Nitrogen, food, and environment

Nitrogen (N) fertilizer is one of the greatest inventions in human history. Smil (2001) estimates that 40% of the current human population would not be alive if the process for turning atmospheric N gas into fertilizer had not been invented. Around half of current global food production can be attributed to the use of N fertilizer (Smil, 2011). Along with its tremendous impact on food production, N fertilizer has potent effects when it escapes to water and air from the agricultural systems where it was applied. For example, plants in coastal waters are, like plants on land, usually N-limited in their growth rate (Vitousek and Howarth, 1991). Coastal waters receiving drainage from intensive agricultural regions (e.g., Gulf of Mexico, Yellow Sea, Baltic Sea) have excessive plant growth that can sometimes be severe enough to lead to dissolved oxygen-depleted as low as to harm marine life (Burkart and James, 1999; Rabalais et al., 2002; Guo et al., 2020; Conley et al., 2011). Nitrogen fertilizer is the main agricultural contributor to global warming potential in the U.S., both through CO₂ produced during its manufacture and N₂O released after its application to farm fields (USEPA, 2016; Davidson, 2009). Although CO₂ from transport and other industries have a much larger global warming impact, the most important reduction advances in agriculture are likely to be from N fertilizer management. In addition, the N₂O from agriculture is now the largest human-driven source of compounds that deplete stratospheric ozone (Ravishankara et al., 2009). Thus, N fertilizer is essential to the U.S. and world food supply but has substantial negative impacts on the environment. There is currently great potential to maintain or increase N benefits in food production while reducing the negative environmental consequences. That potential is the main interest of this committee.

Improving nitrogen use in the U.S.

The U.S. was one of the earliest widespread users of N fertilizer following the conversion of munitions (high in N) factories to fertilizer factories after World War II. In the U.S., more N fertilizer is applied to corn than to all other crops combined; however, it is also used on nearly all non-legume crops. A large proportion of the N fertilizer used in the U.S. is for crops fed to animals (especially corn), and most of that N is excreted by those animals as manure. Efficient management of manure N is also important, both to support crop production and in terms of negative consequences in the environment. Increasing N use efficiency (NUE) from fertilizer and manure has been a key driver increasing crop production efficiency in the U.S. (Houlton et al., 2012). However, the manure N source is often not accounted for and applying fertilizer N to crops in amounts than is needed reduces profitability and increases environmental impact and costs (Hong et al., 2007; Shcherbak et al., 2014).

Considering the availability of the various N sources; “How much N fertilizer is needed?”. This turns out to be a difficult question, and one that is of great interest to this research group. Corn production in the U.S. today relies on large inputs of N fertilizer to meet N demand by the crop (Simons et al., 2014), but also soil N supply. Nitrogen mineralization from soil organic matter (SOM), of which manure can greatly increase, provides a substantial portion of corn’s total N need in many areas (Lynch, 2013), and is supplemented as needed with inorganic N fertilizer, manures, and co-products (Yan et al., 2019). Although more total N is needed by corn as yield increases (Ciampitti and Vyn, 2011), more fertilizer N may not be needed (Shapiro and Wortmann, 2006; Scharf et al., 2006b). The amount of N supplied by the tremendous reserves in soil organic matter can vary widely—this is a purely biological process that is very sensitive to both inorganic and organic inputs, soil temperature, moisture, and oxygen (Sierra, 1997; Fernández et al., 2017). The release of N from organic compounds in manure is similarly complex. Complicating the matter further is the impact of temperature, moisture, and oxygen on N loss via leaching, denitrification, and ammonia volatilization (Scharf and Alley, 1988). The result is that even the optimal N fertilization rate can vary widely from place to place even in a single field (Mamo et al., 2003; Scharf et al., 2005). There is still much to learn and predict both spatially and temporally and spatially about these fundamental processes that control plant N availability and, thus, optimal N fertilizer management.

A key component to improved fertilizer N efficiency and reduced environmental impact is a better understanding and quantification of N mineralization from all sources coupled to crop uptake. Fertilizer N efficiency is normally based on N uptake/yield of unfertilized (check) plots. An important, possibly incorrect, assumption in this approach is that release of organic soil N is unaffected by N fertilization (Jenkinson et al., 1985; Mahal et al., 2019). Not accounting for N fertilizer’s impact on soil organic N release can lead to over-fertilization and increased N loss. While it is commonly accepted that N fertilizer influences soil N mineralization by “priming” processes (Jenkinson et al., 1985; Mahal et al., 2019) little has been done to quantify these effects and incorporate this knowledge into our fertilizer N recommendations or calculations of NUE. Thus, quantifying uptake of fertilizer N by the crop and associated changes in soil N mineralization are paramount to developing sound management approaches that maintain high crop yields while minimizing N losses. Thus, methods to predict the best N fertilizer rate that can account for spatially and temporally variable N loss and release of organic N from soil and manure are a key step toward improved N management. This research group has collected data and published three papers addressing this issue (Scharf et al., 2006a; Laboski et al., 2008; McDaniel et al., 2020). Each of these papers benefited from the regional nature of the group, producing conclusions that were much more robust than can be achieved by a few investigators in limited geographies. The relatively recent addition of microbiological and modeling expertise to the committee adds new approaches that can be used in pursuit of this goal.

However, improved understanding of soil N processes only addresses one side of the equation in the larger picture of environmental sustainability under a changing climate. With the adoption of new cropping systems, including more diverse crops grown in rotation with corn as well as the use of cover crops and integrated livestock systems, a more thorough understanding is needed of N cycle processes to maximize NUE, minimize environmental impact, and protect soil and water resources.

Current knowledge of N-cycle processes and their management

Although the biological underpinnings of most N cycle processes are well characterized, less is known about how these processes interact spatially and temporally within the soil to control fate of N. Because soils are spatially heterogeneous and include biological ‘hot spots’ such as the rhizosphere and decomposing plant residues, the interaction of applied N with SOM pools as mediated by the soil microbiome is rarely considered where sufficient labile N is added as fertilizer. This can lead producers to over-apply N fertilizer. Adequate N supply is required to achieve producer acceptable economic outcomes. Because the relationship between yield and N uptake is usually tightly conserved, achieving ever-higher yields often depends on greater N uptake, which in turn requires greater amounts of available N. However, overapplication of N fertilizer to ensure consistent crop yields has led to decreased NUE (Tilman et al., 2002). Excessive N fertilizer use threatens environmental quality and human health. Emission of greenhouse gases, specifically N₂O, is also attributed to inefficient use of N fertilizers. Fertilizer N also typically accounts for approximately 50% of the fossil energy input into intensively managed crops like corn. Environmental degradation and rising energy costs have become major impediments to both the profitability and sustainability of intensively managed cropping systems.

Climate change science suggests a slight increase in overall precipitation in the U.S. Corn Belt, with a significant increase in intensity and frequency of large rainfall events in the spring/early summer corn growing season (Kellner and Niyogi, 2015). This pattern is consistent with weather events over the past several years. These changes in precipitation patterns will drive increased loss of both mineralized and fertilizer N from soils, via denitrification and leaching (Bowles et al., 2018). Loss directly from soils via denitrification, and in-stream denitrification of nitrate leached to surface waters, will also increase emissions of N₂O. Therefore, one potential indirect consequence of precipitation changes and N loss could be increased N-based greenhouse gas production. Mitigation of these losses requires improved understanding of N release from soil organic N pools to provide better N rate recommendations which can account for variation in N mineralization between years; improved N management practices to reduce N loss and better synchronize soil N availability with crop N demand; and an increase in NUE. Implementation of more diverse crop rotations, cover crops, application of biosolids (manures, biochars etc.) to address climate variability will necessitate updated management practices including N recommendations, timing and placement of fertilizer N, selection of N source and additives to reduced nitrate formation and loss to better synchronize N supply with crop demand. Improved N management practices may also include using crop sensors or other decision tools to guide in-season application, and fertilizer products and additives that reduce loss; all of these practices will aid producers in adapting to climate change, provide environmental benefits, improve crop yields, and give a better economic return to N.

Regional project goals

The long-term goals of this regional project are to better understand how the interactions of soil, weather, climate, and cropping system influence N availability and optimal N management from all N containing inputs. Additionally, we plan to develop tools that help farmers translate this understanding into practice. Over the next five years, we aim to develop one key new piece of knowledge and/or one new management tool that moves N management toward reduced environmental impact while maintaining production benefits. The ultimate success of the project – reduced N loss, efficient N fertilizer use and continued increase in crop yield - lies in grower adoption of N recommendations and management practices developed. This will require a thorough understanding of how practices within a cropping system impact N availability and yield, understanding the producer and adviser decision making process, and development/enhancement of decision tools that will inform N fertilization decisions. Thus, a strong, transformative extension education/outreach program targeted to producers and crop advisors (in conjunction with extension educators, local/state/federal regulatory personnel, and policymakers), is central to this project.