Pepperweed invasion increases nitrogen cycling rates in a managed grassland

Background/Question/Methods

Perennial pepperweed (Lepidium latifolium) is a pervasive 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 a managed grassland. 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), and gravimetric soil moisture content were also measured at each soil depth. Soil oxygen (O₂) concentrations were measured from soil equilibration chambers buried at 10, 30, and 50 cm depth.

Results/Conclusions

Across all soil depths, the pepperweed plots exhibited significantly higher soil NH₄⁺ concentrations, gross mineralization rates, DNRA rates, net N₂O fluxes, and soil O₂ concentrations (p < 0.05, ANOVAs). Gross nitrification rates, MBN, soil NO₃^- concentrations, and soil moisture did not differ significantly between pepperweed-dominated and grass-dominated plots. Across both cover types, soil concentrations of NH₄⁺ and O₂ as well as rates of mineralization, DNRA, and net N₂O emissions were higher in surface soils (0-20 cm) than deeper soils (20-40 and 40-60 cm) (p < 0.05, ANOVAs). Gross mineralization rates in surface soils (0-20 cm) averaged 30.1 ± 4.3 µg g^-1 d^-1 in grass-dominated plots and 81.5 ± 15.3 µg g^-1 d^-1 in pepperweed-dominated plots. Using the entire dataset for regression analyses, gross mineralization rates were positively correlated to soil NH₄⁺ concentrations (R² = 0.54), which averaged 8.4 ± 4.3 µg g^-1 in grass-dominated plots and 27.3 ± 4.4  µg g^-1 in pepperweed-dominated plots at 0-20 cm depth. Gross mineralization rates were also positively correlated to soil O₂ concentrations (R² = 0.36), which ranged from 14.4 to 20.0 % across all plots and depths. Our results suggest that pepperweed increases gross mineralization rates to alter N cycling in invaded soils.