NC1195: Enhancing nitrogen utilization in corn based cropping systems to increase yield, improve profitability and minimize environmental impacts

(Multistate Research Project)

Status: Active

NC1195: Enhancing nitrogen utilization in corn based cropping systems to increase yield, improve profitability and minimize environmental impacts

Duration: 10/01/2016 to 09/30/2021

Administrative Advisor(s):


NIFA Reps:


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 realized over the past 50 years, and needed in the future, can be partially attributed to the intensive and increasing use of N fertilizer (Houlton et al., 2012). Yet, N fertilization comes with both a steep input cost and, particularly when more N is applied than the crop can effectively use, a potentially high environmental cost, such as reduced water quality (Verhoeven et al., 2006), an increase in hypoxic zones off the mouth of our major rivers (Donner and Scavia, 2007), and increased emissions of powerful greenhouse gases (GHG, i.e. nitrous oxide - N2O, Davidson, 2009; Shcherbak et al., 2014). 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 less than 50%.


The relationship between corn yield and N uptake by the plant is strongly correlated (Ciampitti and Vyn, 2011). 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 between yield and fertilizer N need is not (Shapiro and Wortmann, 2006). This is due, in large part, to the varying supply of N provided by the soil to a crop each year. More specifically, this is the rate at which N is mineralized from soil organic matter in a given year, but also widely different potential for N losses from soils during the cropping season (a function of soil properties, crop management systems, as well as local climate).


Though the individual processes which compose the N cycle in soils are now well understood, much less is known about how these processes, cropping systems, climate and N fertilization practices all interact to impact NUE. Nor is there much known on how these N processes interact with carbon and other nutrient cycles. 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, fertilization is known to lead to an increase in the N mineralization (Jenkinson and others 1985), which can result in producers over-applying N fertilizer. But microbial processes can also result in immobilization or sequestering of N in soils (Booth et al., 2005), reducing NUE and crop yield in the year of application, but increasing mineralization at some latter time (otherwise known as a lag effect).


Rainfall and temperature are two important factors controlling most components of N cycling in soils (Brzostek et al., 2012) Thus a coordinated regional research effort which can look at gradients in temperature, precipitation and soil organic matter content across the Cornbelt and how they 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 the same geographical setting. Also there is a large gap in our understanding of the spatial and temporal variability of the soil processes that make N more or less available, preventing progress on predicting field-scale N requirements for corn.


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 determine whether new knowledge and management strategies generated by this group and others, and communicated to stakeholders, can reduce N fertilizer application to corn in the US by 5% over the next five years.


Justification
Corn production in the US today uses large amounts of N fertilizers due to the crop’s large N nutritional requirement (Simons et al., 2014). Soil N mineralization provides a substantial portion of corn’s total N need in many areas (Lynch, 2013), 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 N mineralization. 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 and others 1985, Castellano personal communication). Not accounting for N fertilizer’s impact on soil 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 and others 1985, Castellano personal communication) little has been done to quantify these effects and incorporate this knowledge into our N management recommendations or calculations of fertilizer N use efficiency. 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 N cycle processes are well characterized, less is known about factors controlling N’s fate when these processes interact. The interaction of sources of available N with SOM pools is rarely considered where sufficient labile N is added as fertilizer. This often leads producers to over-apply N fertilizer. This proposal directly addresses the issue of N use efficiency in intensively managed systems. Adequate N supply is required to achieve economic, producer acceptable, yields. Since 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. Fertilizer N has contributed greatly to yield maintenance and increase, in the face of SOM decline around the world. However, overuse of N fertilizer to ensure consistent crop yields has led to decreased crop system N use efficiency (Tilman and others 2002). Excessive N fertilizer use threatens environmental quality and human health. Emission of greenhouse gases, specifically N2O, is 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 US 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 five years. These changes in precipitation patterns will drive 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 also increase emission of N2O. Therefore, one potential indirect consequence of climate change driven precipitation changes and N loss could be increased N-based GHG production.


Mitigation of the N flux from corn fields 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 corn’s N demand; and an increase corn N use efficiency. Improved management practices include N rate recommendation systems, timing and placement of fertilizer N, selection of N source and additives that slow NO3- formation to reduce loss and 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 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, efficient N fertilizer use and continued increase in corn yield - lies in the N recommendations and N management practices developed being adopted and utilized 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 the producer and adviser decision making process, and development of decision tools that will help people make good N fertilization decisions. Thus, a strong, transformative 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.


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. This project will provide information to more accurately determine the contribution of organic N to corn’s 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.

Related, Current and Previous Work

Here we highlight some of the work this multistate committee has been working on. All of the following activities are synergistic. For example, progress in research on Soil N dynamics would help to inform Decision tools. A search of the NIMSS website http://www.nimss.org/ using keywords corn and fertilizer retrieved 325 projects, of the active projects twelve (NCERA_Temp180, NCERA13, WERA_Temp1016, NCERA59, WERA103, NCCC211, NEERA1402, NC1178, NC1182, SERA46, W3147, NCERA103) had some relevance to this project, but did not appear to duplicate our efforts. Consultation with specific groups may lead to future joint meetings, symposia or other outputs where knowledge can be shared.


Soil N dynamics


Optimal N fertilizer rate is known to vary widely across landscapes (Mamo et. al., 2003; Scharf et al., 2005) due to differences in soil N contribution to the crop. Microbial processes largely dictate N transformations in soil and the availability of soil N to crops. Thus, it appears that microbial processes and ecosystems vary widely across landscapes in ways that are poorly understood, but strongly affect how best to manage fertilizer. Our goal is to contribute to a better understanding of the fundamental processes controlling N dynamics in soil, especially mineralization and immobilization, in order to reduce N fertilizer inputs while maintaining yield and protecting the environment. Currently, several members of the committee (Fernández, Laboski, Sawyer, Drijber, and McDaniel) are working on a project to quantify the mineralization potential of 32 different soils over a wide range in climatic conditions across eight Midwestern states from Missouri to North Dakota and from Indiana to Nebraska. We are highly interested in soils that show no yield response to N fertilizer, either transiently or ongoing. Additionally, an understanding of the microbiology behind this would be desirable, and we have an opportunity to address this with committee member Drijber. Presentation and integration of this data across states has led to discussion of potential manuscripts and grant opportunities at our annual meeting.


Several committee members are making contributions to our understanding of N2O fluxes under different management scenarios (Mitchell et al., 2013; Fernández et al., 2015a; Iqbal et al, 2015; Sutradhar et al., 2015; Zhu-Barker et al., 2015; McDaniel et al. in revision). Although nitrous oxide is a minor player in overall soil N dynamics, it is important due to its contribution to atmospheric heat-trapping. Two committee members have promising preliminary data showing that nitrous oxide flux is reduced when nitrogen fertilizer is applied later in the season. These observations formed the basis of a proposal to NIFA from four committee members to further explore this relationship.


Most of the processes that control N form and flux in soil are mediated by microorganisms: mineralization-immobilization, nitrification and denitrification, the latter an important contributor to greenhouse N2O emissions. Committee member Drijber has made several recent contributions to our understanding of how arbuscular mycorrhizal fungi influence and are influenced by plant N uptake and soil N dynamics (Duan et al., 2015; Tian et al., 2013; Jeske, 2012 ). This has important implications to NUE as well as C sequestration, as fungal biomass is a significant contributor to the soil carbon pool and aggregate stabilization. Microbial N transformations are greatly influenced by addition (or removal) of organic source materials (Johnson et al., 2012; Lindsey et al., 2013; Pantoja et al., 2015) and by management- or weather-related modification of the soil environment (Pantoja et al., 2015; Roberts et al., 2015; Toosi et al., 2014, McDaniel and Grandy in revision). These references highlight recent contributions from committee members and have been the subject of much discussion at our annual meetings.


Nitrogen use datasets


Committee members have access to a large dataset generated in 1995-98 by previous NC218 committee members. The dataset includes a total of 70+ sites cropped continuously to maize or in rotation, spanning seven states with variables ranging from basic soil properties and fertility to yield and soil N pools. A second dataset with grain yields by N fertilizer rate from over 1,300 data points across several Midwestern states has been partially analyzed. Plans are to further explore this data using multivariate and regression techniques, then summarize for publication (McDaniel, Drijber & others). These data have been used previously by committee members to address practical soil testing questions related to NUE (Scharf et al., 2006; Laboski et al., 2008).


In addition to these existing datasets, many committee members have additional datasets that could contribute to committee objectives. One strong interest that has come up in the last few meetings is to use these data to evaluate the effects of different agronomic practices on N use efficiency. The committee has plans to create a template that will allow individual members to enter their data; these data can then be combined across all members. Currently the resources are not available within the group to harness the power of searchable databases. Partnering with existing databases, for example the ARS GRACEnet, REAP and Stewards database, is an option to explore if funding becomes available. These databases will be consulted to aid in template development for sharing data among our group.


Decision tools


Ultimately, better N management will require better decision tools. We know that managing N fertilizer the same in all fields, or even all parts of a field, is not the right thing to do. It remains difficult to predict the right thing to do. As a result, few farmers try to vary N rate from field to field or within fields. Many in the group have a strong interest in this objective. It is the historical strength of previous NC nitrogen committees. There is great potential to both leverage existing projects and develop new proposals to evaluate new decision tools (or potential improvements in old ones).


An earlier NC nitrogen research committee found that corn leaf color measured with a hand-held meter was a good predictor of optimal N fertilizer rate, while many soil tests performed poorly (Scharf et al., 2006). Follow up work from the committee found that a new soil test that was getting lots of media attention was not very accurate (Laboski et al., 2008).


The calibration developed by the committee for the color meter is both a decision tool that can help farmers to make better N rate decisions (objective 3), and a strategy to use the plant as an integrator of soil N processes and dynamics (objective 1). Subsequent research provided independent confirmation of the predictive relationship between meter readings and optimal fertilizer rate (Hawkins et al., 2007).


A weakness of the hand-held color meters is that they are not well-suited to help farmers manage the high levels of spatial variability in N status and dynamics that are found in their fields (Mamo et. al., 2003; Scharf et al., 2005). Canopy reflectance sensors apply the same concept but are more useful as decision tools because measurements are automated and continuous while driving a fertilizer applicator through the field. Committee members have contributed to a number of important developments with using sensors as decision tools over the past 5 years (Li et al., 2016; Pena-Yewtukhiw et al., 2015; Raper et al., 2013; Barker and Sawyer, 2012; Scharf et al., 2011). Our committee will probably continue to contribute useful advances in this area.


Computer simulation models were introduced commercially by three companies in 2014 to support nitrogen rate decisions. This is an area of interest to the committee, we have had several discussions regarding the possibility of evaluating these models relative to other decision tools, and several committee members have already undertaken such evaluations. These simulation models mathematically describe nitrogen and water behavior over soils in time to predict availability of N to crops. In a similar vein, committee members have also contributed to greater understanding of how soil properties and weather affect optimal N rate decisions through their effects on N mineralization and N loss (Tremblay et al., 2012).

Objectives

  1. Gain a more thorough understanding of the influence of macro- and micro-scale ecosystems and landscape properties on soil N dynamics.
  2. Explore optimization of nitrogen management practices through interrogation of corn nitrogen use efficiency datasets from studies conducted across the North Central region.
  3. Develop nitrogen management decision-making tools for crop advisers and growers.

Methods

A wide range of methods will be used. Programs, equipment, and expertise for many committee members are mainly field-based, and for a few are mainly lab-based. An emerging area of expertise for several committee members is measurement of nitrous oxide flux under different management scenarios, contributing to objective 1.

Only a few committee members get any funding associated with their membership, thus activities will mainly be based on grants that can be obtained. In some cases, a sub-group of the committee will submit a proposal together (there is one such proposal from four members that is currently in review; few if any grant opportunities are at a large enough scale to support a proposal from the entire committee). In other cases, committee members will identify places that two funded projects can dovetail to enhance the outcome of one or both.

Methods that will be employed are thus dependent on which funding efforts are successful. Substantial continuity with methods previously employed by the committee and its members is expected. Likely methods include field trials of nitrogen treatments (timing, source, rate, placement, loss inhibitors), field trials of other management practices (cover crops, rotations, tillage practices), microbiological studies of soils and rhizospheres, measurement of N2O flux, measurement of soil inorganic N content/dynamics, measurement of system parameters (soil temperature and moisture, plant spectral properties) that may help to explain observed N dynamics and behavior.

Measurement of Progress and Results

Outputs

  • Refereed scientific papers are the central output. The committee has a good history in this regarding nearly all of the papers in the Literature Cited section are important recent contributions from the committee.
  • Presentations to scientific and agricultural audiences are another important output. Committee members have been highly active in this area.
  • Written communications aimed at agricultural audiences is a fourth important output, helping to increase understanding and awareness of nitrogen dynamics and management.
  • Development or refinement of decision tools is critical output area. The tools can be put in the hands of farmers and farm advisors to help them improve the management of nitrogen.

Outcomes or Projected Impacts

  • Scientific papers and presentations from the committee will substantially improve scientific understanding related to one or more project objectives.
  • Farmers and farm advisors will have a better understanding of nitrogen dynamics and management due to committee research and outreach.
  • Nitrogen management practices will improve in at least some agricultural sectors due to increased understanding, or due to the development or refinement of N decision tools.

Milestones

(2016): Organize into interest sub-groups to plan either 1) grant proposals or 2) mining of existing data

(2017):Prepare manuscript from cross-state N mineralization dataset. Once published data, will be made publicly available.

(2017):Synthesis of data for publication from any groups mining existing long-term data set.

(2018):Joint studies initiated here or earlier based on successful proposals (proposals may continue, but probably only studies initiated by 2018 will be completed during the life of the project).

(2010):A key specific goal of this regional project is to determine whether new knowledge and management strategies generated by this group and others, and communicated to stakeholders, can reduce N fertilizer application to corn in the US by 5% over the next five years. To validate this we will explore fertilizer consumption trends compiled by USDA-ERS (http://www.ers.usda.gov/data-products/fertilizer-use-and-price.aspx.) at the end of this project cycle.

Projected Participation

View Appendix E: Participation

Outreach Plan

Several committee members (Fernández, Laboski, Mengel, Sawyer, Scharf, and Steinke) have Extension appointments and are regularly engaged in outreach as in important part of their job duties. These individuals will take the lead in outreach activities, particularly with outcomes that are of importance to farmers and farm advisors. Decision tools are expected to fall into this category. Face-to-face meetings, field days, newsletters, farm press, radio interviews, webinars, and individual websites are all likely to be used as outreach media.

Organization/Governance

The voting membership of the Technical Committee will consist of the official representative from each participating AES or cooperating agency group. The Administrative Advisor and NIFA representative will serve as non-voting ex-officio members of the committee. The participation of additional interested people from member AES and agencies is encouraged. Membership in the Technical Committee is not limited to states within the North Central Region.


The Executive Committee of the technical committee will consist of a Chairperson, Secretary, and committee Member-at-Large. Members of the Executive Committee must be official members of the Technical Committee. Each year the Technical Committee will elect a new Member-at-Large for a three year leadership term. In year two the member-at-large will automatically move up to secretary, and in year three will become Chair. At the discretion of the chair, the Administrative Advisor and other additional members can be designated as ex-officio members of the Executive Committee.


The chair sets the agenda and presides at the annual meeting, and any other meetings as deemed necessary. The current chair is responsible for preparing the annual report and seeing it is posted on the NIMSS website (www.nimss.org). In addition, the current chair is responsible for preparation of any documents required for the mid-term review, preparation and up-loading of the five year project proposal, and appointing working committees as needed.


The secretary will keep the official minutes of all meetings of the Technical Committee and see that they are up-loaded to the official NCRA project website in a timely manner. They will preside over any meetings the chair is unable to attend and will become chair of the Technical Committee in the event that the elected committee chair is for any reason unable to continue in this capacity.


The Member-at-Large is responsible for all local arrangements for meetings of the committee. They will also assume the duties of the secretary in the event that the secretary is unable to attend a meeting, or is required to assume the responsibilities of the chair.


The Executive Committee will review and make recommendations concerning the conduct of business by the technical Committee. The Technical Committee will have a regularly scheduled annual meeting. In addition, the Chair may call other meetings of the Technical Committee and the Administrative Adviser as deemed necessary.

Literature Cited

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Booth, M.S., J.M. Stark and E. Rastetter. 2005. Controls on nitrogen cycling in terrestrial ecosystems: a synthetic analysis of literature data. Ecol. Monogr. 75(2), pp.139-157.


Brzostek, E.R., J.M. Blair, J.S. Dukes, S.D. Frey, S.E. Hobbie, J.M. Melillo, et al. 2012. The effect of experimental warming and precipitation change on proteolytic enzyme activity: positive feedbacks to nitrogen availability are not universal. Global Change Biol. 18: 2617-2625.


Davidson, E.A. 2009. The contribution of manure and fertilizer nitrogen to atmospheric nitrous oxide since 1860. Nature Geoscience 2:659-662.


Ciampitti, I.A. and T.J. Vyn. 2011. A compressive study of plant density consequences on nitrogen uptake dynamics of maize plants from vegetative to reproductive stages. Field Crops Res. 121:2–18.


Donner, S.D. and D. Scavia. 2007. How climate controls the flux of nitrogen by the Mississippi River and the development of hypoxia in the Gulf of Mexico. Limn. & Oceanogr. 52(2), pp.856-861.


Duan, J., H. Tian, R.A. Drijber and Y. Gao. 2015. Systemic and local regulation of phosphate and nitrogen transporter genes by arbuscular mycorrhizal fungi in roots of winter wheat (Triticum aestivum L.). Plant Physiol. & Biochem. 96:199-208.


Fernández, F.G., R.E. Terry and E.G. Coronel. 2015a. Nitrous oxide emissions from anhydrous ammonia, urea, and polymer-coated urea in Illinois cornfields. J. Environ. Qual. 44: 415–422.


Fernández, F.G., E. Wilson, K.P. Fabrizzi and S. Naeve. 2015b. In-situ nitrogen mineralization for corn and soybean in poorly drained soils with and without tile-drainage. In: Annual Meetings Abstracts [CD-ROM]. ASA, CSSA, and SSSA, Madison, WI.


Hawkins, J.A., J.E. Sawyer, D.W. Barker and J.P. Lundvall. 2007. Using relative chlorophyll meter values to determine nitrogen application rates for corn. Agron. J. 99:1034–1040.


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Iqbal, J., D.C. Mitchell, D.W. Barker, F. Miguez, J.E. Sawyer, J. Pantoja and M.J. Castellano. 2015. Does nitrogen fertilizer application rate to corn affect nitrous oxide emissions from the rotated soybean crop? J. Environ. Qual. 44:711-719.


Jenkinson, D.S., R.H. Fox, and J.H. Rayner. 1985. Interactions between fertilizer nitrogen and soil nitrogen – the so-called ‘priming’ effect. Journal of Soil Science 36:425-444.


Jeske, E.S. 2012. Soil microbial community response to maize grown for high yield in rotation with soybeans. PhD Thesis, University of Nebraska, Lincoln.


Johnson, H.J., J.B. Colquhoun, A.J. Bussan and C.A.M. Laboski. 2012. Estimating nitrogen mineralization of composted poultry manure, organic fertilizers, and green manure crops for organic sweet corn production on a sandy soil under laboratory conditions. Hort Tech. 22:37-43.


Kellner, O. and D. Niyogi. 2015. Climate Variability and the U.S. Corn Belt: ENSO and AO Episode-Dependent Hydroclimatic Feedbacks to Corn Production at Regional and Local Scales. Earth Interact. 19: 1-32.


Laboski, C.A.M., J.E. Sawyer, D.T. Walters, L.G. Bundy, R.G. Hoeft, G.W. Randall and T.W. Andraski. 2008. Evaluation of the Illinois Soil Nitrogen Test in the North Central region of the United States. Agron. J. 100:1070-1076.


Li, A., B.D. Duval, R. Anex, P. Scharf, J.M. Ashtekar, P.R. Owens and C. Ellis. Sensor-based variable-rate nitrogen fertilization of corn maintains yield while reducing life-cycle losses of nitrogen to the environment. J. Environ. Qual. (accepted).


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McDaniel, M.D., R.G. Simspon, B.P. Malone, A.B. McBratney, B. Minasny, and M.A. Adams. In revision. Understanding spatio-temporal variability of soil CH4 and N2O fluxes is required to scale from chambers to the field. Global Change Biol.


Mitchell, D.C., M.J. Castellano, J.E. Sawyer and J. Pantoja. 2013. Cover crop effects on nitrous oxide emissions: Role of mineralizable carbon. Soil Sci. Soc. Am. J. 77:1765-1773.


Pantoja, J.L., K.P. Woli, J.E. Sawyer, D.W. Barker and M. Al-Kaisi. 2015. Stover harvest and tillage system effects on corn response to fertilizer nitrogen. Soil Sci. Soc. Am. J. 79:1249-1260.


Pena-Yewtukhiw, E.M, J.H. Grove and G.J. Schwab. 2015. Fertilizer nitrogen rate prescription, interpretational algorithms, and individual sensor performance in an array. Agron. J. 107:1691-1700.


Raper, T.B. J.J. Varco and K.J. Hubbard. 2013. Canopy-based normalized difference vegetation index sensors for monitoring cotton nitrogen status. Agron. J. 105:1345-1354.


Roberts, B.A., F.B. Fritschi, W.R. Horwath and S. Bardhan. 2015. Nitrogen Mineralization Potential as Influenced by Microbial Biomass, Cotton Residues and Temperature. J. Plant Nutr. 38:311-324.


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Scharf, P.C., D.K. Shannon, H.L. Palm, K.A. Sudduth, S.T. Drummond, N.R. Kitchen, L.J. Mueller, V.C. Hubbard and L.F. Oliveira. 2011. Sensor-based nitrogen applications out-performed producer-chosen rates for corn in on-farm demonstrations. Agron. J. 103:1683-1691.


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Tian, H., R.A. Drijber, J.L. Zhang and X.L. Li. 2013. Impact of long-term nitrogen fertilization and rotation with soybean on the diversity and phosphorus metabolism of indigenous arbuscular mycorrhizal fungi within the roots of maize (Zea mays L.). Agric. Ecosys. & Environ. 164:53-61.


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Toosi, E.R., J.P. Schmidt, and M.J. Castellano. 2014. Soil temperature is an important regulatory control on dissolved organic carbon supply and uptake of soil solution nitrate. Eur. J. Soil Biol. 61:68-71.


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Verhoeven, J.T., B. Arheimer, C. Yin and M.M. Hefting. 2006. Regional and global concerns over wetlands and water quality. Trends Ecol. Evol. 21: 96-103.


Zhu-Barker, X., W.R. Horwath and M. Burger. 2015. Knife-injected anhydrous ammonia increases yield-scaled N2O emissions compared to broadcast or band-applied ammonium sulfate in wheat. Agric. Ecosys. Environ. 212:148:157.

Attachments

Land Grant Participating States/Institutions

CA, IA, IL, KS, KY, MI, MN, MO, NE, TN, UT, WI

Non Land Grant Participating States/Institutions

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