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Nitrogen fertilizer use in California: Assessing the data, trends and a way forward

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Authors

Todd S. Rosenstock, World Agroforestry Centre (previously UC Davis)
Daniel Liptzin, University of New Hampshire (previously UC Davis)
Johan Six, ETH-Zurich (previously UC Davis)
Thomas P. Tomich, UC Davis

Publication Information

California Agriculture 67(1):68-79. https://doi.org/10.3733/ca.E.v067n01p68

Published online January 01, 2013

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Abstract

Nitrogen fertilizer is an indispensable input to modern agriculture, but it also has been linked to environmental degradation and human health concerns. Recognition of these trade-offs has spurred debate over its use. However, data limitations and misinformation often constrain discussion, cooperative action and the development of solutions. To help inform the dialogue, we (1) evaluate existing data on nitrogen use, (2) estimate typical nitrogen fertilization rates for common crops, (3) analyze historical trends in nitrogen use, (4) compare typical nitrogen use to research-established guidelines and (5) identify cropping systems that have significant influence on the state's nitrogen cycle. We conclude that a comprehensive grower self-monitoring system for nitrogen applications is required to improve nitrogen-use information and to better support evidence-based decision making. The discussion here presents a primer on the debate over nitrogen fertilizer use in California agriculture.

Full text

Nitrogen fertilizer is an essential resource for agriculture, and its use has undoubtedly benefited California and its citizens. However, overuse of nitrogen fertilizer threatens the health of the state's agricultural, human and natural resources. On the one hand, nitrogen is necessary for crop growth and development, and thus nitrogen fertilizer use supports California's robust agricultural economy and rural society. On the other hand, applying nitrogen in excess has been linked to water and air pollution, depletion of the ozone layer, climate change and numerous human health concerns (Galloway et al. 2003; Millennium Ecosystem Assessment 2005).

Tractor applies fertilizer to cole crop plants near Pigeon Point Lighthouse, San Mateo County. Nitrogen fertilizer is an essential resource for agriculture, but its overuse can threaten human health and the environment.

Tractor applies fertilizer to cole crop plants near Pigeon Point Lighthouse, San Mateo County. Nitrogen fertilizer is an essential resource for agriculture, but its overuse can threaten human health and the environment.

The trade-offs that nitrogen fertilizer use present to society have been documented in California for more than 50 years (Harding et al. 1963; Proebsting 1948). It is worth noting that fertilizer is just one way humans add reactive nitrogen into the environment, and other activities such as fossil fuel combustion and waste discharge contribute to the aforementioned concerns. However, a forthcoming report indicates that inorganic nitrogen fertilizer use is responsible for the largest fraction, by far, of new nitrogen introduced into California's environment each year (Liptzin and Dahlgren, unpublished data).

The amount of inorganic (chemical) nitrogen fertilizer sold in California has risen dramatically over the past 70 years (fig. 1). By the 1970s, nitrogen fertilizer sales — and presumably use — exceeded 400,000 tons of nitrogen contained in inorganic fertilizer per year, and in the subsequent decade sales grew more than 25% to more than 500,000 tons of nitrogen per year. Between 1980 and 2001, the average amount of nitrogen sold per year was no longer increasing significantly, but annual sales have surpassed 600,000 tons of nitrogen in some years. Large upward trends in fertilizer sales in the last half of the twentieth century are not unique to California; similar increases are evident throughout the developed world (Millennium Ecosystem Assessment 2005). As nitrogen fertilizer use has expanded, so has the evidence documenting the negative consequences of reactive nitrogen on human health and the environment (Davidson et al. 2012; Townsend et al. 2003).

Statewide sales of nitrogen fertilizer, 1945–2008. Because there is no explanation for the 50% rise in sales from 2001 to 2002, the largest 1-year change since estimates began, there is reason to question the accuracy of data since 2001. Source: California Department of Food and Agriculture.

Fig. 1. Statewide sales of nitrogen fertilizer, 1945–2008. Because there is no explanation for the 50% rise in sales from 2001 to 2002, the largest 1-year change since estimates began, there is reason to question the accuracy of data since 2001. Source: California Department of Food and Agriculture.

Today, nitrogen in general and nitrogen fertilizer use specifically both figure prominently in regulatory discourse. Federal and state agencies tasked with protecting air and water quality as well as with mitigating climate change are evaluating the causes, consequences and costs of agricultural nitrogen use. Examples of this concern in California include the UC Center for Watershed Sciences' report to the California Legislature on nitrate in drinking water, the Central Coast Regional Water Quality Control Board's (RWQCB) renewal process for the Irrigated Agricultural Lands Waiver, the Climate Action Reserve's nitrogen fertilizer reduction protocol, the Central Valley RWQCB's Irrigated Lands Regulatory Program, the Central Valley SALTS program andthe Central Valley RWQCB's General Order for Dairy Waste Dischargers. The latter, for instance, regulates nitrogen fertilizer application on croplands associated with dairies, constraining its use.

Background and scope of this article

This article reports research from one part of the California Nitrogen Assessment (see sidebar page 70). Assessments are an increasingly common method scientists use to analyze existing data sets and gain a big-picture view of what is known and what is scientifically uncertain.

The best example of an assessment is the global effort that led to reports by the Intergovernmental Panel on Climate Change (Ash et al. 2010; IPCC 2007; Millennium Ecosystem Assessment 2005). Recently, the Integrated Nitrogen Committee published an assessment of nitrogen in the United States (Integrated Nitrogen Committee 2011).

Here the authors assess existing knowledge on inorganic nitrogen fertilizer flows, practice and policy in California agriculture — knowledge that has only now been integrated and analyzed as a whole. They examine how statistics are generated, identify sources of uncertainty and compare and interpret data.

Scope. The research scope is limited to inorganic nitrogen fertilizer. Dairy manure, for instance, is not , although it is a high priority for attention by scientists and policymakers — and is included in the larger California Nitrogen Assessment ( http://nitrogen.ucdavis.edu ). Dairy manure application adds about 200,000 tons of nitrogen to California soil per year, an amount equivalent to more than one-third of the annual inorganic nitrogen sold in recent years, and it is applied to a relatively small number of forage crops.

Limits. The authors examine soil nitrogen cycling processes, which include exchanges of nitrogen between the soil and either air or water. However, the discussion is intentionally general; it does not capture nitrogen transformation or emissions under various soil, crop and water management conditions. Analysis and experiments are needed to draw conclusions regarding the fate of nitrogen in specific fertilized and irrigated systems.

Stakeholder questions addressed. This article addresses stakeholder questions about nitrogen management practices in cropping systems. It presents the best available information that applies to these questions:: How is nitrogen fertilizer currently being used? What are the current nitrogen rate recommendations? Are those recommendations adequate for present-day cropping conditions? More information on the stakeholder process can be found at http://nitrogen.ucdavis.edu . — Editors

It is important that credible and comprehensive scientific information on nitrogen use be available to support evidence-based policy-making. Without information based on sound science, nitrogen policies may be poorly prescribed, ineffective, cause unintended consequences or even be counterproductive. Stakeholders recognize this and have identified the need for more information on inorganic nitrogen fertilizer use as a high priority task ( http://nitrogen.ucda-vis.edu ).

Accurate data on nitrogen fertilizer use are difficult to come by, however. Either nitrogen fertilizer use is simply not tracked at relevant scales, as is most often the case, or the data sources are inconsistent (see discussion of grower and expert surveys below). Despite the fact that this data scarcity makes current estimates of nitrogen fertilizer use uncertain, the estimates still serve as an input to policy discussions. For example, the Intergovernmental Panel on Climate Change (IPCC) suggests that estimated application of nitrogen fertilizer to cropland is a key parameter to use in approximating cropland emissions of nitrous oxide, a potent greenhouse gas.

Because of the relationships among fertilizer use, crop yields, resource degradation and the current policy environment in California, information on nitrogen use is in high demand now and will become of even greater importance as policies are developed in the future. The objective of this research is to assess the available information on nitrogen use in California by (1) identifying data sources and their limitations, (2) establishing average nitrogen application rates by crop, (3) determining historical trends in nitrogen use (within the context of changes in crop yield) and (4) comparing how average nitrogen application rates articulate with nitrogen rate guidelines. We go on to show that these results identify crops that have significant influence on nitrogen use, and we suggest this information can then be used to set priorities for research, outreach or policy. This evaluation of the current state of knowledge on nitrogen fertilizer use is part of a broader assessment of nitrogen in California, the California Nitrogen Assessment (see box below).

What is the California Nitrogen Assessment?

The California Nitrogen Assessment (CNA) is a comprehensive effort to examine existing knowledge on nitrogen science, policy and practice in California. Researchers have collected and synthesized a large body of data to analyze patterns and trends in nitrogen inputs, outputs and storage throughout the state. The aim is to more effectively link science with action and inform policy and field-level practice.

The CNA includes:

  • Identification of underlying drivers (e.g., regulations, population growth) and direct drivers (e.g., fertilizer use, soil management and fuel combustion) that affect stocks and flows of nitrogen in California agriculture.

  • Calculation of a mass balance to examine how nitrogen moves through California agroecosystems and the state as a whole (including agriculture, sewage, industry and transportation).

  • Evaluation of the state of knowledge about nitrogen's impacts on ecosystem health and human well-being.

  • A suite of practices and policy options and the potential effects each would have on agriculture, the environment and human health.

  • Communications to help the public understand the nitrogen cycle and to help decision makers at the farm and public policy levels.

The CNA is a project of the Agricultural Sustainability Institute at UC Davis and the UC Sustainable Agriculture Research and Education Program.

For more information:

General information on California Nitrogen Assessment (CNA) http://nitrogen.ucdavis.edu

Basics of nitrogen biogeochemistry and the CNA's mass balance http://nitrogen.ucdavis.edu/research/nitrogen/n-science/n-biogeochemistry

Information on stakeholder involvement, review and questions http://nitrogen.ucdavis.edu/research/nitrogen/n-stakeholders/nitrogen-stakeholders

Major funding for the California Nitrogen Assessment is provided by a grant from the David and Lucile Packard Foundation. Work on the assessment began in January 2009 and will continue through 2013. Institutional partners are the UC Agricultural Issues Center and the Kearney Foundation of Soil Science. — Editor

Scientific assessments, such as the California Nitrogen Assessment, have become a common method scientists use to inform policymakers on complex social and environmental issues. Instead of generating new research, these assessments analyze existing bodies of research, data and models. Assessments generate insights through the synthesis and integration of available information from multiple scientific disciplines to distinguish that which is known and well established from that which is unknown and scientifically uncertain. Assessments piece together the best available information to inform discussions, systematically calling out uncertainty. The assessment of nitrogen fertilizer use reported here relied on standard assessment methods, such as engaging stakeholders to frame the scientific question, aggregating available information and identifying sources of uncertainty (Ash et al. 2010).

The nitrogen cycle

There are no easy solutions to managing the trade-offs associated with agricultural nitrogen; this is due to (1) the complexity of the nitrogen cycle in general (fig. 2) and (2) the mobility and diversity of soil nitrogen compounds in particular. The vast majority of nitrogen in soils is in soil organic matter and hence does not pose an immediate threat to the environment or humans. This soil organic matter serves as a nitrogen reservoir, and each year a fraction of this nitrogen is mineralized to ammonium. Soil microbes can then turn ammonium into nitrate via the process of nitrification. Both forms of nitrogen, ammonium and nitrate, are available for plant uptake. Mineralization supplies as much as half or more of the nitrogen to crops (Gardner and Drinkwater 2009). The reverse process (immobilization) entails the integration of the inorganic nitrogen produced by mineralization into the living biomass of plants and microbes.

The nitrogen cycle. Nitrogen in the environment is highly mobile and readily transformed into various compounds by physical, chemical and biological processes. Arrows indicate major nitrogen-cycling processes, which continuously produce diverse nitrogen compounds in the environment.

Fig. 2. The nitrogen cycle. Nitrogen in the environment is highly mobile and readily transformed into various compounds by physical, chemical and biological processes. Arrows indicate major nitrogen-cycling processes, which continuously produce diverse nitrogen compounds in the environment.

Glossary: Nitrogen in soils

Nitrogen may enter the soil through rainfall, lightning, nitrogen fixation by soil organisms, plant and animal decomposition, or manures and commercial fertilizers. It may be lost by plant removal, volatilization, leaching or erosion. It transforms continuously in soil, air and water.

Ammonification (mineralization): During decomposition of plant or animal material, specialized bacteria transform nitrogen to ammonia (NH3) or ammonium (NH4+); the latter is useful to plants.

Ammonium (NH4+): This form of nitrogen can be used by plants, or converted to nitrate by bacteria (and then taken up by plants). It is a positively charged ion (cation), attracted to negatively charged soil clay. For this reason, it is not leached to a great extent.

Denitrification: In this anaerobic process, other specialized bacteria change nitrate back to nitrogen gas, reducing pollution of groundwater but increasing nitrogen oxides in the air. Denitrification occurs only when oxygen is low, such as during flooding and in clay soils. Because most California soils are coarse and well-drained, denitrification occurs less often, and soils are more vulnerable to nitrate contamination of water supplies by leaching.

Nitrification, nitrite (NO2) and nitrate (NO3): Specialized bacteria change ammonia to nitrite, and still others change nitrite to nitrate. Both processes are nitrification, and they are aerobic, occurring only when oxygen is present. Nitrate is the principal form of nitrogen used by plants. Because it is a negatively charged ion (anion) and is not attracted to soil clay, it leaches easily and is a water pollutant. Nitrate-enriched groundwater can also contribute to algal blooms in streams, although most such blooms result from nitrogen- and phosphorus-enriched surface runoff.

Nitrogen gas (N2): Dinitrogen gas occurs when two nitrogen atoms form a very strong, trivalent chemical bond; it comprises 78% of the atmosphere. Although largely inert, nitrogen gas can be "fixed" into biologically useful forms in the soil (see first paragraph).

Nitrogen loss (leaching, erosion): Nitrogen losses from the soil system occur by plant removal, denitrification, leaching, volatilization and erosion. Plant removal by crops is fertilization. Erosion and leaching can contribute to ground- and surface water pollution.

Nitrogen, organic (nitrogen in living or once-living things): “Organic nitrogen” refers to a nitrogen compound that had its origin in living material and is still part of a carbon-chain complex. It can enter soil as decomposed plant or animal tissue. It is not available to plants until microorganisms transform it to ammonium (NH4+).

Nitrogen, reactive: Reactive nitrogen is all nitrogen other than dinitrogen gas (N2).

Volatilization: Soil microorganisms convert ammonium nitrogen to ammonia gas in soils with a high pH, that is a pH greater than 7.5. Such soils are not common in California.

Glossary sources include an article by Thomas Harter in the July/August 2009 Southwest Hydrology. Janet White

Nitrogen compounds can also be released from the crop root zone through multiple processes. Leaching relates to the physical movement of nitrate downward through the soil profile. Volatilization is a physiochemical process that emits gaseous ammonia. Denitrification is a microbial-mediated release of inert dinitrogen gas and potentially nitrogen oxides including nitrous oxide. It is the emission of these nitrogen compounds that threatens the health of California's environment and human population.

The rate at which nitrogen cycling occurs in soils is a function of a multitude of abiotic (precipitation and temperature), biotic (microbial communities) and human-mediated (such as tillage and nitrogen fertilizer application rate) factors.

Fertilizer and excess nitrogen

Adding inorganic nitrogen fertilizer to soil promotes high plant productivity and long-term soil fertility (Ladha et al. 2011), but this can also cause large surpluses of nitrogen in the environment. This excess nitrogen can lead to environmental degradation by percolation (leaching) through the root zone and into groundwater, through surface runoff into waterways, or via emissions of nitrogen gases such as ammonia, nitric oxide or nitrous oxide into the atmosphere. Gaseous and water-borne nitrogen may be related to nitrogen fertilizer application rates in linear and nonlinear ways, which means application rates alone are not always enough to determine how much is lost to the environment (Broadbent and Rauschkolb 1977; Hoben et al. 2011; Linquist et al. 2012). Recent evidence suggests that the best indicator of potential nitrogen loss into the environment is the “surplus” nitrogen, which is the difference between the nitrogen applied as fertilizer and the nitrogen taken up by the crop (Van Groenigen et al. 2010). Therefore, both nitrogen application rate and nitrogen surplus, which is calculated after the crops are harvested, are important factors for predicting where nitrogen loss should be highest.

Fertilizer trucks transport liquid ammonia throughout the state. Adding inorganic nitrogen fertilizer to soil promotes high plant productivity and long-term soil fertility, but can also lead to excess nitrogen in the environment and environmental degradation.

Fertilizer trucks transport liquid ammonia throughout the state. Adding inorganic nitrogen fertilizer to soil promotes high plant productivity and long-term soil fertility, but can also lead to excess nitrogen in the environment and environmental degradation.

Sonja Brodt, Daniel Liptzin and Todd Rosenstock learn about fertilizer production in California from Ken Johnson of TSI Fertilizer Manufacturing in Dixon. Large upward trends in fertilizer sales in the last half of the twentieth century are evident throughout the developed world.

Sonja Brodt, Daniel Liptzin and Todd Rosenstock learn about fertilizer production in California from Ken Johnson of TSI Fertilizer Manufacturing in Dixon. Large upward trends in fertilizer sales in the last half of the twentieth century are evident throughout the developed world.

Nitrogen-fertilizer-use data

Data on nitrogen fertilizer use in California are scarce and fragmented. Typically, data are less available and more variable at finer spatial resolutions. The following identifies the primary sources of data available for statewide and county nitrogen use and nitrogen application rates by crop, and discusses some of the inherent limitations of these data sources.

Statewide nitrogen fertilizer use.

Fertilizer data are collected by the California Department of Food and Agriculture (CDFA) and reported at the state and county levels. Since fertilizer sales are only recorded when a licensed fertilizer dealer sells to an unlicensed buyer, these data provide a rough approximation of the total inorganic nitrogen applied statewide, assuming no stockpiling or interstate transfer of fertilizing materials (fig. 1). Annual data are available dating back to 1945. However, there are additional reasons to question the accuracy of these data. Perhaps the most obvious is the unexplainable 50% jump in sales between 2001 and 2002, the largest 1-year change since annual estimates began. And the reported sales remained abnormally high in the following 5 years (2003 to 2007). Because there is no explanation for this large jump in reported fertilizer sales statewide — neither its root cause nor an apparent accounting error — we have little confidence in the data reported since 2001.

County nitrogen fertilizer use.

While fertilizer sales data are reported to CDFA at the county level, the precision of these data is problematic. County fertilizer data portray a geographic distribution of sales unlikely to match actual use for most counties. This is due to the method of data collection, which neglects fertilizer transported from one county to another. For example, more than 20% of total statewide nitrogen sales were reported to have taken place in San Joaquin County. It is entirely possible that this value can be attributed to the large quantity of ammonia delivered to the Port of Stockton and redistributed from there. County-level sales data may be an appropriate proxy for nitrogen applications in counties where one does not suspect significant transport of nitrogen into or out of the county, but it is not possible to be certain with the current data collection system.

Nitrogen fertilizer use by crop.

There is neither a comprehensive source of information nor current estimates of average nitrogen applications by crop in California. The most complete source of data in California is a 1973 survey of approximately 120 UC experts and affiliates about nitrogen application rates on 45 commodities (Rauschkolb and Mikkelsen 1978). (The term “expert” in this article refers to UC employees — faculty, farm advisors and facility managers — but we acknowledge there are many other sources of expertise.) However, these rates are unlikely to be the same today due to changes in irrigation technology, tillage, cultivars and countless other management practices since the 1970s. While a few other expert estimates are available, they generally cover fewer crops than the 1973 survey (Miller and Smith 1976; Zhang et al. 2009)

Data direct from growers are largely unavailable. In a few instances, surveys have been conducted (Hartley and van Kessel 2003), though they sometimes omit asking for (Lopus et al. 2010) or reporting (Dillon et al. 1999) nitrogen application rates. The only systematic source of nitrogen application data based on grower surveys is the USDA Agricultural Chemical Use Program reports (USDA NASS 2010a). The USDA surveys growers for nitrogen fertilizer application rates for major crops on a rotating schedule, with an emphasis on field crops. As a result, surveys on nutrient use for each crop only occur intermittently — sometimes with significant time elapsing between information being gathered for certain crops. For example, almond was surveyed in 1999 and 2009. Though long-term trends may be detectable from such data, there is the distinct possibility that they may be obscured by year-to-year variability in data that is not quantified and therefore cannot be taken into account. Furthermore, some crops that contribute significantly to California's agricultural economy are not customarily surveyed in any state (such as fresh-market tomatoes), not surveyed in California (such as corn) or not surveyed for nutrient use (such as nursery and greenhouse plants).

A farmworker applies fertilizer to nursery crops in Winters in the Central Valley. At present, there is neither a comprehensive source of information nor current estimates of average nitrogen applications by crop in California.

A farmworker applies fertilizer to nursery crops in Winters in the Central Valley. At present, there is neither a comprehensive source of information nor current estimates of average nitrogen applications by crop in California.

Assessing crop nitrogen use

Developing new estimates of nitrogen use by crop is critical to informing the research, outreach and policy agenda on nitrogen fertilizer use. Surveys are resource intensive, and their design and scale may make it difficult to achieve a representative sample, especially in the diverse California agricultural landscape. In addition, the California Nitrogen Assessment had little success in an effort to survey UCCE employees about nitrogen use, and commodity boards about nitrogen research; the response rate was less than 7% and less than 15%, respectively. In place of a new survey, we developed and utilized a new approach to estimate an average nitrogen application rate by crop based on available data. The premise underlying this assessment was to smooth out some of the uncertainties and variation in these data by aggregating across sources. We compiled the available information from expert and grower sources into a database according to the methods described below.

For each crop, we first averaged the available expert data since 2000 and then averaged the grower data since 1999. Utilizing nitrogen estimates that date from 1999 or 2000 was necessary to increase the sample sizes, as a result of the limited number of expert responses available over the time period for each crop.

Expert data.

Expert opinions of nitrogen fertilizer use were taken from UC Agricultural and Resource Economics (ARE) Cost and Return Studies that have been conducted from 2000 to the present (UCD 2010). Studies of each crop were selected to represent variations in California's agricultural regions (such as the Imperial Valley versus the Salinas Valley) as well as the breadth of management practices (such as furrow versus drip irrigation). Compiling studies that span the geographic and production continuum was important because of the potential differences in nitrogen application with the various environmental conditions and production techniques.

Not all of the available studies were included in the database. Some studies were omitted because studies of the same crop often recycle the descriptions and estimates of nitrogen use until management practices change significantly and thus inclusion of every study would have skewed the estimate. An average of two studies were included for each crop, but the number of studies included ranged from one to five. Data were averaged to provide a representative value of nitrogen fertilizer use for each crop based on expert opinion.

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Author notes

The authors thank C. Snyder, C. Garnache, S. Ogburn, C. Bishop, T. Harter, J. Dickey, five anonymous reviewers and the editors for their comments on earlier versions of the manuscript. The David and Lucile Packard Foundation supported this research as part of the California Nitrogen Assessment.

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Nitrogen fertilizer use in California: Assessing the data, trends and a way forward

Todd S. Rosenstock, Daniel Liptzin, Johan Six, Thomas P. Tomich
Webmaster Email: wsuckow@ucanr.edu

Nitrogen fertilizer use in California: Assessing the data, trends and a way forward

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Authors

Todd S. Rosenstock, World Agroforestry Centre (previously UC Davis)
Daniel Liptzin, University of New Hampshire (previously UC Davis)
Johan Six, ETH-Zurich (previously UC Davis)
Thomas P. Tomich, UC Davis

Publication Information

California Agriculture 67(1):68-79. https://doi.org/10.3733/ca.E.v067n01p68

Published online January 01, 2013

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Abstract

Nitrogen fertilizer is an indispensable input to modern agriculture, but it also has been linked to environmental degradation and human health concerns. Recognition of these trade-offs has spurred debate over its use. However, data limitations and misinformation often constrain discussion, cooperative action and the development of solutions. To help inform the dialogue, we (1) evaluate existing data on nitrogen use, (2) estimate typical nitrogen fertilization rates for common crops, (3) analyze historical trends in nitrogen use, (4) compare typical nitrogen use to research-established guidelines and (5) identify cropping systems that have significant influence on the state's nitrogen cycle. We conclude that a comprehensive grower self-monitoring system for nitrogen applications is required to improve nitrogen-use information and to better support evidence-based decision making. The discussion here presents a primer on the debate over nitrogen fertilizer use in California agriculture.

Full text

Nitrogen fertilizer is an essential resource for agriculture, and its use has undoubtedly benefited California and its citizens. However, overuse of nitrogen fertilizer threatens the health of the state's agricultural, human and natural resources. On the one hand, nitrogen is necessary for crop growth and development, and thus nitrogen fertilizer use supports California's robust agricultural economy and rural society. On the other hand, applying nitrogen in excess has been linked to water and air pollution, depletion of the ozone layer, climate change and numerous human health concerns (Galloway et al. 2003; Millennium Ecosystem Assessment 2005).

Tractor applies fertilizer to cole crop plants near Pigeon Point Lighthouse, San Mateo County. Nitrogen fertilizer is an essential resource for agriculture, but its overuse can threaten human health and the environment.

Tractor applies fertilizer to cole crop plants near Pigeon Point Lighthouse, San Mateo County. Nitrogen fertilizer is an essential resource for agriculture, but its overuse can threaten human health and the environment.

The trade-offs that nitrogen fertilizer use present to society have been documented in California for more than 50 years (Harding et al. 1963; Proebsting 1948). It is worth noting that fertilizer is just one way humans add reactive nitrogen into the environment, and other activities such as fossil fuel combustion and waste discharge contribute to the aforementioned concerns. However, a forthcoming report indicates that inorganic nitrogen fertilizer use is responsible for the largest fraction, by far, of new nitrogen introduced into California's environment each year (Liptzin and Dahlgren, unpublished data).

The amount of inorganic (chemical) nitrogen fertilizer sold in California has risen dramatically over the past 70 years (fig. 1). By the 1970s, nitrogen fertilizer sales — and presumably use — exceeded 400,000 tons of nitrogen contained in inorganic fertilizer per year, and in the subsequent decade sales grew more than 25% to more than 500,000 tons of nitrogen per year. Between 1980 and 2001, the average amount of nitrogen sold per year was no longer increasing significantly, but annual sales have surpassed 600,000 tons of nitrogen in some years. Large upward trends in fertilizer sales in the last half of the twentieth century are not unique to California; similar increases are evident throughout the developed world (Millennium Ecosystem Assessment 2005). As nitrogen fertilizer use has expanded, so has the evidence documenting the negative consequences of reactive nitrogen on human health and the environment (Davidson et al. 2012; Townsend et al. 2003).

Statewide sales of nitrogen fertilizer, 1945–2008. Because there is no explanation for the 50% rise in sales from 2001 to 2002, the largest 1-year change since estimates began, there is reason to question the accuracy of data since 2001. Source: California Department of Food and Agriculture.

Fig. 1. Statewide sales of nitrogen fertilizer, 1945–2008. Because there is no explanation for the 50% rise in sales from 2001 to 2002, the largest 1-year change since estimates began, there is reason to question the accuracy of data since 2001. Source: California Department of Food and Agriculture.

Today, nitrogen in general and nitrogen fertilizer use specifically both figure prominently in regulatory discourse. Federal and state agencies tasked with protecting air and water quality as well as with mitigating climate change are evaluating the causes, consequences and costs of agricultural nitrogen use. Examples of this concern in California include the UC Center for Watershed Sciences' report to the California Legislature on nitrate in drinking water, the Central Coast Regional Water Quality Control Board's (RWQCB) renewal process for the Irrigated Agricultural Lands Waiver, the Climate Action Reserve's nitrogen fertilizer reduction protocol, the Central Valley RWQCB's Irrigated Lands Regulatory Program, the Central Valley SALTS program andthe Central Valley RWQCB's General Order for Dairy Waste Dischargers. The latter, for instance, regulates nitrogen fertilizer application on croplands associated with dairies, constraining its use.

Background and scope of this article

This article reports research from one part of the California Nitrogen Assessment (see sidebar page 70). Assessments are an increasingly common method scientists use to analyze existing data sets and gain a big-picture view of what is known and what is scientifically uncertain.

The best example of an assessment is the global effort that led to reports by the Intergovernmental Panel on Climate Change (Ash et al. 2010; IPCC 2007; Millennium Ecosystem Assessment 2005). Recently, the Integrated Nitrogen Committee published an assessment of nitrogen in the United States (Integrated Nitrogen Committee 2011).

Here the authors assess existing knowledge on inorganic nitrogen fertilizer flows, practice and policy in California agriculture — knowledge that has only now been integrated and analyzed as a whole. They examine how statistics are generated, identify sources of uncertainty and compare and interpret data.

Scope. The research scope is limited to inorganic nitrogen fertilizer. Dairy manure, for instance, is not , although it is a high priority for attention by scientists and policymakers — and is included in the larger California Nitrogen Assessment ( http://nitrogen.ucdavis.edu ). Dairy manure application adds about 200,000 tons of nitrogen to California soil per year, an amount equivalent to more than one-third of the annual inorganic nitrogen sold in recent years, and it is applied to a relatively small number of forage crops.

Limits. The authors examine soil nitrogen cycling processes, which include exchanges of nitrogen between the soil and either air or water. However, the discussion is intentionally general; it does not capture nitrogen transformation or emissions under various soil, crop and water management conditions. Analysis and experiments are needed to draw conclusions regarding the fate of nitrogen in specific fertilized and irrigated systems.

Stakeholder questions addressed. This article addresses stakeholder questions about nitrogen management practices in cropping systems. It presents the best available information that applies to these questions:: How is nitrogen fertilizer currently being used? What are the current nitrogen rate recommendations? Are those recommendations adequate for present-day cropping conditions? More information on the stakeholder process can be found at http://nitrogen.ucdavis.edu . — Editors

It is important that credible and comprehensive scientific information on nitrogen use be available to support evidence-based policy-making. Without information based on sound science, nitrogen policies may be poorly prescribed, ineffective, cause unintended consequences or even be counterproductive. Stakeholders recognize this and have identified the need for more information on inorganic nitrogen fertilizer use as a high priority task ( http://nitrogen.ucda-vis.edu ).

Accurate data on nitrogen fertilizer use are difficult to come by, however. Either nitrogen fertilizer use is simply not tracked at relevant scales, as is most often the case, or the data sources are inconsistent (see discussion of grower and expert surveys below). Despite the fact that this data scarcity makes current estimates of nitrogen fertilizer use uncertain, the estimates still serve as an input to policy discussions. For example, the Intergovernmental Panel on Climate Change (IPCC) suggests that estimated application of nitrogen fertilizer to cropland is a key parameter to use in approximating cropland emissions of nitrous oxide, a potent greenhouse gas.

Because of the relationships among fertilizer use, crop yields, resource degradation and the current policy environment in California, information on nitrogen use is in high demand now and will become of even greater importance as policies are developed in the future. The objective of this research is to assess the available information on nitrogen use in California by (1) identifying data sources and their limitations, (2) establishing average nitrogen application rates by crop, (3) determining historical trends in nitrogen use (within the context of changes in crop yield) and (4) comparing how average nitrogen application rates articulate with nitrogen rate guidelines. We go on to show that these results identify crops that have significant influence on nitrogen use, and we suggest this information can then be used to set priorities for research, outreach or policy. This evaluation of the current state of knowledge on nitrogen fertilizer use is part of a broader assessment of nitrogen in California, the California Nitrogen Assessment (see box below).

What is the California Nitrogen Assessment?

The California Nitrogen Assessment (CNA) is a comprehensive effort to examine existing knowledge on nitrogen science, policy and practice in California. Researchers have collected and synthesized a large body of data to analyze patterns and trends in nitrogen inputs, outputs and storage throughout the state. The aim is to more effectively link science with action and inform policy and field-level practice.

The CNA includes:

  • Identification of underlying drivers (e.g., regulations, population growth) and direct drivers (e.g., fertilizer use, soil management and fuel combustion) that affect stocks and flows of nitrogen in California agriculture.

  • Calculation of a mass balance to examine how nitrogen moves through California agroecosystems and the state as a whole (including agriculture, sewage, industry and transportation).

  • Evaluation of the state of knowledge about nitrogen's impacts on ecosystem health and human well-being.

  • A suite of practices and policy options and the potential effects each would have on agriculture, the environment and human health.

  • Communications to help the public understand the nitrogen cycle and to help decision makers at the farm and public policy levels.

The CNA is a project of the Agricultural Sustainability Institute at UC Davis and the UC Sustainable Agriculture Research and Education Program.

For more information:

General information on California Nitrogen Assessment (CNA) http://nitrogen.ucdavis.edu

Basics of nitrogen biogeochemistry and the CNA's mass balance http://nitrogen.ucdavis.edu/research/nitrogen/n-science/n-biogeochemistry

Information on stakeholder involvement, review and questions http://nitrogen.ucdavis.edu/research/nitrogen/n-stakeholders/nitrogen-stakeholders

Major funding for the California Nitrogen Assessment is provided by a grant from the David and Lucile Packard Foundation. Work on the assessment began in January 2009 and will continue through 2013. Institutional partners are the UC Agricultural Issues Center and the Kearney Foundation of Soil Science. — Editor

Scientific assessments, such as the California Nitrogen Assessment, have become a common method scientists use to inform policymakers on complex social and environmental issues. Instead of generating new research, these assessments analyze existing bodies of research, data and models. Assessments generate insights through the synthesis and integration of available information from multiple scientific disciplines to distinguish that which is known and well established from that which is unknown and scientifically uncertain. Assessments piece together the best available information to inform discussions, systematically calling out uncertainty. The assessment of nitrogen fertilizer use reported here relied on standard assessment methods, such as engaging stakeholders to frame the scientific question, aggregating available information and identifying sources of uncertainty (Ash et al. 2010).

The nitrogen cycle

There are no easy solutions to managing the trade-offs associated with agricultural nitrogen; this is due to (1) the complexity of the nitrogen cycle in general (fig. 2) and (2) the mobility and diversity of soil nitrogen compounds in particular. The vast majority of nitrogen in soils is in soil organic matter and hence does not pose an immediate threat to the environment or humans. This soil organic matter serves as a nitrogen reservoir, and each year a fraction of this nitrogen is mineralized to ammonium. Soil microbes can then turn ammonium into nitrate via the process of nitrification. Both forms of nitrogen, ammonium and nitrate, are available for plant uptake. Mineralization supplies as much as half or more of the nitrogen to crops (Gardner and Drinkwater 2009). The reverse process (immobilization) entails the integration of the inorganic nitrogen produced by mineralization into the living biomass of plants and microbes.

The nitrogen cycle. Nitrogen in the environment is highly mobile and readily transformed into various compounds by physical, chemical and biological processes. Arrows indicate major nitrogen-cycling processes, which continuously produce diverse nitrogen compounds in the environment.

Fig. 2. The nitrogen cycle. Nitrogen in the environment is highly mobile and readily transformed into various compounds by physical, chemical and biological processes. Arrows indicate major nitrogen-cycling processes, which continuously produce diverse nitrogen compounds in the environment.

Glossary: Nitrogen in soils

Nitrogen may enter the soil through rainfall, lightning, nitrogen fixation by soil organisms, plant and animal decomposition, or manures and commercial fertilizers. It may be lost by plant removal, volatilization, leaching or erosion. It transforms continuously in soil, air and water.

Ammonification (mineralization): During decomposition of plant or animal material, specialized bacteria transform nitrogen to ammonia (NH3) or ammonium (NH4+); the latter is useful to plants.

Ammonium (NH4+): This form of nitrogen can be used by plants, or converted to nitrate by bacteria (and then taken up by plants). It is a positively charged ion (cation), attracted to negatively charged soil clay. For this reason, it is not leached to a great extent.

Denitrification: In this anaerobic process, other specialized bacteria change nitrate back to nitrogen gas, reducing pollution of groundwater but increasing nitrogen oxides in the air. Denitrification occurs only when oxygen is low, such as during flooding and in clay soils. Because most California soils are coarse and well-drained, denitrification occurs less often, and soils are more vulnerable to nitrate contamination of water supplies by leaching.

Nitrification, nitrite (NO2) and nitrate (NO3): Specialized bacteria change ammonia to nitrite, and still others change nitrite to nitrate. Both processes are nitrification, and they are aerobic, occurring only when oxygen is present. Nitrate is the principal form of nitrogen used by plants. Because it is a negatively charged ion (anion) and is not attracted to soil clay, it leaches easily and is a water pollutant. Nitrate-enriched groundwater can also contribute to algal blooms in streams, although most such blooms result from nitrogen- and phosphorus-enriched surface runoff.

Nitrogen gas (N2): Dinitrogen gas occurs when two nitrogen atoms form a very strong, trivalent chemical bond; it comprises 78% of the atmosphere. Although largely inert, nitrogen gas can be "fixed" into biologically useful forms in the soil (see first paragraph).

Nitrogen loss (leaching, erosion): Nitrogen losses from the soil system occur by plant removal, denitrification, leaching, volatilization and erosion. Plant removal by crops is fertilization. Erosion and leaching can contribute to ground- and surface water pollution.

Nitrogen, organic (nitrogen in living or once-living things): “Organic nitrogen” refers to a nitrogen compound that had its origin in living material and is still part of a carbon-chain complex. It can enter soil as decomposed plant or animal tissue. It is not available to plants until microorganisms transform it to ammonium (NH4+).

Nitrogen, reactive: Reactive nitrogen is all nitrogen other than dinitrogen gas (N2).

Volatilization: Soil microorganisms convert ammonium nitrogen to ammonia gas in soils with a high pH, that is a pH greater than 7.5. Such soils are not common in California.

Glossary sources include an article by Thomas Harter in the July/August 2009 Southwest Hydrology. Janet White

Nitrogen compounds can also be released from the crop root zone through multiple processes. Leaching relates to the physical movement of nitrate downward through the soil profile. Volatilization is a physiochemical process that emits gaseous ammonia. Denitrification is a microbial-mediated release of inert dinitrogen gas and potentially nitrogen oxides including nitrous oxide. It is the emission of these nitrogen compounds that threatens the health of California's environment and human population.

The rate at which nitrogen cycling occurs in soils is a function of a multitude of abiotic (precipitation and temperature), biotic (microbial communities) and human-mediated (such as tillage and nitrogen fertilizer application rate) factors.

Fertilizer and excess nitrogen

Adding inorganic nitrogen fertilizer to soil promotes high plant productivity and long-term soil fertility (Ladha et al. 2011), but this can also cause large surpluses of nitrogen in the environment. This excess nitrogen can lead to environmental degradation by percolation (leaching) through the root zone and into groundwater, through surface runoff into waterways, or via emissions of nitrogen gases such as ammonia, nitric oxide or nitrous oxide into the atmosphere. Gaseous and water-borne nitrogen may be related to nitrogen fertilizer application rates in linear and nonlinear ways, which means application rates alone are not always enough to determine how much is lost to the environment (Broadbent and Rauschkolb 1977; Hoben et al. 2011; Linquist et al. 2012). Recent evidence suggests that the best indicator of potential nitrogen loss into the environment is the “surplus” nitrogen, which is the difference between the nitrogen applied as fertilizer and the nitrogen taken up by the crop (Van Groenigen et al. 2010). Therefore, both nitrogen application rate and nitrogen surplus, which is calculated after the crops are harvested, are important factors for predicting where nitrogen loss should be highest.

Fertilizer trucks transport liquid ammonia throughout the state. Adding inorganic nitrogen fertilizer to soil promotes high plant productivity and long-term soil fertility, but can also lead to excess nitrogen in the environment and environmental degradation.

Fertilizer trucks transport liquid ammonia throughout the state. Adding inorganic nitrogen fertilizer to soil promotes high plant productivity and long-term soil fertility, but can also lead to excess nitrogen in the environment and environmental degradation.

Sonja Brodt, Daniel Liptzin and Todd Rosenstock learn about fertilizer production in California from Ken Johnson of TSI Fertilizer Manufacturing in Dixon. Large upward trends in fertilizer sales in the last half of the twentieth century are evident throughout the developed world.

Sonja Brodt, Daniel Liptzin and Todd Rosenstock learn about fertilizer production in California from Ken Johnson of TSI Fertilizer Manufacturing in Dixon. Large upward trends in fertilizer sales in the last half of the twentieth century are evident throughout the developed world.

Nitrogen-fertilizer-use data

Data on nitrogen fertilizer use in California are scarce and fragmented. Typically, data are less available and more variable at finer spatial resolutions. The following identifies the primary sources of data available for statewide and county nitrogen use and nitrogen application rates by crop, and discusses some of the inherent limitations of these data sources.

Statewide nitrogen fertilizer use.

Fertilizer data are collected by the California Department of Food and Agriculture (CDFA) and reported at the state and county levels. Since fertilizer sales are only recorded when a licensed fertilizer dealer sells to an unlicensed buyer, these data provide a rough approximation of the total inorganic nitrogen applied statewide, assuming no stockpiling or interstate transfer of fertilizing materials (fig. 1). Annual data are available dating back to 1945. However, there are additional reasons to question the accuracy of these data. Perhaps the most obvious is the unexplainable 50% jump in sales between 2001 and 2002, the largest 1-year change since annual estimates began. And the reported sales remained abnormally high in the following 5 years (2003 to 2007). Because there is no explanation for this large jump in reported fertilizer sales statewide — neither its root cause nor an apparent accounting error — we have little confidence in the data reported since 2001.

County nitrogen fertilizer use.

While fertilizer sales data are reported to CDFA at the county level, the precision of these data is problematic. County fertilizer data portray a geographic distribution of sales unlikely to match actual use for most counties. This is due to the method of data collection, which neglects fertilizer transported from one county to another. For example, more than 20% of total statewide nitrogen sales were reported to have taken place in San Joaquin County. It is entirely possible that this value can be attributed to the large quantity of ammonia delivered to the Port of Stockton and redistributed from there. County-level sales data may be an appropriate proxy for nitrogen applications in counties where one does not suspect significant transport of nitrogen into or out of the county, but it is not possible to be certain with the current data collection system.

Nitrogen fertilizer use by crop.

There is neither a comprehensive source of information nor current estimates of average nitrogen applications by crop in California. The most complete source of data in California is a 1973 survey of approximately 120 UC experts and affiliates about nitrogen application rates on 45 commodities (Rauschkolb and Mikkelsen 1978). (The term “expert” in this article refers to UC employees — faculty, farm advisors and facility managers — but we acknowledge there are many other sources of expertise.) However, these rates are unlikely to be the same today due to changes in irrigation technology, tillage, cultivars and countless other management practices since the 1970s. While a few other expert estimates are available, they generally cover fewer crops than the 1973 survey (Miller and Smith 1976; Zhang et al. 2009)

Data direct from growers are largely unavailable. In a few instances, surveys have been conducted (Hartley and van Kessel 2003), though they sometimes omit asking for (Lopus et al. 2010) or reporting (Dillon et al. 1999) nitrogen application rates. The only systematic source of nitrogen application data based on grower surveys is the USDA Agricultural Chemical Use Program reports (USDA NASS 2010a). The USDA surveys growers for nitrogen fertilizer application rates for major crops on a rotating schedule, with an emphasis on field crops. As a result, surveys on nutrient use for each crop only occur intermittently — sometimes with significant time elapsing between information being gathered for certain crops. For example, almond was surveyed in 1999 and 2009. Though long-term trends may be detectable from such data, there is the distinct possibility that they may be obscured by year-to-year variability in data that is not quantified and therefore cannot be taken into account. Furthermore, some crops that contribute significantly to California's agricultural economy are not customarily surveyed in any state (such as fresh-market tomatoes), not surveyed in California (such as corn) or not surveyed for nutrient use (such as nursery and greenhouse plants).

A farmworker applies fertilizer to nursery crops in Winters in the Central Valley. At present, there is neither a comprehensive source of information nor current estimates of average nitrogen applications by crop in California.

A farmworker applies fertilizer to nursery crops in Winters in the Central Valley. At present, there is neither a comprehensive source of information nor current estimates of average nitrogen applications by crop in California.

Assessing crop nitrogen use

Developing new estimates of nitrogen use by crop is critical to informing the research, outreach and policy agenda on nitrogen fertilizer use. Surveys are resource intensive, and their design and scale may make it difficult to achieve a representative sample, especially in the diverse California agricultural landscape. In addition, the California Nitrogen Assessment had little success in an effort to survey UCCE employees about nitrogen use, and commodity boards about nitrogen research; the response rate was less than 7% and less than 15%, respectively. In place of a new survey, we developed and utilized a new approach to estimate an average nitrogen application rate by crop based on available data. The premise underlying this assessment was to smooth out some of the uncertainties and variation in these data by aggregating across sources. We compiled the available information from expert and grower sources into a database according to the methods described below.

For each crop, we first averaged the available expert data since 2000 and then averaged the grower data since 1999. Utilizing nitrogen estimates that date from 1999 or 2000 was necessary to increase the sample sizes, as a result of the limited number of expert responses available over the time period for each crop.

Expert data.

Expert opinions of nitrogen fertilizer use were taken from UC Agricultural and Resource Economics (ARE) Cost and Return Studies that have been conducted from 2000 to the present (UCD 2010). Studies of each crop were selected to represent variations in California's agricultural regions (such as the Imperial Valley versus the Salinas Valley) as well as the breadth of management practices (such as furrow versus drip irrigation). Compiling studies that span the geographic and production continuum was important because of the potential differences in nitrogen application with the various environmental conditions and production techniques.

Not all of the available studies were included in the database. Some studies were omitted because studies of the same crop often recycle the descriptions and estimates of nitrogen use until management practices change significantly and thus inclusion of every study would have skewed the estimate. An average of two studies were included for each crop, but the number of studies included ranged from one to five. Data were averaged to provide a representative value of nitrogen fertilizer use for each crop based on expert opinion.

Grower da

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Author notes

The authors thank C. Snyder, C. Garnache, S. Ogburn, C. Bishop, T. Harter, J. Dickey, five anonymous reviewers and the editors for their comments on earlier versions of the manuscript. The David and Lucile Packard Foundation supported this research as part of the California Nitrogen Assessment.

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