FIGURE 3.9. Idealized plot of lignocellulase activity (heavy dashed line) in relation to litter mass loss (solid line) through time. Lignocellulase activity traces a "bell-shaped" pattern over the course of litter decomposition, peaking (in this example) when cumulative mass loss reaches 45%. The dashed horizontal lines at 20% and 80% mass loss highlight breakpoints in the mass loss curve. During the early stages of litter decomposition rapid mass loss is often largely attributable to leaching and mineralization of soluble litter constituents. In the middle stage, lignocellulose degradation predominates. Throughout the late stages, the accumulation of humic condensates depresses microbial activity, stabilizing the remaining material (from Sinsabaugh et al., 1994).

mass remaining, the accumulation of humic condensates depresses microbial activity, stabilizing the remaining material (Fig. 3.9) (Sinsabaugh et al., 1994).

Many models of soil organic matter (SOM) decomposition are based on first order kinetics that assume that decomposition rate of a particular carbon pool is proportional to pool size and a simple decomposition constant (dC/dt = kC). In reality, SOM decomposition is catalyzed by extracellular enzymes that are produced by the microorganisms. A theoretical model to explore the behavior of a decomposition-microbial growth system that operates by exoenzyme catalysis used the following relationship: DC = K*d EnzC, where DC = decomposition of polymeric material to produce available C; K*d is a single decomposition constant, and EnzC = exoenzyme pool (Schimel and Weintraub, 2003). An enzyme kinetics analysis showed there must be some mechanisms to produce a nonlinear response of decomposition rates to enzyme concentration. This nonlinearity induces carbon limitation, regardless of the potential carbon supply. In a linked carbon and nitrogen version of the model, adding a pulse of carbon to a nitrogen-limited system increases respiration, while adding nitrogen decreases respiration (with carbon redirected from waste respiration to microbial growth). Previous conclusions drawn in the literature have assumed that the lack of a respiratory response by soil microbes to added nitrogen indicates that they are not nitrogen limited. This model of Schimel and Weintraub (2003) suggests that, while total carbon flow may be limited by the functioning of the exoenzyme system, in fact microbial growth may be nitrogen limited. This important finding should be the subject of several laboratory and field studies in the near future.

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