Thesis

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.

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