Resistance, resilience, and redundancy of soil microbial community structure and function: Effects across disturbance and plant diversity gradients in agroecosystems

Background/Question/Methods Ecosystem and global biogeochemical models usually assume that microbially-mediated processes are controlled solely by abiotic factors and that microbial community structure has no impact on ecosystem process rates, whereas model parameters are often adjusted to account for richness, diversity and community composition of plants.  This assumption, if false, will affect the accuracy and interpretation of biogeochemical models used to predict the impact of land use intensification and climate change on ecosystems.  Our objectives were to address the resistance, resilience, and redundancy of soil microbial community structure and function via the following questions: Does microbial community structure impact ecosystem process rates in response to environmental change or disturbance? Are there predictable relationships between plant diversity, microbially-mediated soil processes, and the resistance, resilience, and redundancy of soil microbial communities in agroecosystems?  We sampled soil from the Farming Systems Project in Beltsville, MD, a long term cropping systems experiment with five agricultural production systems that represent a gradient of disturbance and plant species diversity.  We conducted a 300-day, in vitro soil incubation to monitor the response of the microbial community structure, and C and N mineralization to glucose or lignin additions to test the degree of microbial community stability.

Results/Conclusions Changes in microbial community structure and biomass were measured by molecular (TRFLP) and biochemical (PLFA) methods on twelve dates.  We found microbial community structure differed among systems and these structural differences resulted in significantly different processing rates of nitrogen and carbon. We found all systems to be sensitive to perturbation (glucose > lignin).  The system with highest plant diversity was least resistant to perturbation as indicated by change in process rate from control, whereas systems with low plant diversity were most resistant to perturbation.  Systems with low disturbance were less resistant than comparable systems with high disturbance.  All systems were resilient, in that they returned to control levels of process rate by 112 days post perturbation, but the expression of resilience (rate of return to control rates) differed significantly among systems.  The systems with highest plant diversity and systems with relatively low disturbance, showed the greatest expression of resilience.  These results suggest soil microbial communities developed in agroecosystems with high plant diversity and low disturbance have the capacity to be strongly responsive to environmental change but also show a strong degree of resilience to perturbation.