Water sources and nitrogen relations of grasses and shrubs in phreatophytic communities of the Great Basin Desert

Water exports have been occurring in Owens Valley, California for almost a century, leading to questions about possible interactions between groundwater availability and community composition. While many studies of vegetation change in Owens Valley focused on depth to groundwater as the primary factor influencing competition among grasses and shrubs, nutrient relations may also play an important role. We evaluated access to groundwater and nitrogen (N) in grasses and shrubs in Owens Valley with measurements of water isotopes and N concentrations, isotopes, and fluxes. We found that grasses accessed isotopically enriched soil water at shallow depths while shrubs utilized groundwater throughout the growing season. However, N relations showed a different pattern among functional types. Halophytic species showed consistently high leaf N that was also more isotopically enriched than other species. Soil N was also highly isotopically enriched in the surface soil, suggesting that halophytes accessed shallow N pools. In general, there was a consistent relationship between leaf C:N and leaf N isotopes across all species and sites. These results suggest that access to isotopically enriched, surface N is a strong control on N uptake, and that N relations play an important role in vegetation composition in this ecosystem.