Effects of gaseous nitrogen dioxide deposition on plant nitrogen status and performance in key serpentine grassland species

Background/Question/Methods   Atmospheric nitrogen (N) deposition has the potential to strongly influence plant N status and performance either directly by stimulating growth or indirectly by influencing competition in plant communities. However, the role of N deposition and the mechanisms driving successful invasion and persistence of exotic species in serpentine grasslands remain unclear. Lolium multiflorum (Italian ryegrass), the most common invasive plant species in Bay Area serpentine grasslands, exhibits a high growth rate and low resource-use efficiency typical of invaders. Thus, increased N deposition may be a key factor allowing L. multiflorum to successfully invade and persist in a formerly low-resource environment that has resisted invasion for centuries. Using a growth chamber system, we exposed several native species in monoculture and in combination with L. multiflorum to different concentrations of gaseous nitrogen dioxide (NO₂; a common N pollutant) and soil NH₄NO₃ to simulate ~10 years of accumulated N from atmospheric deposition. Measurements included δ¹³C and δ¹⁵N, specific leaf area, stomatal conductance, photosynthetic rate, and foliar and whole plant N, P, and Ca concentration. The objective of this study was to test how increased N deposition affects plant nutrition and the relative performance of key native and exotic species in a CA serpentine grassland.

Results/Conclusions   All native species showed increased biomass accumulation when grown in monoculture in response to added gaseous NO₂ and soil NH₄NO₃. Total biomass accumulation was higher in L. multiflorum exposed to soil NH₄NO₃ additions compared to natives, but we observed no response under NO₂ exposure. Photosynthetic capacity and stomatal conductance were higher in L. multiflorum than in natives when exposed to both gaseous NO₂ and soil NH₄NO₃ additions. L. multiflorum increased in both foliar and whole plant N content in response to gaseous NO₂ and soil NH₄NO₃ additions, while natives showed little or no response to additional N inputs. Results suggest that at concentrations commonly observed in the San Francisco Bay Area, NO₂ may act as a nutrient and stimulate greater biomass accumulation and greater allocation of N to photosynthetic tissue in invasive L. multiflorum than in native serpentine grassland species. Our study contributes understanding of how physiological differences among species affect both the dynamics of N uptake in the serpentine ecosystem and the outcomes of competition between native and exotic species experiencing anthropogenic N loading.