New reactive nitrogen enters California largely in the form of fertilizer, imported animal feed, and fossil fuel combustion. While some of that nitrogen contributes to productive agriculture, excess nitrogen from those sources contributes to groundwater contamination and air pollutants in the form of ammonia, nitric oxides, and nitrous oxide. In addition to statewide calculations, the magnitude of nitrogen flows was examined for eight subsystems: cropland; livestock; urban land; people and pets; natural land; atmosphere; surface water; and groundwater. Understanding the major nitrogen contributors will help policy-makers and nitrogen users, like farmers, prioritize efforts to improve nitrogen use.

nitrogen flows

Stakeholder Questions

Stakeholder-generated questions addressed in this chapter include the following:

  • What are the relative contributions of different sectors to N cycling in California?
  • What are the relative amounts of different forms of reactive nitrogen in air and water?
  • Are measurements of gaseous losses and water contamination accurate?


Main Messages

Synthetic fertilizer is the largest statewide import (519 Gg N yr-1) of nitrogen (N) in California.The predominant fate of this fertilizer is cropland including cultivated agriculture (422 Gg N yr-1) and environmental horticulture (44 Gg N yr-1).However, moderate amounts of synthetic fertilizer are also used on urban land for turfgrass (53 Gg N yr-1).

Excretion of manure is the second largest N flow (416 Gg N yr-1) in California.The predominant (72%) source of this N is dairy production, with minor contributions from beef, poultry and horses.A large fraction (35%)  of this manure is volatilized as ammonia (NH3) from livestock facilities (97 Gg N yr-1) and after cropland application (45 Gg N yr-1). However, there is limited evidence for rates of ammonia volatilization from manure.  While liquid dairy manure must be applied very locally (within a few kilometers of the source), the solid manure from dairies and other concentrated animal feeding operations can be composted to varying degrees and transported much longer distances (>100 km).However, because of the increased regulation of dairies in the Central Valley (see Chapter 8), it will soon be possible to determine what fraction of the dairy manure is used on the dairy farm compared to what is exported based on the nutrient management plans produced for each dairy.

Synthetically fixed N dominates the N flows to cropland.  Synthetic fertilizer (466 Gg N yr-1)is the largest flow of N to cropland, but a large fraction of N applied in manure and irrigation water to cropland is also originally fixed synthetically.  On average, we estimated that 69% of the N added annually to cropland statewide is derived from synthetic fixation.  

The biological N fixation that occurs on natural land (139 Gg N yr-1) has become completely overshadowed by the reactive N related to human activity in California.While this flow was once the major source of new reactive (i.e., biologically available) N to California, it now accounts for less than 10% of new imports at the statewide level. The areal rate (8 kg N ha-1 yr-1) representing the sum of all N inputs to natural lands, including N deposition, is an order of magnitude lower than either urban or cropland. 

The synthetic fixation of chemicals for uses other than fertilizer is a moderate (71 Gg N yr-1) N flow.  These chemicals include everyday household products such as nylon, polyurethane, and acrylonitrile butadiene styrene plastic (ABS).These compounds have been tracked to some degree at the national level (e.g., Domene and Ayres 2001).The true breadth and depth of their production, use, and disposal is poorly established.

Urban land is accumulating N.Lawn fertilizer, organic waste disposed in landfills, pet waste, fiber (i.e wood products), and non-fertilizer synthetic chemicals are all accumulating in the soils (75 Gg N yr-1), landfills (68 Gg N yr-1), and other built areas associated with urban land (123 Gg N yr-1).

Nitrogen exports to the ocean (39 Gg N yr-1) from California rivers accounts for less than 3% of statewide N imports.In part, this low rate of export is due to the fact that a major (45%) fraction of the land in California occurs in closed basins with no surface water drainage to the ocean.While concentrations of nitrate in some rivers can be quite high, the total volume of water reaching the ocean is quite low.

Direct sewage export of N to the ocean (82 Gg N yr-1) is more than double the N in the discharge of all rivers in the state combined.Because of the predominantly coastal population, the majority of wastewater is piped several miles out to the ocean.A growing number of facilities (> 100) in California appear to be using some form of N removal treatment prior to discharge.

Nitrous oxide (N2O) production is a moderate (38 Gg N yr-1) export pathway for N.Human activities produce 70% of the emissions of this greenhouse gas while the remainder is released from natural land.Agriculture (cropland soils and manure management) was a large fraction (32%) of N2O emissions in the state.

Ammonia is not tracked as closely as other gaseous N emissions because it is not currently regulated in the state.While acute exposures to NH3 are rare, both human health and ecosystem health are potentially threatened by the increasing regional emissions and deposition of NH3. However, rigorous methods for inventorying emissions related to human activities as well as natural soil emissions are currently lacking.

Atmospheric N deposition rates in parts of California are among the highest in the country, with the N deposited predominantly as dry deposition.The Community Multiscale Air Quality (CMAQ) model predicts that 66% of the deposition is oxidized N and 82% of the total deposition is dry deposition not associated with precipitation events.In urban areas and the adjacent natural ecosystems of southern California, deposition rates can exceed 30 kg N ha-1 yr-1, but deposition is, on average, 5 kg N ha-1 yr-1 statewide.

The atmospheric N emitted as NOx or NH3 in California is largely exported via the atmosphere downwind (i.e., east) from California.  Approximately 65% of the NOx and 73% of the NH3 emitted in California is not redeposited within state boundaries, making California a major source of atmospheric N pollution.  Further, atmospheric exports of N are more than 20 times higher than riverine N exports. 

Leaching from cropland (333 Gg N yr-1) was the predominant (88%) input of N to groundwater. It appears that N is rapidly accumulating in groundwater with only half of the annual N inputs extracted in irrigation and drinking water wells or removed by denitrification in the aquifer.On the whole, groundwater is still relatively clean, with a median concentration ~ 2 mg N L-1  throughout the state. However, there are many wells in California that already have nitrate concentrations above the Maximum Contaminant Level (10 mg NO3−-N L−1). Because of the time lag associated with groundwater transport (decades to millennia), the current N contamination in wells is from past activities and current N flows to groundwater will have impacts far into the future.

The amount of evidence and level of agreement varies between N flows. The most important sources of uncertainty in the mass balance calculations are for major flows with either limited evidence or low agreement or both.  Based on these criteria, biological N fixation on cropland and natural land, the fate of manure, denitrification in groundwater, and the storage terms are the most important sources of uncertainty.  

In many ways, the N flows in California are similar to other parts of the world. In a comparison with other comprehensive mass balances -- the Netherlands, United States, South Korea, China, Europe, and Phoenix - California stands out in its low surface water exports and high N storage, primarily in groundwater and urban land. Further, when compared to these other regions of varying size, California has a relatively low N use on both a per-capita and, especially, on a per-hectare basis.