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The California Nitrogen Assessment
Challenges and Solutions for People, Agriculture, and the Environment
By Thomas P. Tomich, Sonja B. Brodt, Randy A. Dahlgren, Kate M. Scow
UNIVERSITY OF CALIFORNIA PRESSCopyright © 2016 The Regents of the University of California
All rights reserved.
Introducing the California Nitrogen Assessment
K. THOMAS, D. LIPTZIN, AND T. P. TOMICH
M. COLEY, R. DAHLGREN, B. HOULTON, K. SCOW, AND A. WHITE
What Is This Chapter About?
This chapter provides background information to understand the issues related to nitrogen use and management in California agriculture, presents an overview of the assessment approach and implementation, and outlines the goals of the California Nitrogen Assessment (CNA). Assessments emphasize legitimacy, credibility, and relevancy; the research and stakeholder engagement process is as important as the results and outputs produced. An assessment looks at existing knowledge and reduces complexity by synthesizing what is known and widely accepted and differentiating it from that which is unknown or not agreed upon. This chapter describes how the CNA engaged with stakeholders to establish research priorities and outputs that meet the needs of a wide range of perspectives, including farmers, government, and environmental and health organizations.
Nitrogen is necessary to sustain all life and is often the primary nutrient limiting productivity. Nitrogen is also a critical nutrient required to sustain agriculture in California and the global food supply. As a component of synthetic and organic fertilizer, nitrogen is critical to plant growth and the expansion of crop production. With estimates for worldwide population growth and rising per capita incomes and agricultural production, the demand for N is likely to intensify in coming years to meet growing global demands for food. This is also true for California; the state produces more than half of the fruits, nuts, and vegetables grown in the United States and 21% of the dairy commodities.
Since 1960, the amount of nitrogen used in agriculture has doubled on the planet, as has food production, with excess nitrogen being released to the environment. The majority of nitrogen imports to California are in the forms of fertilizer, imported animal feed, fossil fuel combustion, and biological nitrogen fixation. While much of that nitrogen contributes to productive agriculture, excess nitrogen from those sources contributes to surface water and groundwater contamination and air pollutants such as ammonia and nitrous oxide, a potent greenhouse gas. Striking the right balance between the benefits nitrogen provides to our food supply and the costs it can have on our health and environment demands a critical look at the trade-offs involved.
This is the first comprehensive accounting of nitrogen flows, practices, and policies for California agriculture at the statewide level. The assessment identifies key drivers of nitrogen use decisions and examines how these drivers influence the statewide mass balance of nitrogen — how much enters the state through new sources and, ultimately, the multiple ways these compounds enter and affect the environment. The assessment tracks nitrogen's impacts on environmental health and human well-being, and examines technological and policy options to minimize nitrogen leakage while sustaining the vitality of agriculture.
Assessments reduce complexity through the synthesis and integration of a large body of existing information, providing a valuable method for focusing efforts and systematically calling out uncertainty. Assessments are designed to be legitimate in the eyes of key stakeholders, relevant to decision-makers' needs, and scientifically credible.
In an assessment, understanding what is not known is just as important as assessing what is known. We evaluated the quality of the data and note when results are based on very reliable information or data that are less reliable (e.g., due to gaps in information or disagreement in the literature). We employed "reserved wording" to describe uncertainty. Questions lacking good data were highlighted where more research and record keeping are needed. The online Appendices describe the sources and approaches used in the assessment, evaluate the level of uncertainty of this information, and highlight where information gaps exist.
The assessment used multiple avenues to engage with more than 350 stakeholders across 50 organizations. Through a participatory research design, stakeholders identified more than 100 nitrogen-related questions that were used to direct research and synthesize priorities, provided data and examples of on-the-ground practices and management options, and developed four "scenarios" for the future of nitrogen in California agriculture.
The assessment's findings underwent a multistage peer review process. This included consecutive reviews by (1) over 50 scientific experts, (2) the Stakeholder Advisory Committee (SAC), and (3) an open public comment period. A group of 10 review editors (9 chapter review editors and 1 overall review editor) ensured all comments received appropriate attention and responses from authors.
1.0. The California Nitrogen Assessment
Since 1960, the amount of nitrogen (N) used in agriculture has doubled on the planet, as has food production (Galloway et al., 2004; MA, 2005a; Vitousek et al., 1997). With expectations for continued growth in the human population and per capita consumption, and intensifying utilization of natural resources such as freshwater, striking a balance between the benefits and costs of human nitrogen use will require significant trade-offs and a critical investigation of technological and policy options to minimize nitrogen losses to the environment while sustaining the vitality of agriculture.
Tracking trends in nitrogen flows, understanding key inputs and outputs, evaluating management options, and devising new regulatory structures that are sensitive to cross-system interactions will be essential to ensuring this balance is attained. Despite increasing awareness of the importance of these trade-offs, a lack of cohesive knowledge that gives a big-picture view of California's nitrogen system still hampers effective decision-making from policy options to field-level practices.
The CNA is a comprehensive evaluation of existing knowledge about nitrogen science, practice, and policy in the state. Broadly, the goals of the assessment were to:
1. Gain a comprehensive view of N flows in California, with emphasis on agriculture's roles.
2. Provide useful insights for stakeholders into the balance between the benefits of agricultural nitrogen and the effects of excess nitrogen in the environment.
3. Compare options, including practices and policies, for improving the management of nitrogen and mitigating the negative impacts of excess nitrogen in the environment.
4. Move beyond "academic business-as-usual" to more effectively link science with action and to produce information that informs both policy and field-level practices.
The assessment is targeted towards a broad audience with diverse and often conflicting perspectives. Throughout the assessment, the level of detail varies, with emphasis placed on those issues and topics identified as being of greatest interest to our stakeholders. We do not offer recommendations, but rather endeavor to synthesize the current scientific understanding, point out gaps in knowledge, and present a balanced understanding of the issues and options for moving forward on key concerns.
1.1. Understanding Nitrogen and Its Global Implications
Dinitrogen gas (N2) comprises 78% of Earth's atmosphere. However, this form of N is largely unreactive. While all biological species require nitrogen for growth and development, the transformation of N into reactive nitrogen forms (Nr) is required to make it biologically available (Box 1.1). The biological and physical transformations that comprise the global nitrogen cycle can be categorized into three groups, all of which can result from biotic and abiotic processes: (1) the fixation of atmospheric N2, (2) the transformations among forms of solid, dissolved, and gaseous forms of Nr in the air, land, and water, and (3) the production of N2, largely as a result of denitrification (Box 1.2).
Nitrogen gas is converted to Nr naturally in ecosystems through the process of nitrogen fixation. A small amount of nitrogen is fixed abiotically by lightning in the atmosphere and during vegetation fires, but the majority of natural nitrogen fixation is a result of biological activity (Chapter 4). Once Nr has been fixed in an ecosystem, the N can be assimilated by organisms and converted into living biomass. When the organisms die, the various organic forms of N are transformed back into ammonia (ammonification) and nitrate (nitrification) by specialized groups of microorganisms. Other microorganisms denitrify the nitrate to N2 with nitrogen oxide gases (NO and N2O) as intermediate products of the series of enzymatic reactions. In addition to denitrification, ecosystems can also lose N during leaching or surface water runoff containing dissolved forms of organic N and nitrate (NO3-). While most ecosystems tend to retain most of their N, some N is transported between ecosystems in groundwater and surface water and in the atmosphere. Superimposed on the natural flows of N, humans created several new flows of Nr and altered the magnitude of others. Fossil fuel combustion, synthetic fertilizer use, increased concentration of livestock, sewage dis charge, cultivation of legumes, irrigation, frequency and magnitude of wildfires, and groundwater pumping all alter amount, distribution, and flows of Nr in the environment (figure 1.1). These flows have both positive and negative consequences for ecosystem health and human well-being. The N cascade conceptual model illustrates that a single molecule of N can have multiple positive and negative effects from the time that it is fixed to the time it is returned to the atmosphere as N2. Depending on the particular fate of the N that escapes its intended purpose, the costs to ecosystems and human health will vary (see Chapter 5; Birch et al. 2011; Compton et al., 2011).
Human activity has dramatically reshaped the global N cycle in terms of the magnitude of the production of Nr, availability of N in ecosystems, and rates of N cycling. Currently, humans create more Nr than do all of the planet's natural processes combined (Rockstrom et al., 2009), and it has been suggested that the anthropogenic changes to the N cycle have already crossed a "planetary boundary," or threshold for stability of Earth system processes (Rockstrom et al., 2009). However, the timescale of these changes is small relative to their magnitude. Historically, farmers in the United States relied on planting legumes, local recycling of human and animal wastes, or import of inorganic nitrates to replace N loss in soils. Limited by the amount of naturally occurring Nr available for food production, many realized that the natural rate of Nr replenishment in soils would not match the rate of global population growth. This interest in sourcing additional Nr for food production led to the invention of the Haber–Bosch process in 1913. This technology gave humans the ability to produce Nr on an industrial scale and removed the need to rely solely on naturally occurring Nr for food production (Galloway et al., 2004).
The Haber–Bosch process is one of three pathways by which humans produce Nr. Additionally, fossil fuel combustion creates gaseous forms of Nr because there is N in the fuel (in the case of coal) and because the combustion process can lead to reactions between N2 and oxygen in the atmosphere. Finally, humans plant vast acreage of leguminous crops for several reasons — many take advantage of legumes' N-fixing properties to enrich soil. Leguminous crops also are harvested widely for both human and animal consumption.
Roughly half the human population on earth is supported by Haber–Bosch-produced nitrogen fertilizer (Davidson et al., 2012), making this one of the greatest innovations ever. There has been an exponential increase in the use of the Haber–Bosch process since the 1940s, revolutionizing agriculture and allowing for abundant fertilizer supplies to support growing human populations. Davidson et al. (2012) found that for 2008, 56% of the major sources of natural and anthropogenic nitrogen inputs to the United States came from agriculture (synthetic N fertilizer [Galloway et al., 2008] and crop biological N fixation). Globally, anthropogenic sources of newly fixed nitrogen now exceed natural terrestrial sources by at least 50% (Galloway et al., 2008). With continued growth in human population forecasted at 9.6 billion people by 2050 (Gerland et al., 2014), the implications for increased food production and nitrogen use, as well as resulting effects on the environment and human well-being, are substantial.
Nitrogen's effect on the broader environment also includes unintended consequences for human and ecosystem health. Only 55% of the intentionally fixed N in the United States makes its way into an intended product (i.e., food, fiber, energy, industry) (Houlton et al., 2013). The N efficiency of the most common grain crops is typically less than 50% (Cassman et al., 2002), which allows the remaining N to escape from the soil as nitrate or in various gaseous forms. This escaped N can alter ecosystem services and damage human health: eutrophication and anoxic "dead zones" in surface waters and coastal areas; harmful algae blooms; high fluxes of nitrous oxide, a potent greenhouse gas; loss of plant biodiversity; enhancing competition from invasive species; and nitrate contamination of drinking water (see Chapter 5). While nitrogen interacts with human health and well-being in a variety of ways, the trade-offs involved in agricultural nitrogen use highlight many of the key sustainability issues related to the challenges of the twenty-first century: global climate change, depletion of fossil fuels, and mounting pressure on land, air, and water resources from growing human population and rising incomes. For example, work in Europe estimates that the environmental and human health costs of excess N now exceed the annual benefits of N use for crop production (Sutton et al., 2011b). Houlton et al. (2013) estimate that agricultural N spillover results in air-quality damages in the United States that exceed $16 billion each year. Recent studies suggest that an increase in public and private funding on the order of $17–34 million per year over many decades will be needed to implement required nitrate mitigation projects for water systems in the Tulare Lake Basin and Salinas Valley (Honeycutt et al., 2012).
1.2. Why California?
California provides an excellent location to study nitrogen because of its diversity. Its ecosystems range from deserts to alpine tundra. Its population is concentrated in large metropolitan areas, but the majority of the state is rural. California agriculture has both a large livestock and crop component. Further, California is the source of the majority of production for many fruit, nut, and vegetable crops for the United States, and thus carries a lot of the nitrogen burden for many non-Californians. In addition, California is actively dealing with many of the challenges confronting agriculture throughout the United States and internationally: population growth and urbanization (Landis and Reilly, 2004; Williams et al., 2005); changing demographics in rural communities (Bradshaw and Muller, 1998); flood control and water demand for irrigation (Tanaka and Sato, 2005); maintaining air, soil, and water quality; coping with climate change (Cavagnaro et al., 2006; Hayhoe et al., 2004); responding to domestic and international markets (AIC, 2006); and facing increasing regulation. Thus, California exemplifies the biophysical and social context in which this assessment will be both locally relevant and nationally and internationally significant — it encompasses extreme diversity in its agricultural production systems and climatic regions and landscapes, is subject to enormous population pressures and complex social problems, and has pioneered innovative environmental policies.
Excerpted from The California Nitrogen Assessment by Thomas P. Tomich, Sonja B. Brodt, Randy A. Dahlgren, Kate M. Scow. Copyright © 2016 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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