Human activities have greatly altered the global nitrogen cycle and these changes are apparent in water quality, air quality, ecosystem and human health. However, the relative magnitude of the sources of new reactive nitrogen and the fate of this nitrogen is not well established. Further, the biogeochemical aspects of the nitrogen cycle are often studied in isolation from the economic and social implications of all the transformations of nitrogen. The California Nitrogen Assessment is an interdisciplinary project whose aim is evaluating the current state of nitrogen science, practice, and policy in the state of California. Because of the close proximity of large population centers, highly productive and diverse agricultural lands and significant acreage of undeveloped land, California is a particularly interesting place for this analysis. One component of this assessment is developing a mass balance of nitrogen as well as identifying gaps in knowledge and quantifying uncertainty. The main inputs of new reactive nitrogen to the state are 1) synthetic nitrogen fertilizer, 2) biological nitrogen fixation, and 3) atmospheric nitrogen deposition. Permanent losses of nitrogen include 1) gaseous losses (N2, N2O, NHx, NOy), 2) riverine discharge, 3) wastewater discharge to the ocean, and 4) net groundwater recharge. A final term is the balance of food, feed, and fiber to support the human and animal populations.
Results/Conclusions
The largest input of new reactive nitrogen to California was nitrogen fertilizer, but both nitrogen fixation and atmospheric deposition contribute significantly. In addition, even with highly productive agricultural lands, the large populations of people and animals require a net influx of food and feed to the state. On the export side, the riverine nitrogen loads smaller than many more mesic climates. Because many of the large population centers are on the coast, approximately three times more nitrogen is discharged directly from wastewater treatment plants into the ocean compared to the major watersheds in the state combined. Gas losses are being estimate through a combination of bottom up approaches using field data, emissions inventories, and numerical models. Methods for assessing groundwater loading as well as increases in soil and vegetation pools are still being developed. These calculations along with the analysis of management and policy tools will help elucidate the spatial location or activities that would be best to target to reduce the negative consequences of human alteration of the nitrogen cycle.