Long-term fertilization increases soil but not plant or microbial N in a Chihuahuan Desert Grassland

Abstract. Although the negative consequences of increased nitrogen (N) supply on plant communities and soil chemistry are well known, most studies have focused on mesic grasslands, and the fate of added N in arid and semi-arid ecosystems remains unclear. To study the impacts of long-term increased N deposition on ecosystem N-pools, we sampled a 26-year-long fertilization (10 g N m^-2 yr^-1) experiment in the northern Chihuahuan Desert at the Sevilleta National Wildlife Refuge (SNWR) in New Mexico. To determine the fate of the added N, we measured multiple soil, microbial, and plant N pools in shallow soils at three time points across the 2020 growing season. We found small but significant increases with fertilization in soil available (NO₃^--N and NH₄⁺-N), yet the soil microbial and plant communities do not appear to be taking advantage of the increased N-availability, with no changes in biomass or N-content in either community. However, there were increases in total soil N with fertilization, suggesting increases in microbial or plant N earlier in the experiment. Ultimately, the majority of the N added in this multi-decadal experiment was not found in the shallow soil, nor the microbial or plant community, and is likely to have been lost from the ecosystem entirely.