Print PageHumans have greatly altered the global dynamics of reactive forms of nitrogen (N). The global increase in reactive N has occurred intentionally through the planting of legumes and synthetic fixation of fertilizer and unintentionally by combusting fossil fuels which produces a mixture of reactive N gases. While some of this N is associated with food production, a large fraction of reactive N has resulted in decreased air and water quality. Because of the close proximity of dense urban areas, highly productive cropland, and significant acreage of natural land, California is a particularly interesting place for examining N flows. We focus on identifying the sources of N to the land surface (excluding the inputs to humans and domesticated animals) in California and estimating the loss pathways. We use two approaches for this analysis. First, we compiled literature estimates of inputs and outputs from three land covers (cropland, natural land, and urban land) and used a mass balance approach to constrain the magnitude of the N flows. Secondly, we use the calculated N surplus by substracting N harvested in crops from the estimated inputs and modeled the partitioning of the outputs into gasesous and dissolved losses based on soil and climate parameters.
Results/Conclusions
Although cropland covered only 5 million hectares, or 12% of the area, it accounted for more than 70% of the 1300 Gg of N inputs to the land surface in California. Synthetic fertilizer (50%), livestock manure (26%), and N fixation by alfalfa (18%) were the dominant sources of the 950 Gg of N inputs to cropland. The 270 Gg of N inputs to natural lands, which cover 80% of the state, were split evenly between atmospheric deposition and biological N fixation. In urban land soils, atmospheric deposition and synthetic fertilizer for turfgrass supplied the new N. By far, the largest loss pathway (529 Gg N) was the harvest of crops. Gaseous emissions dominated in natural lands while dissolved losses dominated in cropland. However,the mass balance approach indicated significant soil storage of N in all three land cover types. The modeling approach, which assumed steady state N conditions, suggested that gaseous and dissolved losses were approximately equally divided. Further work will partition the gaseous losses into nitric oxide, nitrous oxide, and dinitrogen gases. While these calculations focus on land surface and not the people and livestock, the results clearly show the dominance of agriculture on N dynamics in California.
