Bromus tectorum invasion alters nitrogen dynamics in an undisturbed arid grassland ecosystem

The nonnative annual grass Bromus tectorum has successfully replaced native vegetation in many arid and semiarid ecosystems. Initial introductions accompanied grazing and agriculture, making it difficult to separate the effects of invasion from physical disturbance. This study examined N dynamics in two recently invaded, undisturbed vegetation associations (C₃ and C₄). The response of these communities was compared to an invaded/ disturbed grassland. The invaded/disturbed communities had higher surface NH₄⁺ input in spring, whereas there were no differences for surface input of NO₃⁻. Soil inorganic N was dominated by NH₄⁺, but invaded sites had greater subsurface soil NO₃⁻. Invaded sites had greater total soil N at the surface four years post‐invasion in undisturbed communities, but total N was lower in the invaded/disturbed communities. Soil δ¹⁵N increased with depth in the noninvaded and recently invaded communities, whereas the invaded/disturbed communities exhibited the opposite pattern. Enriched foliar δ¹⁵N values suggest that Bromus assimilated subsurface NO₃⁻, whereas the native grasses were restricted to surface N. A Rayleigh distillation model accurately described decomposition patterns in the noninvaded communities where soil N loss is accompanied by increasing soil δ¹⁵N; however, the invaded/ disturbed communities exhibited the opposite pattern, suggesting redistribution of N within the soil profile. This study suggests that invasion has altered the mechanisms driving nitrogen dynamics. Bromus litter decomposition and soil NO₃⁻ concentrations were greater in the invaded communities during periods of ample precipitation, and NO₃⁻ leached from the surface litter, where it was assimilated by Bromus. The primary source of N input in these communities is a biological soil crust that is removed with disturbance, and the lack of N input by the biological soil crust did not balance N loss, resulting in reduced total N in the invaded/disturbed communities. Bromus produced a positive feedback loop by leaching NO₃⁻ from decomposing Bromus litter to subsurface soil layers, accessing that deep‐soil N pool with deep roots and returning that N to the surface as biomass and subsequent litter. Lack of new inputs combined with continued loss will result in lower total soil N, evidenced by the lower total soil N in the invaded/disturbed communities.