Principal Investigator:
Joshua P. Schimel
Dept. Ecology, Evolution and Marine Biology
University of Califronia
Santa Barbara, CA, 93106
Duration of the Project: 2 Years
There are several contradictions apparent in our understanding of soil organic
matter processing: 1) microbes often appear C limited, even in C-rich soils,
and 2) C-based studies often conclude C limitation, while N-based studies often
conclude N-limitation. We believe that a resolution to these lies in our understanding
and models of soil organic matter dynamics: models use 1st order
kinetics on SOM concentration. However SOM breakdown is catalyzed by exoenzymes,
and to accurately model catalyzed reactions, catalyst concentration must
be part of the rate equation. We developed a simple model in which SOM breakdown
is a function of exoenzymes. The model suggests a number of surprising and counter-intuitive
results that explain the contradictions above, including that in an N limited
soil, adding C increases respiration while adding N decreases it. We will test
5 hypotheses that grow from the model predictions. We will analyze the exoenzyme
kinetics of SOM breakdown and how non-linear enzyme kinetics produces C limitation
even in the midst of plenty, how the disconnection between actual polymer breakdown
and microbial growth affects the response to stress events, and how enzyme driven
C flow interacts with possibly N limited microorganisms to produce counter intuitive
responses to C and N additions. We will use 4 soils that range from C to N limitation.
We will use applications of specific enzymes and analysis of C flow from 14C
labeled polymers to evaluate the role of exoenzymes and microbes in SOM processing.
If the hypotheses are supported, it will require rethinking of our understanding
of C flow in the soil system, and of experiments testing the resource limiting
microbial growth and activity. This work directly addresses the Kearney mission
to "Understand mechanisms and processes governing the storage and flow
of carbon in soils that support California's diverse ecosystems."