Empirical and modeling approaches to setting critical loads for N deposition in southern California shrublands

Southern California deserts and coastal sage scrub (CSS) are undergoing vegetation-type conversion to exotic annual grassland, especially in regions downwind of urban areas that receive high N, primarily as dry deposition. To determine critical loads (CL) of N that cause negative impacts, we measured plant and soil responses along N deposition gradients, fertilized vegetation at different N levels, and used biomass production output from the DayCent model. N deposition gradients were identified from the CMAQ model and compared with measured N deposition values. CSS receives N deposition as high as 30 kg ha-1yr- 1, while the desert has levels up to 16 kg ha-1yr-1. Unlike more mesic ecosystems where critical loads are determined by changes in soil chemistry or biogeocycling, these arid and semiarid ecosystems are subject to increases in exotic species production, loss of native species diversity, and increased fire risk at relatively low CL's for N of 5 - 10 kg ha-1yr-1. For instance, a gradient survey in CSS showed that exotic grass increased and native plant species and arbuscular mycorrhizal species richness declined by almost half above 11 kg N ha-1yr-1. Fertilization studies in desert creosote bush scrub showed a significant increase in exotic species biomass with 5 kg N ha-1yr-1 in a wet year, and biomass output from DayCent modeling indicated an increased fire risk from exotic grasses with 1 T per ha production during years with moderate to high precipitation at 5 kg N ha-1yr-1. The difference in CL between desert and CSS may be related to the responsiveness of native vs. exotic plant species to N, as well as the degree to which precipitation and soil N limits plant growth in the two vegetation types.