The role of dissolved organic nitrogen and nitrogen reallocation along a precipitation gradient in US grasslands

Background/Question/Methods

Central US grassland communities vary across a longitudinal precipitation gradient from semiarid shortgrass steppe in the western portion, through the central mixed-grass prairie, to the sub-humid tallgrass prairie in the east. There are strong interactions among water availability, nitrogen (N) dynamics and plant community structure in this region. Climate change and N cycling dynamics are currently undergoing large changes worldwide, and thus it is important to understand the interactions between them. While aboveground net primary production (ANPP) increases linearly across the precipitation gradient, multiple studies have shown that field estimated rates of net N-mineralization are relatively constant across this gradient.  Our major objective in this research was to identify the source of N used to build plant biomass across the grassland precipitation gradient.  We addressed two potential sources: dissolved organic N, and N reallocation from perennial tissues. As a first step, we quantified the amount of dissolved organic nitrogen (DON) in grassland soils that could be available for plant acquisition, positing that soils at the wet end of the gradient supply most N in organic form, that plants may be able to take up. We also performed a greenhouse experiment that we coupled with an isotope mixing model to determine the amount of leaf N reallocated from root tissue, hypothesizing that with increasing N limitation at the wet end of the gradient, dominant grasses can reallocate more N from root tissues, thus reducing the total amount of inorganic N uptake in a growing season.  

Results/Conclusions

We found that dissolved organic nitrogen increases across the precipitation gradient and could be an important source of N at the wet end of the gradient. Further, greenhouse-grown grasses were able to build substantial aboveground biomass in nitrogen-free soils, suggesting N reallocation from perennial tissues.  Our combined results show that the relationship between ANPP and N dynamics in grasslands is more complex than once perceived; this relationship must be better understood in order to model future grassland dynamics under changing climate and N regimes.