as a component of human-induced global change. Successful invaders may alter soil conditions, but the effect of exotic species on soil microbial communities has not been studied. We studied two exotic understory plant species (Japanese barberry [Berberis thunbergii] and Japanese stilt grass [Microstegium vimineum]) in hardwood forests in northern New Jersey, USA. We sampled bulk and rhizosphere soils under the two exotic species, as well as under a co-occurring native species (blueberry [Vaccinium spp.]). We indexed the structure (by measuring phospholipid fatty acid [PLFA] profiles) and function (by measuring enzyme activities and substrate-induced respiration [SIR] profiles) of microbial communities in the sampled soils. Soils under the three species differed in microbial community structure and function. These differences were observed in both the rhizosphere and bulk soil samples. Differences in the structural variables were correlated to differences in the functional variables as demonstrated by canonical correlation analysis. These results indicate that successful exotic invasive species can have profound effects on the microbial community of the soil.

Exotic plant species alter the microbial community structure and function in the soil

The mineralization of nitrogen and phosphorus from plant residues provides an important input of inorganic nutrients to the soil, which can be taken up by plants. The dynamics of nutrient mineralization or immobilization during decomposition are controlled by different biological and physical factors. Decomposers sequester carbon and nutrients from organic substrates and exchange inorganic nutrients with the environment to maintain their stoichiometric balance. Additionally, physical losses of organic compounds from leaching and other processes may alter the nutrient content of litter. In this work, we extend a stoichiometric model of litter nitrogen mineralization to include (1) phosphorus mineralization, (2) physical losses of organic nutrients, and (3) chemical heterogeneity of litter substrates. The enhanced model provides analytical mineralization curves for nitrogen and phosphorus as well as critical litter carbon : nutrient ratios (the carbon : nutrient ratios below which net nutrient release occurs) as a function of the elemental composition of the decomposers, their carbon-use efficiency, and the rate of physical loss of organic compounds. The model is used to infer the critical litter carbon : nutrient ratios from observed nitrogen and phosphorus dynamics in about 2600 litterbag samplings from 21 decomposition data sets spanning artic to tropical ecosystems. At the beginning of decomposition, nitrogen and phosphorus tend to be immobilized in boreal and temperate climates (i.e., both C:N and C:P critical ratios are lower than the initial ratios), while in tropical areas nitrogen is generally released and phosphorus may be either immobilized or released, regardless of the typically low phosphorus concentrations. The critical carbon : nutrient ratios we observed were found to increase with initial litter carbon : nutrient ratios, indicating that decomposers adapt to low-nutrient conditions by reducing their carbon-use efficiency. This stoichiometric control on nutrient dynamics appears ubiquitous across climatic regions and ecosystems, although other biological and physical processes also play important roles in litter decomposition. In tropical humid conditions, we found high critical C:P ratios likely due to high leaching and low decomposer phosphorus concentrations. In general, the compound effects of stoichiometric constraints and physical losses explain most of the variability in critical carbon : nutrient ratios and dynamics of nutrient immobilization and release at the global scale.

Stoichiometric controls on carbon, nitrogen, and phosphorus dynamics in decomposing litter

Because plant litter decomposition is directly mediated by extracellular enzymes (ectoenzymes), analyses of the dynamics of their activity may clarify the mechanisms that link decomposition rates to substrate quality and nutrient availability. We investigated this possibility by placing arrays of white birch sticks at eight upland, riparian, and lotic sites on a forested watershed in northern New York. For 3 yr, samples were analyzed for mass loss, protein, total Kjeldahl nitrogen (TKN), and total phosphorus (TP) accumulation, and the activity of 11 classes of extracellular enzymes involved in C, N, and P cycling. The relationship between lignocellulase activity and mass loss did not differ among sites. TKN and TP immobilization exhibited some spatial variation; rates of accumulation per 1% loss of initial mass, estimated from linear regressions, ranged from 2.2 to 4.4 mg/g OM for TP and from 43 to 139 mg/g OM for TKN, with maximum concentrations reached at °80% mass loss. The relationship between the activities of acid phosphatase (AcPase) and N—acetylglucosaminidase (NAGase), enzymes involved in the acquisition of P and N from organic sources, and mass loss displayed even greater variation among sites; the slopes of linear regressions relating mass loss and temporally integrated activity ranged from 0.019 to 0.135 activity—months per mass loss point and 0.107 to 0.775 activity—months per mass loss point, respectively, suggesting that edaphic rather than substrate quality factors were regulating activity. The extent of N limitation at each site was inferred by plotting TKN accumulation, defined as the slope of the linear regression TKN concentration vs. mass loss, in relation to NAGase activity accumulation, defined as the slope of the linear regression cumulative NAGase activity—months vs. mass loss. P limitation at each site was similarly assessed from an analogous plot of TP accumulation in relation to AcPase activity accumulation. Low N or P accumulation in conjunction with high acquisition activity was taken as an indication of nutrient limitation while the converse indicated surfeit. The diagrams suggested that decomposition at the upland hemlock and lotic sites, which displayed intermediate rates of OM loss (zero order k = 0.29 g/mo and 0.23 g/mo, respectively), was primarily N limited, while the riparian sites, which had the lowest rates of OM loss (k = 0.14 g/mo), appeared to be P limited. Relative to the others, OM loss at the upland deciduous sites (k = 0.38 g/mo) was not limited by either N or P. The concordance of field observations with predictions based on ectoenzyme regulation mechanisms suggest that enzyme activity assays in conjunction with nutrient concentration measurements may be a useful indicator of nutrient limitation. An economic model is proposed that directly links N and P availability to litter decomposition rates on the basis of microbial allocation of resources of extracellular enzyme production.

Wood decomposition: nitrogen and phosphorus dynamics in relation to extracellular enzyme activity'

Because plant litter decomposition is directly mediated by extracellular enzymes, analyses of the dynamics of their activity may clarify the mechanisms that link decomposition rates to substrate quality and to temperature, moisture and nutrient availability patterns. We investigated this possibility by placing arrays of white birch ice-cream sticks at eight upland, riparian and lotie sites on a forested watershed in northern New York. For 3 yr, samples were analyzed for mass loss, protein, nitrogen and phosphorus accumulation and the activity of 11 classes of extracellular enzymes involved in lignocellulose degradation and nutrient cycling. Despite considerable heterogeneity both within and between sites, decomposition rates were closely related to the activity of lignocellulose-degrading enzymes. A statistical model was developed that accounted for 94% of the variance in mass loss rates as a function of the temporally-integrated activity of these enzymes. Models of this type contribute to our understanding of scale integration and may facilitate the estimation of decomposition rates among landscape units.

D. Repert

Papers

Wood decomposition: nitrogen and phosphorus dynamics in relation to extracellular enzyme activity'

Wood decomposition over a first-order watershed: mass loss as a function of lignocellulase activity.