Modeling soil carbon and nitrogen cycling based on microbial ecology

Microbial decomposers in the soil are responsible for the largest flux of CO2 from terrestrial ecosystems to the atmosphere. Soil microbial activity is usually investigated at the bulk soil level, both in empirical and in modeling studies, which makes it impossible to understand mechanisms emerging from complex microbial interactions at the microscale. In past years I developed and tested an individual-based computer model, which simulates decomposing litter or soil at microbial-relevant scales (µm). With this model, we showed that:

• Microbial community dynamics alleviate stoichiometric constraints during litter decay: The interaction of functional microbial groups feeding on specific substrates lead to an adaptation of the microbial community, which accelerates nitrogen (N) recycling in litter with low N content and thus alleviates microbial N limitation [1].
• Social interactions among individual microbes lead to a self-regulation of the overall microbial activity at the community level. Our results indicate that the ubiquitous presence of seemingly “useless” opportunistic microbes buffers the response of soil respiration to changing environmental conditions and is key for the long-term accumulation of the vast carbon (C) and N stocks in present day soils [2].
• Detachment of microbes from their limiting element under droughts explain soil CO2 flux at rewetting: 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 examine the biological and physical dynamics leading up to a CO2 pulse after drying-rewetting [3].
• Self-organizing spatial patterns lead to the formation of carbon and nitrogen “hotspots” in soil: Long-term runs of the model toward equilibrium lead to the formation of self-organizing spatial patterns under certain conditions (Turing pattern). This pattern formation fosters the separation of C and N-rich areas in the model. Based on the theory of Turing conditions, I found that pattern formation was driven by parameters related to the relative speed of microbial dispersal versus diffusion of labile substrates in the soil. These parameters (microbial turnover rate and carbon use efficiency) have a larger impact on steady state soil C stock and C/N ratio, than amount or C/N ratio of substrate inputs. Pattern formation enhances the ability of the system to cope with C and N imbalances between substrate and microbes by establishing a spatial order in local C and N flows. The possibility for such self-organizing pattern to occur in real soil is yet to be analyzed.

SA-YSSP 2014-15 Research Theme 8: Modelling mechanisms influencing above- and below-ground diversity and productivity in plant communities

Plants share a considerable amount of the carbon (C) that they gain by photosynthesis with soil microbes in the form of root exudates, release of allelopathic substances or direct mutualistic investments. The amount and composition of this plant belowground “investment” shapes the composition of the local soil microbial community, which in turn feeds back on the fitness of the host plant and its neighboring plants. A better understanding of this below-ground feedback on plant health and community dynamics is necessary to assess the full consequences and potentials of human manipulations in agricultural settings, which potentially alter plant below-ground carbon release (i.e., selection for specific plant genotypes, herbicides).

Within the SA-YSSP 2014-15, as a Postdoctoral associate for the Research Theme 8 (Students: Hung-Yu Chaun, Anette Alleman, Supervisors: Gergely Boza, Ulf Dieckmann, Wijnand Swart), I participated in the joint development of a model on plant-microbe interactions which links quantity and quality of root exudations to soil microbial community dynamics and its feedback to plant performance. 

Last edited: 17 February 2015