Responses of nitrifying microbes in Southern California native plant communities to additions of nitrogen

Southern California native plant communities, such as coastal sage scrub (CSS) and chaparral, are subject to a wide range of atmospheric nitrogen deposition produced from anthropogenic sources. The effect ofN deposition on the soils of two such plant communities-Santa Margarita Ecological Reserve (CSS) and Sky Oaks Field Station (chaparral) -was recently studied (90, 92). An in situ N-addition experiment has been conducted at these two sites to simulate deposition of atmospheric nitrogen compounds. Permanent plots (1 0 x 1Om) were exposed either to ambient N deposition alone (the control plots), or received an additional 50 kg N ha"1 yr-1 ofNIW03 fertilizer (experimental plots) during the fall. Microbial biomass and extractable inorganic nitrogen (N03-+ NJ4) concentrations were measured every three months over a one-year period. Extracts of soil samples from theN-added plots at both Santa Margarita and Sky Oaks had significantly higher levels of N03-than the control plots. There were no significant differences in microbial biomass between treatments (control v. manipulated (N-added)) at the two sites; there were significant effects of time on microbial biomass at both sites, however. The rates of microbial transformation of nitrogenous compounds were also examined over a ten-day period of soil incubation under controlled conditions. There were no differences in the potential for net ammonification between the control and VI N-added plots at either site. There was, however, a significant increase in potential nitrification in the N-added plots in chaparral, and both sites showed a significant seasonal effect on net nitrification rates. These results indicate that nitrification rates may increase in response to deposition of anthropogenic N in both coastal sage scrub and chaparral habitats. These fmdings have implications for the potential loss ofN as N03-in runoff into the groundwater or as greenhouse gas products (NO and N20) from microbial nitrification processes. Keywords: Microbial biomass; ninhydrin-reactive N· Nitrogen cycle; ' ammonification; nitrification; mineralization; global climate change.