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Nitrogen dynamics associated with annual grass invasion in a Great Basin ecosystem.
Booth, Mary1,2, Stark, John1, Caldwell, Martyn1, 1 2
ABSTRACT- To examine how annual grasses affect nitrogen cycling relative to perennial vegetation, soil N dynamics were compared among monodominant patches of the exotic annual grass Bromus tectorum, the perennial bunchgrass Elymus elymoides, and the shrub Artemisia tridentata, at Curlew Valley, a salt-desert shrub site in Northern Utah. Total N was higher in near-surface soils under Bromus than under perennials, and isotope-dilution assays determined that both gross mineralization and nitrification rates were higher in Bromus-dominated than Artemisia-dominated soils. Nitrifier populations were higher under Bromus than under perennials. However, soils under Bromus also showed higher gross ammonium and nitrate consumption than Artemisia-dominated soils, suggesting that microbial N immobilization is important for N retention in annualized systems. An autumn root exclusion experiment demonstrated decreasing soil nitrate under germinating Bromus, but elevated concentrations where root uptake was excluded. Similarly, while soil inorganic N concentrations were uniformly low among Bromus, Elymus, and Artemisia stands in the spring, soil nitrate concentrations were elevated in post-senescent Bromus stands. Litterbags containing a common material showed greater mass loss and N mineralization when buried in Bromus stands than in Artemisia stands, suggesting the soil environment under Bromus promotes decomposition. Greater inorganic N availability under Bromus than under perennials appears to support the development of distinct soil faunal and microbial communities, which in turn contribute to patterns of N cycling in the annual grass community.
KEY WORDS: annual grass, nitrogen cycling