Restoration and Plant-Soil Feedbacks of California Ecosystems Under High N Deposition, Invasive Species, and Frequent Fire.

California shrublands and grasslands are highly invaded by exotic annual grasses and forbs that dominate the seedbank and alter biotic and chemical characteristics of soil. Often the exotics have dominated for so long there is no record of the original vegetation, so restoration goals are uncertain. Many of these sites are also subject to anthropogenic nitrogen (N) deposition up to 30 kg N ha^-1yr^-1 that preferentially increases exotic grass productivity. Increased fine fuel from grasses promotes frequent fire, the native seedbank has been depleted, and exotic species dominate the seedbank. A variety of techniques have been tested to restore these lands. Carbon addition to immobilize N was a temporary solution to promote native species establishment under continued N deposition. Other techniques to control exotic annuals include spring fires before seed drop of exotic grasses, solarization, mowing, grazing, and herbicides, often with short-term success. Exotic grasses reinvaded many restored sites, and any restoration technique must be reapplied after several years to maintain native vegetation. Altered mycorrhizal and microbial communities as well as elevated rates of N cycling recovered following removal of exotics and/or reseeding with native species, indicating that soil characteristics may respond quickly to restoration treatments. The soils of sites under low N deposition appear to be resilient because pools of total N and C did not change and fluxes were restored upon removal of exotics. Legislative controls on N deposition are critical to reduce exotic grass productivity and promote recovery of native vegetation. Other restoration challenges include the lack of historic reference areas, the dense exotic seedbank and high rate of reinvasion.