Soil nitrogen cycling is resilient to invasive annuals following restoration of coastal sage scrub

摘要

Southern California coastal sage scrub (CSS) is highly invaded by Mediterranean annual grasses and undergoing extensive restoration efforts. Exotic plant invasion alters ecosystem structure and function through plant-soil feedbacks that can be detrimental to native plants. Assessments of CSS restoration have focused on aboveground plant communities, while belowground effects have received less attention. We examined CSS soil resilience following restoration of native CSS species using ecosystem property divergence in restorations from an invaded state as a measure of exotic plant impacts. We hypothesized that exotic annual plants compete with native species for nutrients and change nutrient cycling, and exotic plant removal and native plant restoration would allow soil recovery under native plant-soil inputs. Nitrogen (N) cycling was resilient but not resistant to vegetation changes. Exotic annual plants increased N mineralization and nitrification but did not affect total soil carbon (C) and N and extractable phosphorus. Extractable N was reduced in invaded plots, and immediately increased following weeding. These changes suggest that exotic plants are directly competing with native plants for N. Impacts to N cycling were reversible after exotic plants were removed and native shrubs reestablished, which may have important implications for recovery of other ecosystems invaded by annual grasses.