PS 22-8 - The interactive effects of exotic grass (Lehmann lovegrass) removal and nitrogen amendments on Chihuahuan Desert grassland biogeochemistry

Background/Question/Methods

Anthropogenic nitrogen (N) deposition can promote the success of exotic plant species, whose presence can further impact the fitness and growth of native plant communities. In the southwestern USA, the prolific invasive grass Eragrostis lehmanniana (Lehmann lovegrass) has taken over thousands of acres of native vegetation. There is some evidence that E. lehmanniana outcompetes native plants under N enriched conditions. However, it is unclear if E. lehmanniana influences biogeochemical processes in invaded ecosystems. It is also uncertain whether the effects on biogeochemistry may be due to interactions between E. lehmanniana and N deposition. This study assesses the possible interactive effects between N deposition and exotic grass presence on biogeochemical processes in a southwestern semi-arid grassland experiencing heightened levels of atmospheric N deposition and E. lehmanniana invasion. We evaluated a suite of soil biogeochemical responses for three years to a factorial E. lehmanniana removal and resource addition experiment. Resource additions simulated realistic (modeled) N deposition (0, 4, or 8 kg-N-ha^-1) and assessed soil N saturation by stimulating N uptake with carbon (C) additions. Our primary questions were, ^1. does E. lehmanniana impact biogeochemical processes, and ^2. are the effects on biogeochemistry linked to interactions between N addition and E. lehmanniana?

Results/Conclusions

We identified direct and interactive effects of invasive grass removal and N addition treatments on biogeochemistry. Soil microbial function was only affected by the invasive grass removal treatment. Here, the removal of E. lehmannianareduced C or P acquiring enzyme potentials regardless of resource addition treatment. While soil microbial biomass C, N, and phosphorous (P) were unaffected by either treatment, the effect of the invasive grass removal treatment alone on C and P enzymes suggests that E. lehmanniana presence enhances C and P cycling. In contrast, the effects on soil ecosystem properties were associated with interactive effects. The removal of E. lehmanniana increased the availability of soil nitrate, phosphate, and extractable total N but only when N was added simultaneously. Additionally, while the removal of E. lehmanniana meant the removal of the dominant plant species (and biomass), soil resources only accumulated when N was added. Thus, this accumulation of resources was likely due to reduced plant uptake and direct nutrient inputs, which allowed for continued soil microbial activity. Understanding how exotic grasses influence biogeochemistry directly and how these effects may also interact with N deposition is essential to determining appropriate invasion management strategies.