NC1195: Enhancing nitrogen utilization in corn based cropping systems to increase yield, improve profitability and minimize environmental impacts (NC1032/218)
Statement of Issues and Justification
Issues: Designing efficient, economically sound and environmentally friendly corn (Zea mays L.) based cropping systems is a prerequisite to remaining competitive in today s global agricultural market place. The dilemma facing US corn producers and policy makers today is that the steady increase in corn yield over the past 50 years is partially attributed to the increasing use of N fertilizer, yet N fertilization comes with both a steep input cost and potentially high environmental cost. This is particularly true when more N is applied than the crop can effectively use, and adverse environmental consequences such as reduced ground and surface water quality, an increase in hypoxic zones off the mouth of our major rivers and increased emissions of powerful greenhouse gases (GHG) such as N2O can occur. Unfortunately, after nearly a century of research to develop precise N fertilizer recommendations and efficient N management systems, fertilizer N use efficiency (NUE) worldwide is still significantly less than 50%.The relationship between corn yield and N uptake by the plant is strongly correlated. As yield potential increases the plant requires more N to produce the vegetation and grain associated with higher yield. However, while the relation between increasing corn yield and N uptake is tightly correlated, the relationship is not linear, and the relationship between yield and fertilizer N need is not nearly as well correlated. This is due in part to the varying capacity of soils to supply N to a crop each year. Particularly the rate at which N is mineralized from organic materials in the soil, and changes in potential N loss during the cropping season, a function of soil properties and crop management systems, as impacted by climate.
Though the individual processes which compose the N cycle in soils are well understood, less is known about how these processes, cropping systems, climate and N fertilization practices all interact to impact NUE. The interaction of sources of available N with soil organic matter and crop residue for example is rarely considered when making N fertilizer rate recommendations. However, N fertilization is known to lead to an increase in the mineralization of soil organic N, which can result in producers over-applying N fertilizer. But the same microbial processes responsible for mineralization can also result in immobilization or sequestering of N in soils, reducing NUE and crop yield in the year of application, and increasing mineralization, and potential N loss, at some latter time.
Rainfall and temperature are two important factors controlling most components of N cycling in soils. Thus a coordinated regional research effort which can look at gradients in temperature, precipitation and soil organic matter content across the Midwest, Mid-South and Great Plains and how these factors impact processes controlling both N mineralization and N losses from soils, is much more likely to arrive at a deeper understanding of N cycling and develop more efficient N management practices and increase NUE, than a series of independent, individual investigator projects conducted in a restricted geographical setting.
The long-term general goals of this regional project are to better understand how the interactions of soil, climate, cropping system and N fertilization practices impact NUE, and develop better N rate and management recommendations for growers. If these recommendations are utilized, growers will more efficiently utilize N fertilizers to meet the needs of increasing crop yield, while minimizing any potentially adverse effects on the environment. A key specific goal of this regional project is to use this new knowledge obtained to reduce the N fertilizer application to corn in the US over the next decade. Justification: Corn production in the US today uses large amounts of N fertilizers due to the crops large N nutritional requirement. Soil N mineralization provides a substantial portion of corns total N need in most areas, and is supplemented as needed with inorganic fertilizer and organic manures and co-products. A key component to improved fertilizer N efficiency and reduced environmental impact is a better understanding and quantification of mineralized soil N release. Fertilizer N efficiency is normally calculated as (N uptake of fertilized plots-N uptake of unfertilized plots)/fertilizer N applied. An important, possibly incorrect, assumption in this approach is that release of organic soil N is unaffected by N fertilization. While it is commonly accepted that N fertilizer influences soil N mineralization by priming soil carbon (C) processes, little has been done to quantify these effects and incorporate this knowledge into our N management recommendations or calculations of fertilizer N use efficiency. Not accounting for N fertilizers impact on soil N release can lead to over-fertilization and increased N loss. 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 corn yields while minimizing N losses.
Though individual N cycle processes are well characterized, less is known about factors controlling Ns fate when these processes interact. The interaction of sources of available N with SOM pools is rarely considered when sufficient labile N is added as fertilizer. Most estimates of N mineralization are done in unfertilized conditions and lead to underestimates of mineralization and can lead to producers over-applying N fertilizer. This project plans to directly address the issue of N mineralization and how this impacts N fertilizer need and NUE in intensively managed systems.
Other processes leading to N loss can also impact NUE. Leaching, volatilization and denitrification are all important processes which are impacted by climate and soil water relations. A standard method used by many farmers to minimize the impact of these N loss mechanisms on yield, is add a little extra N as insurance. However, excessive N fertilizer use threatens environmental quality and human health. Soil nitrate moving to the Mississippi River and Gulf of Mexico is blamed for the increase in the hypoxic or dead zone noted off the mouth of the Mississippi River in summer. Emission of greenhouse gases, specifically N2O, through denitrification, is attributed to inefficient use of N fertilizers. Fertilizer N use also accounts for approximately 50% of the fossil energy input into intensively managed crops like corn. Thus environmental degradation and rising energy costs have become major impediments to both the profitability and sustainability of intensively managed corn systems in the US.
Climate change science suggests a slight increase in overall precipitation in the US Corn Belt, with a significant increase in intensity and frequency of large rainfall events in the spring/early summer corn growing season will result from continued climate change. This pattern is consistent with recently observed weather events. If these predicted changes in precipitation patterns were to continue, it would lead to increased loss of both mineralized and fertilizer N from soils, via denitrification and leaching. Loss directly from soils via denitrification, and in-stream denitrification of NO3- leached to surface waters, will increase emission of N2O. Therefore, one potential indirect consequence of climate change driven precipitation changes and N loss could be increased N-based greenhouse gas production.
Mitigation of the N flux from corn fields requires improved understanding of N release from soil organic N pools and the ability to adjust N rate recommendations from year to year to account for variation in N mineralization between years; improved N management practices to reduce fertilizer N loss and better synchronize soil and fertilizer N availability with corns N demand; and an increase in corn N use efficiency. Improved management practices include improved N rate recommendation systems that account for variations in N mineralization, and improved use of timing and placement of fertilizer N, selection of N source and additives that slow NO3- formation and/or ammonia volatilization, to reduce N loss and to better synchronize N supply with N demand. Improved N management practices may also include using crop sensors or other decision tools to guide in-season application, allowing a better prediction of N needs. All of these practices will aid corn producers in adapting to climate change, provide environmental benefits, improve corn yield, and give a better economic return to N.
The ultimate success of the project - reduced N loss, more efficient N fertilizer use and continued increase in corn yield - lies in the N recommendations and N management practices developed being adopted and utilized by corn growers across the corn producing regions of the U.S. This will require a thorough understanding of how these practices impact N availability and yield, understanding of the producer and adviser decision making process, and development of decision tools that will help people make good N fertilization decisions. Thus, a strong, extension education/outreach program targeted to producers and crop advisors (in addition to extension educators, local/state/federal regulatory personnel, and policy makers), is embedded in this project, since many of the current project team members have joint research and extension appointments.
The long-range prosperity of the U.S. agricultural and food system is increasingly tied to concerns over environmental impact including climate change. Unused N fertilizer represents a reduction in profitability, can cause environmental degradation and can impact global climate change. Over-application of fertilizer N is often the result of difficulty in predicting the amount of plant available N supplied from mineralization of soil organic N, or over estimating the potential for N loss. This project will provide information to more accurately determine the contribution of organic N to corns N needs, and resulting fertilizer requirement. In addition, decision making tools will assist growers in determining how best that fertilizer can be applied to result in high utilization by the target crop, and minimal loss to the environment. Improved N management across the U.S. Corn Belt will make important contributions to reduced N2O emissions, reduced NO3- movement to surface and groundwater, and will still result in high levels of corn productivity.
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