Linking microbial dynamics and soil carbon storage in a temperate forest exposed to chronic nitrogen additions

Background/Question/Methods Soils contain two-thirds of the carbon stored in the terrestrial biosphere.  They also harbor a phylogenetically and physiologically diverse microbial community whose activities play a key role in the formation and stabilization of soil organic matter.  Extensive research has been done to characterize, quantify, and model soil organic matter pools and their turnover; however, a significant limitation to fully understanding soil C dynamics, and hence the balance between stabilization/destabilization of soil organic matter, is our limited understanding of the molecular mechanisms underlying microbial transformations of organic matter.  Thus there is increasing interest in integrating –omics information (i.e., genomics, transcriptomics, proteomics), biogeochemical process data, and ecosystem models to better understand the relationship between microbial community structure and ecosystem C cycling and storage.  In this talk, I will present molecular and process-level data collected from the Chronic Nitrogen Addition Experiment at Harvard Forest and discuss how nitrogen additions appear to be altering soil microbial and carbon cycling dynamics.  I will also discuss the technical and conceptual gaps that continue to limit our ability to fully integrate across scales (i.e., genes to ecosystems).
Results/Conclusions Results from Harvard Forest indicate that chronic nitrogen additions to a mixed hardwood forest stand have increased forest floor mass and soil carbon stocks across the soil profile; suppressed litter decay; decreased microbial biomass, especially the fungal component; decreased microbial enzyme activity, particularly for enzymes responsible for protein and lignin decay; and altered microbial community structure.  These results, along with those from several other recent studies, suggest that microbial responses to long-term nitrogen additions (either via fertilization or nitrogen deposition) play an important role in the observed carbon accumulation in these soils.