The Implications of Microbial Functional Diversity on the Response of Soil Organic Matter Decomposition to Nitrogen Fertilization: A Theoretical Model

Abstract. Nitrogen (N) fertilization decreases carbon (C) decomposition, but there remain competing hypotheses underlying this response including reduced belowground C allocation, shifts in microbial diversity, and altered microbial function. Further, current soil organic matter (SOM) decomposition models lack the ability to test these hypothesized mechanisms.  To theoretically test these mechanisms, we integrated the Carbon Acquisition Ecological Strategies (CAES) framework into the Carbon, Organisms, Rhizosphere, and Protection in the Soil Environment (CORPSE) model. CAES represents a pathway for integrating functional diversity into microbial explicit soil decomposition models, with three microbial functional groups that specialize in decomposing and utilizing a different SOM pool: primary decomposers (1° decomposers) target complex SOM, secondary decomposers (2° decomposers) target microbial necromass, and passive consumers target simple SOM. We then used the refined model (Functional Group-CORPSE) to theoretically test hypotheses by leveraging long term data from a 30 year, watershed scale N fertilization experiment at the Fernow Experimental Forest in Parsons, WV. We performed modeling experiments to evaluate the microbial community response to varying C and N inputs, and to simulate hypothesized mechanisms of microbial response to N deposition. Varying substrate composition led to distinct microbial communities, reflecting substrate preferences between functional groups. However, only when both the function and abundance of complex C decomposers was reduced could FG-CORPSE capture the SOM dynamics observed at the Fernow. Collectively, our results show that FG-CORPSE enables the simulation of both functional and compositional shifts in microbial communities to facilitate the testing of microbe centric hypotheses for decomposition and C storage under N enrichment.