Perennial Pepperweed Invasion Alters Nitrogen Cycling Rates in a Drained Pasture

Pepperweed (Lepidium latifolium) is a pervasive perennial exotic species that has spread throughout the western United States, invading natural and agricultural systems. Pepperweed has been documented to increase soil microbial enzyme activity associated with nitrogen (N) mineralization, but the effect of pepperweed on soil N cycling has not been determined. The objective of this study was to determine if pepperweed impacts gross N cycling rates and nitrous oxide (N₂O) emissions in an irrigated pasture soil. We used ¹⁵N pool dilution and tracer techniques to measure rates of gross N mineralization, gross nitrification, dissimilatory nitrate (NO₃^-) reduction to ammonium (NH₄⁺) (DNRA), and net N₂O fluxes from replicate plots (n = 6 per cover type) dominated by pepperweed versus dominated by an invasive annual grass (Hordenum murinem) with no pepperweed present. Because pepperweed has extensive root systems, we measured gross N cycling rates at three depths (0-20, 20-40, 40-60 cm) to determine pepperweed effects through the soil profile. Soil NH₄⁺ and NO₃^- concentrations, microbial biomass N (MBN) were also measured at each soil depth. Across all soil depths, the pepperweed landform exhibited significantly higher soil NH₄⁺ concentrations, gross mineralization rates, and DNRA rates. Gross nitrification rates, MBN, net N₂O fluxes, and soil NO₃^- concentrations did not differ significantly between pepperweed-dominated and grass-dominated plots. Across both cover types, soil concentrations of NH₄⁺ as well as rates of mineralization and DNRA rates were higher in surface soils (0-20 cm) than deeper soils. Our results suggest that pepperweed increases gross mineralization rates in invaded soils.