Increases in population levels, food production, and energy consumption have approximately doubled the annual input of reactive nitrogen (N_r) to ecosystems globally, causing nutrient imbalances in terrestrial systems and polluting aquatic environments. However, a substantial fraction of inputs to terrestrial landscapes often is missing in mass balance budgets that use streams or rivers as export systems. Denitrification and long-term N_r sequestration in organic matter likely account for a large share of this deficit, but factors controlling these processes (e.g., temperature and precipitation) have been difficult to quantify or only assessed for specific regions, i.e., river basins in eastern North America. We present a spatially explicit analysis of N_r inputs for 25 catchments in the Central Valley (CV) of California, USA, where increased N_r levels have degraded water quality and impaired freshwater and estuarine ecosystems, particularly in the San Francisco Bay-Delta Complex. Rates and spatial distribution of inputs from atmospheric deposition, sewage effluent, N-fixation, and applications of inorganic and manure fertilizers were estimated for the late 1990s through early 2000s.

Results/Conclusions

Estimates of net N_r input ranged from 8.1 to 120.7 kg N ha^-1 yr^-1. Inorganic and manure fertilizers together accounted for 33% and 66% of total N_r inputs in northern (> 38.5˚ N) and southern catchments, respectively. N-fixation and atmospheric deposition accounted for nearly all of the remaining N_r inputs; sewage effluent never accounted for more than 3.2% of total input to individual catchments. Comparison of measured river export from 1999 to 2003 with inputs suggests that many CV catchments (18 of 25 in the study) retain or remove > 90% of anthropogenic N_r inputs on an annual basis. Percent of N_r inputs exported by streams or rivers in CV catchments declined exponentially with increasing mean annual temperature (p=0.002; r² = 0.37), showing a similar pattern to that of river basins in eastern North America. However, catchment topography as well as latitudinal distribution of agriculture may alternatively explain the pattern of N_r export. Management and large-scale modeling implications of anthropogenic N_r inputs in the CV also will be discussed.