Litter and soil C and N mineralization dynamics for a coastal sage scrub ecosystem exposed to chronic experimental N deposition

Background/Question/Methods: Atmospheric N deposition is estimated to be 20-45 kgN ha^-1 yr^-1 in polluted southern Californian urban shrublands.  High N input has the potential to alter soil organic matter and litter quantity and quality, which can influence decomposition and N mineralization.  Using soil and litter collected from a long-term in situ N addition experiment in a southern Californian coastal sage scrub ecosystem, we have been conducting a laboratory incubation experiment over the past 120 days to test the hypothesis that N addition will increase initial rates of C and N mineralization.  Intact soil cores were collected from field plots that have been exposed to either ambient or elevated N (50 kgN ha^-1 yr^-1) over an 8 year period.  Surface litter was removed from the soil cores and replaced with litter of Salvia mellifera E. Greene collected from the ambient or added N field plots described above. 

Results/Conclusions: Salvia mellifera litter exposed to added N had significantly (p < 0.05) higher N concentration and a lower C:N ratio than litter collected from control plots.  However, litter N exposure had no effect on microbial respiration.  Rather, microbial respiration, measured as soil CO₂ efflux, was significantly higher on added N plots over the first week of incubation, but over time microbial respiration became higher for control soil, resulting in a significant interaction between soil N exposure and time.  Similarly, net N mineralization was significantly higher in soil exposed to added N, while litter N exposure did not have a strong effect on N mineralization dynamics.  While the experiment is still being conducted, our results indicate that chronic N deposition acts to increase in soil C storage and N availability in these semi-arid shrublands.