Constraints on soil enzymes and microbial biomass: Implications for decomposition.
Allison, Steven*,1, 1 University of California, Irvine, CA
ABSTRACT- In many ecosystems, constraints on extracellular enzymes and decomposer populations may allow soil carbon to accumulate and persist (the Brown Ground hypothesis). I used a simulation model to explore how changes in nutrient availability affect microbial enzyme production and the breakdown of complex molecules, and compared these predictions to experimental results. The model predicted that nutrient availability, particularly nitrogen supply, constrains enzyme production, microbial growth, and CO2 respiration. Experimental data largely confirmed this prediction for microbial utilization of complex substrates, but real microbes were more flexible in their uptake and use of simple substrates than predicted by the model. However, the model over-predicted the response of microbial biomass and enzyme activities to changes in nutrient inputs relative to the experimental system. Additional measurements of the loss of enzyme activity from soils supplemented with enzymes and exposed to gamma-irradiation suggested that turnover rates for enzyme pools are not homogeneous as assumed in the model. In real soils, active microbes and enzymes associated with fresh organic matter inputs probably have much higher turnover rates than microbes and enzymes adsorbed to mineral surfaces. This multi-pool concept should be incorporated into ecosystem models to reflect the control of organic matter decomposition by small but active microbial biomass and enzyme pools. These results also help explain why estimates of microbial biomass and enzyme activity in bulk soils may be poor predictors of the rates of important ecosystem processes.
Key words: extracellular enzymes, mineralization, soil carbon, decomposition
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