Patterns in Nitrogen Cycling Across Diverse California Soils Subjected to Nitrogen Inputs

California receives the highest rates of anthropogenic nitrogen (N) deposition in the western United States. Changes in N cycling with N deposition are likely to be strongly influenced by native soil C and N content. We performed a ten-month long laboratory incubation using soils from 33 California ecosystems, including deserts, wetlands, grasslands, shrublands and forests, which ranged widely in soil C (0.07-5.40%) and N (0.009-0.35%) content. Plant-free soils were maintained at field capacity and four treatment levels of N (0, 5, 20, 80 kg-N/ha/y equivalent) were added as ammonium nitrate solution weekly. We measured gross N cycling rates, dissimilatory NO3 reduction to NH4 (DNRA), and soil C and N content initially and after ten months. Soil N cycling rates did not respond to the N addition treatments but differed by ecosystem both initially and after incubation. Initially, wetlands and forests had the highest gross mineralization rates, and shrublands had the highest gross nitrification rates. After ten months, desert soils exhibited lower gross nitrification rates and wetland soils exhibited higher DNRA rates compared to all other ecosystems (p< 0.001). Inorganic N pools and soil C content were strongly predictive of N cycling rates though the controls on N cycling rates generally differed before and after incubation. Across biomes, initial gross mineralization rates were best predicted by soil NH4 and NO3 concentrations whereas final rates were best predicted by final C content (R2= 0.39 and 0.43, respectively). Initial gross nitrification rates were best predicted by the combination of soil NH4 concentrations, total soil N content, and DNRA rates (R2= 0.49). However, final gross nitrification rates were best predicted by initial C content and final NO3 concentrations (R2= 0.73). In contrast, both initial and final DNRA rates were best predicted by final soil NH4 and NO3 concentrations (R2= 0.83 and 0.73, respectively). Neither final soil NH4 and NO3 concentrations nor final soil C content were correlated with N addition. These results suggest that microbial processes were relatively insensitive to N inputs in the long-term laboratory incubation.