The effects of C and N availability on soil microbial activity and biomass

Background/Question/Methods:  With the increase of population and automobile use in the semi-arid southwest, the chaparral shrublands of southern California region experience high amounts of atmospheric Nitrogen-deposition. This large input of atmospheric N from dry deposition accumulates on the soil surfaces and becomes available as a pulse following winter rainfall, and there is substantial evidence that soil C and N cycling and storage is affected by these anthropogenic N pulses.  Thus, it is our objective to examine how microbial activity (respiration, N mineralization, and nitrification) and N and C storage is affected by N input and to evaluate if microbes are co-limited by C and N.  We hypothesize that the addition of C to N-enriched soil will significantly increase microbial retention of N and microbial activity.  This hypothesis is being evaluated using fully-factorial field and laboratory experiments where chaparral and coastal sage scrub soils are provided exogenous N (NH4NO3), C (sucrose), N+C, and control (no added N or C).   

Results/Conclusions: Preliminary results indicate that C input significantly increased net N immobilization and microbial N storage in coastal sage scrub soil but not chaparral, while added N had the opposite result, stimulating net immobilization and microbial N uptake in chaparral soil but not coastal sage scrub.  These results suggest differences in the N and C limitation of coastal sage scrub and chaparral communities, and have important implications for soil N storage and cycling in areas with high atmospheric N input.