Direct drivers of California’s nitrogen cycle

by Zixin Lu last modified Jun 10, 2016 01:39 PM
The release of nitrogen the environment is in part a consequence of the inherent properties of the N cycle but is greatly affected by human decisions. This chapter assesses those human and natural processes that directly alter N cycling (hereafter referred to as ‘direct drivers’). The chapter considers trends in these on-the-ground actions that influence N use, leakages, and emissions, following on our examination of underlying drivers and leading to calculations of the relative magnitude of N flows in the state.

We document six activities that have and will continue to shape California's N cycle:

1) fertilizer use on croplands;
2) feed and manure management;
3) fossil fuel combustion;
4) industrial processes (e.g. chemicals, explosives, and plastics);
5) wastewater management; and
6) land use, land cover, and land management

Stakeholder Questions

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

  • What are the current N rate recommendations? Are current nitrogen application guidelines appropriate for present-day cropping conditions?
  • How is nitrogen use efficiency (NUE) determined and what are the most efficient and inefficient production systems?


Main Messages

Everyday actions of Californians radically alter the nitrogen (N) cycle. Activities such as eating, driving, and even disposing of waste modify N stocks and flows, transferring N statewide and influencing N dynamics beyond California’s border. Six actions fundamentally change N cycling in the state. Each of these drivers has intensified since 1980.

Direct drivers catalyze specific N transformations and transfers between environmental systems. There are close relationships
between a direct driver and the N cycle. Some direct drivers are much more important than others for specific impacts. Fertilizer use dominates nitrate (NO3) leaching and nitrous oxide (N2O) emissions. Fuel combustion drives volatilization of nitrogen oxides (NOx). The spatial distributions of activities create distinct regional patterns of consequences (both benefits and costs).

Fertilizer use—inorganic and organic—represents the most significant N cycle modification. Sales of chemical N fertilizers
have increased considerably since World War II and have risen by at least 40% since 1970, but consumption has leveled off in the past 20 years. Increases in agricultural productivity have been even greater. N fertilizer has been critical for the growth of California’s agricultural industry and rural economy. Despite progress, inorganic N fertilizer application rates (kg ha−1) increased an average of 25% between 1973 and 2005. Data show the majority of California crops recover well below half of applied N, with some crops capturing as little as 30%. Similar or even lower N recovery rates are found when organic N sources are used. Differences between the NUE in research trials at plot and field scale and statewide averages suggest there may be substantial potential for improvement in fertilizer management.

Until recently, manure management decisions were made without much regard to N consequences. The breadth of techniques used, limitations in available information, and large variability among operations, especially for San Joaquin Valley dairies, make any conclusion about changes in manure management practices tentative. Surveys, however, suggest the recent adoption of manure management techniques helps manage nutrients more effectively. It is important to note that optimal manure N handling is the consequence of many processes and thus must be considered as a system.

Fuel combustion increased significantly, but emissions declined steadily since 1980. Over the past 30 years, sales of diesel and gasoline fuel, size of the vehicle fleet (both passenger cars and heavy-duty trucks), and the number of stationary sources (e.g., energy production and industry) increased measurably, often doubling. Emissions however have been controlled by aggressive technology forcing regulations. This is most evident in the declining importance of the small vehicle fleet for NOx emissions by comparison to diesel engines.

Ammonia (NH3) is an ingredient in a variety of industrial products - including plastics, nylons, chemical intermediaries, and explosives. However, much of its use and related impacts are poorly documented.
In addition to the release of N compounds during production, the longevity of N-derived industrial products (varying from spatulas to counter tops) results in a latent pool of N in human settlements. Slow degradation means they are a long-term threat to human and environmental health. Industrial use may be as large as 55% of inorganic N fertilizer use annually.

About 77% of food N will enter wastewater collection systems and about 50% of wastewater is dispersed in the environment without N removal treatment. This includes wastewater treatment plants with limited nitrification, leakage from sewers, and wastewater infiltration systems. Recent attempts to control N pollution have increased the level of treatment practiced at municipal wastewater facilities throughout California. In 2008, nearly 50% of wastewater treatment facilities reported performing at least advanced secondary treatment and 20% performed tertiary treatment processes. On-site wastewater systems treat the wastewater of more than 3.5 million Californians, with approximately 12,000 new units installed each year. Despite relatively small potential N emissions, improperly sited or malfunctioning systems can cause N discharge hot spots.

Changes in land cover, land use, and land management fundamentally alter N cycling in ways only recently appreciated.  Change can result from a shift in land cover or simply a change in the intensity of use; both have occurred in California. The net effect of urbanization and agricultural relocation/expansion has led to a 1% decrease in total agricultural land over the same time frame. This shift in land cover has been accompanied with an intensification of use. In croplands, the mix of crops produced has changed from relatively N extensive to N intensive species.  Field crops were still grown on 53% of cropland in 2007 (largely because of the land area dedicated to alfalfa) but this is a significant decrease from 74% in 1970. Simultaneously, the dairy cow population has doubled and the broiler population has tripled in conjunction with higher flock/herd size, concentrating N rich feed in California and amplifying manure N handling concerns. Change can result from a shift in land cover or change in intensity of use. Urbanization has caused agriculture to relocate, often to lands more marginally suited for these systems. The net effect of urbanization and agricultural relocation/expansion has led to a 1% decrease in total agricultural land between 1972 and 2000. This has been accompanied by an intensification of use. The mix of crops produced has changed from relatively N-extensive to N-intensive species. Field crops were still grown on 53% of cropland in 2007 (largely because of the land area dedicated to alfalfa), but this is a significant decrease from 74% in 1970. Simultaneously, the dairy cow population has doubled and the broiler population has tripled, concentrating N-rich feed in California and amplifying manure N handling concerns.

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