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. Much of the landscape is also subject to anthropogenic nitrogen (N) deposition up to 30 kg N ha-1yr-1that 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. Exotic grasses preferentially form associations with species of arbuscular mycorrhizal fungi known as fine endophytes, and extractable nutrients are depleted. Agricultural lands are frequently set aside for restoration as habitat for endangered species, and are also dominated by exotic grasses. These soils may harbor agricultural pathogens that invade native plant roots. Active restoration is required to control exotic species and reintroduce native species.
Results/Conclusions
Altered mycorrhizal and microbial communities as well as rates of N cycling recovered following restoration, indicating that soil characteristics may recover if there is residual inoculum of native microbial communities. Native annuals generally had positive to neutral feedbacks when grown either in their own soil or soil from invasive plants, suggesting populations of annuals tend not to form negative feedbacks. Mycorrhizal inoculum overcame pathogen impacts in agricultural soils. The soils of sites under low N deposition appear to be resilient because pools of total N and C did not change and nutrient flux rates were restored upon removal of exotics. However, exotic grasses often reinvade restored sites, and must be periodically controlled to maintain native vegetation. In regions of the landscape with high N deposition, legislative controls on air quality are critical to reduce exotic grass productivity and promote recovery of native vegetation.