Modeling soil carbon and nitrogen cycling based on microbial ecology

Introduction

The feedback of the terrestrial carbon cycle to global environmental change is among  the greatest uncertainties with respect to predictions of future rise of atmospheric carbon dioxide (CO₂). Soil CO₂ emissions, driven by microbial decomposition of organic matter, are highly sensitive to changes in temperature, moisture, and carbon (C) and nitrogen (N) availability. Despite their importance, the mechanisms driving microbial processes in soil remain mostly unclear and are only poorly represented in current biogeochemical models. In particular, only a little is known about how and whether microbial community dynamics influence the response of soil CO₂ release to changing environmental conditions.

Methodology

We investigated the link between microbial community dynamics and organic matter decomposition with an individual- and process-based microbial micro-scale model. The model simulates competitive and synergistic interactions between individual microbes belonging to different functional groups in a spatially structured environment (i.e., a two-dimensional grid, representing small-scale "microsites" in the soil). By carrying out specific decomposition processes, microbes in the model constantly alter their own environment, which feeds back to community composition and lead to a dynamic link between functional community structure and C and N turnover rates.

Results

i) Microbial community dynamics alleviate stoichiometric constraints during litter decay. The interaction of functional microbial groups feeding on specific substrates leads to an adaptation at the community level, which accelerates N recycling in litter with high initial C : N ratios and thus alleviates microbial N limitation. This mechanism allows microbial decomposers to overcome large imbalances between resource and biomass stoichiometry without the need to decrease carbon use efficiency, which is in contrast to the predictions of traditional stoichiometric mass balance equations [1]. 

ii) The effect of social interactions among microbial decomposers on biogeochemical cycles. A powerful self-regulating mechanism emerges in the model when microbial decomposer communities include “cheaters” (opportunistic microbes that benefit from the catalytic activities of others), which alleviates the effect of changing environmental conditions, such as temperature or nutrient availability, on decay rates. Our results further indicate that the ubiquitous presence of microbial “cheaters” in decomposer communities prevent otherwise high C and N losses from terrestrial systems, thereby fostering ecosystem N retention and the long-term build-up of terrestrial C and N stocks [2]. 

Drivers of CO₂ flux under fluctuating rainfall patterns. Large rainfall events after long dry periods result in a flux of CO₂ from soil that is larger than current models predict (the “Birch” effect). In a collaboration based on a Young Scientists Summer Program (YSSP) project (Sarah Evans, University of Colorado, 2012) we examined the biological and physical dynamics leading up to a CO₂ pulse after drying-rewetting [3].

Conclusion

Our results indicate that microbial community dynamics play a significant role for C and N cycling in terrestrial ecosystems. Identifying microbial community-driven mechanisms as a basis for their implementation into biogeochemical models is important for accurately predicting terrestrial C fluxes in response to changing environmental conditions.

Last edited: 15 April 2014