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

Background/Question/Methods

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 microbial activity.  We considered how changes in PDC inputs and SOM-N pool size together may alter microbial activity, 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 activity 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), measuring microbial activity and gross N cycling. 

Results/Conclusions

Structural equation modeling of the field data showed that gross ammonification was both positively and directly related to microbial activity and SOM-N pool size. Gross ammonification was positively and indirectly related to SOM-N pool size and belowground PDC inputs, via microbial activity.  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.