Interaction between nitrogen and carbon availability on microbial activity and biomass in chaparral soils of southern California.

A large portion of nitrogen deposition on southern California’s chaparral and coastal sage scrub (CSS) is due to anthropogenic sources. The implications of increasing soil nitrogen, and the relationship between soil nitrogen and carbon on soil microorganism growth and activity, are not well understood. Possible interactions between N inputs and soil C availability on soil respiration and microbial biomass were assessed in chaparral and CSS plots that have been experimentally treated with N for about 12 years. Soil (microbial + autotrophic) respiration and microbial biomass were measured in four conditions: in plots exposed to added and ambient N, and within these plots, under shrubs and in open spaces, which represent microsites with differing N and C availability. We measured soil respiration and microbial biomass in these conditions every 3 months for a period of 1 year to test the hypothesis that respiration and microbial biomass would (1) increase in plots with higher C and N availability and (2) be higher during the winter and spring because of higher soil water availability. Our results indicate that soil respiration was significantly higher under shrubs but not in plots exposed to added N while microbial biomass was significantly higher in plots exposed to added N but not under shrubs. Soil respiration and microbial biomass were higher in the summer months than during the winter and spring months. These results were observed for both CSS and chaparral, indicating that the effects of long-term N exposure on soil microbial activity and biomass may be general for semiarid shrublands. While speculative, the N-induced increase in microbial biomass, without an increase in activity (respiration), suggests that N exposure has altered the soil microbial community. A change in the soil microbial community has important implications for soil N and C cycling and storage, especially in semi-arid chaparral ecosystems subject to large inputs of atmospheric N.