From meadows to lawns: Comparing soil nitrogen dynamics across a montane to urban semi-arid watershed
Urbanization substantially increases nitrogen (N) inputs and hydrologic losses relative to wildland ecosystems, although the fate of urban N additions to lawns and remnant grasslands remains contested. Snowmelt is the dominant period of infiltration in montane semi-arid ecosystems, and impacts of snow cover on soil temperature dynamics can affect nitrogen cycling. Here, we compared soil N dynamics between a fertilized and irrigated urban lawn and nearby natural riparian meadow in Salt Lake City, Utah, in the context of a snow manipulation experiment.
Snow removal increased freeze/thaw events but had no impact on indices of N cycling, microbial biomass, or soil oxygen (O2). Mineral N concentrations were similar between sites despite lawn fertilization, but dissolved organic N (DON) was consistently four-fold greater in the lawn (2.1 ± 0.1 mg N l-1). Modeled leaching losses were similar and minimal (< 2 kg N ha-1 y-1) at both sites due to differences in hydraulic conductivity and water infiltration. Lawn soil O2 fluctuated between 20.9 and 1.6 % following snowmelt and irrigation, but varied little in the meadow. The lawn also had a greater prevalence of reducing microsites as indicated by iron speciation. Post-snowmelt potential denitrification was six-fold greater in the lawn than the meadow and comparable to previous wetland measurements. Trends in potential denitrification were echoed by patterns of natural abundance nitrate isotopes in soil extractions. Nitrate d18O and d15N values increased from the meadow to lawn and co-varied linearly with a slope measuring 1.15 ± 0.09, consistent with denitrification. Lawns can provide hotspots of denitrification that potentially exceed natural riparian ecosystems in semi-arid landscapes, but could also increase DON losses relative to the ecosystems they replace.