Changes in N cycling and microbial N with elevated N in exotic annual grasslands of southern California

The impacts of nitrogen (N) fertilization and N deposition on N mineralization and microbial biomass were studied in exotic annual grasslands in southern California. The goal of the study was to understand how N deposition impacts N availability to the grasslands by studying mineralization in plots in an urban area that has received chronic N deposition for 50 years compared with N fertilized and control plots in a rural area. Fertilized plots had higher net and gross rates of N cycling than did soils from the control. The effect of soil mineral N concentrations on microbial N varied between and within growing seasons. Lower microbial N corresponded to more net N release and higher microbial N corresponded to less net N release. Urban soils often had higher NO₃^- concentrations than did soils from the rural site but there was no difference in NH₄⁺ concentrations. Urban soils also had lower mineral N concentrations than the fertilized soils and mineralization patterns in the high N deposition soils did not resemble those in the fertilized soils, indicating that the levels of N deposition at this site were well below the experimental fertilization rate. The levels of soil mineral N in the rural site were considerably higher than from other studies in the same plots in recent years. This corresponds with rapidly increasing suburbanization of the rural site and increasing N deposition, as suggested from a recent air pollution model. Although the urban and rural soils were not as different in mineral N concentrations as expected, soils in exotic grasslands near urban areas across the region can have mineral N concentrations as high as the fertilized soils, indicating that increased N cycling and altered microbial N may occur under N deposition.