Contrasting effects of short-term nitrogen addition on semi-arid vs. mesic grassland productivity and plant communities

Anthropogenic activities cause the accumulation of biologically reactive nitrogen in ecosystems worldwide, leading to substantial changes in plant community structure and function, particularly in nitrogen-limited grasslands. Responses of plant communities and primary productivity vary depending on the magnitude of eutrophication and climate of the ecosystem, yet the exact form of these relationships is largely unknown. Here, we report results from the first 5 years of an experiment in which nitrogen was added at eight levels, ranging from 0 to 30 g m⁻² at two grassland sites bookending the broad precipitation gradient of the US Central Plains: (1) semi-arid shortgrass steppe and (2) mesic tallgrass prairie. This allowed us to examine the mediating effects of climate on short-term aboveground net primary productivity (ANPP) and plant community responses to nitrogen addition. Although nitrogen addition caused a decrease in plant species richness at both grassland sites, the two sites differed in their responses in ANPP and plant composition. At the shortgrass site, we found no effect of nitrogen addition at any level on ANPP, but compositional change occurred starting at 5 g m⁻². In contrast, ANPP at the tallgrass site increased at 5 g m⁻² then saturated, but no significant compositional change was observed. Collectively, these results provide two key insights: (1) ANPP and plant community responses can be decoupled with short-term nitrogen addition and (2) site-level water limitation can result in contrasting responses of grasslands to 5 years of nitrogen addition, but with these effects manifesting at the same critical load of addition.