The role of ammonia volatilization in controlling the natural 15N abundance of a grazed grassland

Although the variation in natural ¹⁵N abundance in plants and soils is well characterized, mechanisms controlling N isotopic composition of organic matter are still poorly understood. The primary goal of this study was to examine the role of NH₃ volatilization from ungulate urine patches in determining ¹⁵N abundance in grassland plants and soil in Yellowstone National Park. We additionally used isotopic measurements to explore the pathways that plants in urine patches take up N. Plant, soil, and volatilized NH₃¹⁵N were measured on grassland plots for 10 days following the addition of simulated urine. Simulated urine increased ¹⁵N of roots and soil and reduced ¹⁵N of shoots. Soil enrichment was due to the volatilization of isotopically light NH₃. Acid-trapped NH₃¹⁵N increased from –28 (day 1) to –0.3 (day 10), and was lighter than the original urea-N added (1.2). A mass balance analysis of urea-derived N assimilated by plants indicated that most of the N taken up by plants was in the form of ammonium through roots. However, isotope data also showed that shoots directly absorbed ¹⁵N – depleted NH₃-N that was volatilized from simulated urine patches. These results indicate that NH₃ volatilization from urine patches enriches grassland soil with ¹⁵N and shoots are a sink for volatilized NH₃, which likely leads to accelerated cycling of excreted N back to herbivores.