A Microbial Community Perspective on the Regulation of Soil Organic Matter Turnover

Abstract:

Mechanisms driving microbial processes in soil or decomposing litter are still largely unclear and only poorly represented in current biogeochemical models. Soil processes are traditionally investigated from a bird’s eye view, i.e. at scales considerably larger than those relevant to soil microbes, in both empirical and modeling studies, which has limited their understanding and consequently hampered the prediction of microbial organic matter decomposition under changing environmental conditions.

With the aim to envisage the soil as a complex, dynamic system, we developed an individual-based mm-scale soil C and N turnover model, which simulates competitive and synergistic interactions between functionally different microbes in a spatially structured environment (a two-dimensional grid of 10 000 soil ‘microsites’) of decomposing litter or soil.

Analyses of our model show that microbial interactions at the micro-scale drive the emergence of qualitatively new system properties at the macro-scale. In particular, interactions between microbes that actively degrade chemically complex compounds (‘degraders’) and those who do not, but benefit from the activity of others (‘opportunists’), lead to a self-regulation of the overall soil microbial activity: increasing catalytic efficiency of extracellular enzymes released by degraders are outbalanced by increased competition from opportunists, thereby down-regulating the amount of enzymes produced at the community level. As a consequence, decay rates in our model became effectively independent of the catalytic properties of extracellular enzymes.

Our results show that regulations at the community level have the potential to overrule anticipated responses of microbial physiology or extracellular enzyme kinetics to changing environmental conditions. We conclude that current efforts to enhance mechanistic detail in biogeochemical models need to go beyond microbial physiology, and to include mechanisms of community-driven regulation.