Feedback on plant productivity can be constrained by SOM in N-limited grasslands

In many terrestrial ecosystems plant productivity is limited by the availability of mineral nitrogen, which is produced by soil microbial transformations of organic N in soil organic matter (SOM-N). Mineral N availability results from two opposing processes, 1) gross mineral N production (gross ammonification/gross nitrification) and 2) microbial N immobilization. These processes can be influenced by the availability of plant-derived C (PDC) inputs to the microbes, SOM-N pool size, and the size of the microbial community (microbial biomass). We considered how changes in PDC inputs and SOM-N pool size together may alter microbial biomass, mineral N availability, and feedbacks on plant productivity. In areas dominated by one of six tallgrass prairie species along a natural gradient of PDC inputs and SOM-N pool size, we conducted a field survey of microbial biomass and gross ammonification. We also performed greenhouse manipulations of SOM-N pool size and PDC inputs on two species in our study area (Poa pratensis and Schizachyrium scoparium). Structural equation modeling of the field data showed that gross ammonification was both positively and directly related to microbial biomass and SOM-N pool size. Gross ammonification was positively and indirectly related to SOM-N pool size and belowground PDC inputs, via microbial biomass. In the short-term greenhouse study, PDC inputs and SOM-N pool size positively affected gross mineral N production, although only at high SOM-N pool size. If the patterns in the greenhouse can be applied to field conditions, this suggests that SOM-N pool size may constrain plant driven feedbacks on plant productivity by limiting gross mineral N production.