Duration: 10/01/2001 to 09/30/2006

Administrative Advisor(s):

NIFA Reps:

Non-Technical Summary

Statement of Issues and Justification

Nitrate enrichment of ground water and surface waters and their impacts on drinking water quality and hypoxia in the Gulf of Mexico are important water quality issues. Nitrogen (N) use in cropping systems in the North Central Region is one of the major sources of nitrate entering natural waters. To address concerns about Gulf hypoxia, the National Science and Technology Councils Committee on Environment and Natural Resources issued a report identifying a 20% reduction in N loss from farmlands as a component of a strategy to ameliorate the Gulf of Mexico hypoxic zone. Studies to identify the source of nitrate that may contribute to Gulf hypoxia suggest that states within the North Central region are major contributors (Bratkovich et al., 1994; Rabalais et al., 1995; Burkart and James, 1999). Concerns with nitrate enrichment of groundwater continue to grow and are a legitimate public health concern because many municipalities and rural residents depend on groundwater as their primary source of drinking water.

The need to understand and elucidate the role of active carbon (C) and N pools in cropping systems continues to be critical for predicting N mineralization and availability in cropping systems. Jarvis et al. (1996) concluded that better quantification of the N mineralization contribution in cropping systems would help minimize N losses to the environment and allow more accurate recommendations for crop production. If N mineralization can be predicted more reliably, more precise guidance can be provided so that sufficient supplemental N can be applied to optimize crop production without the risks of over application.

Improved understanding of active C and N pools is especially important in view of current efforts to sequester C in agricultural soils as a means of lowering greenhouse gas emissions and minimizing the influence of these emissions on global warming (Rosenberg et al., 1999; CAST, 2000). Specifically, very little information is available on the effects of soil C buildup on N mineralization and the supplemental N needs of crops grown on these soils. Conversely, little is known about changes in C and N mineralization in cropping systems where soil C content is being depleted.

Surveys indicate that farmers are reluctant to adopt improved N management practices that could lower the frequency of excess N application because they perceive a high risk of economic loss if these practices are implemented (Shepard, 2000; WDNR, 2000). Educational programs and materials are needed to illustrate for producers and agricultural professionals the agronomic and environmental impacts of adopting various N management practices. This information should aid producers in making sound research-based decisions about the impacts of adopting improved practices instead of relying on perceptions that may not have a valid research basis.

The members of the NC-218 committee and the projected participants of the proposed project have the range of technical expertise and the appropriate facilities to conduct the proposed research. The regional and cross-regional perspectives and the range of experimental environments provided by the various participants will lead to broad applicability of the findings from the research. In addition, use of a core experiment approach plus the centralized, collaborative arrangements for analytical determinations, data analysis, and interpretation are advantages of the regional research approach.

Impacts from successful completion of the proposed work include improved diagnostic capability to predict crop N needs in the North Central region and in other areas represented by the committee membership. Communication of improved N management techniques to agricultural professional and farmers will lead to more efficient use of N from all sources and a reduction in excess N applications in crop production. This communication will be facilitated by the N management guidance document to be prepared as a part of this proposal and by the substantial Extension and outreach efforts planned by project participants (See Appendix E). Implementation of improved N management, especially avoiding excess N use, will reduce N losses and avoid water quality problems associated with nitrate enrichment.

Related, Current and Previous Work

Results from the NC-201 project showed that preplant (PPNT) and presidedress (PSNT) soil nitrate tests could improve identification of sites where corn would not respond to N fertilization, but use of soil nitrate tests in cropping systems with substantial organic N mineralization was less promising and indicated a need to better understand and quantify N mineralization in cropping systems (Bundy et al., 1999). The objectives and experimental protocol of the NC-218 project were founded on the NC-201 observation that when traditional soil nitrate testing procedures were employed, approximately 25% of sites (n=301) received N fertilizer recommendations with no observed corn N response. One of the principal objectives of the NC-218 project was to obtain a quantitative, real-time measure of soil N mineralization during the course of the growing season as well as a real-time measure of plant N status with the SPAD chlorophyll meter. Soil N mineralization was quantified by coupling on-site soil temperature with laboratory determined kinetic parameters from long-term aerobic incubations conducted on soil sampled from each site. A total of 74 site years of data were collected during the NC-218 project. Each site was exhaustively monitored during the growing season with measurement of soil N mineralization, soil nitrate-N concentration, periodic biomass yield and N uptake measurements as well as absolute and relative SPAD meter readings to monitor crop N status during plant development. The database has been compiled and distributed for analysis by the NC-218 committee.

Soil N mineralization between PPNT and PSNT (V6 to V10 corn growth stages) sampling times averaged 24 kg N/ha but ranged between 0.27 and 129 kg N/ha. In theory, the PSNT test is designed to capture early-season N mineralization however our data shows that soil nitrate-N captured by the PSNT is not always a good measure of soil N mineralization. In addition, we observed that an average of 63% of N mineralized during the growing season (range: 29%78%) occurred after PSNT sampling time. Soil N mineralization was greater where highly labile N sources such as manure or alfalfa residues were amended to soil. These observations suggest that a means of rapidly assessing N mineralization potential should improve N recommendations and fertilizer N use efficiency. To that end, ten mineralization quick tests were also evaluated for each site in the NC-218 database. A number of these show promise in the identification of sites with highly labile N pools but further analysis of this data is needed. For practical application of these findings to production management practices, our intent is to identify site edaphic and substrate characteristics in relation to the most appropriate quick test to estimate N mineralization potential. Possible additional approaches for rapid assessment of N mineralization potential include recently developed tests from Illinois (Mulvaney and Khan, 200_). and Iowa (Acosta-Martinez and Tabatabai, 2000).

In addition to site variation in soil N supply, current soil testing technologies may also be adversely affected by seasonal variation in crop N demand and physiological N use efficiency. The NC-218 database contains the appropriate plant N uptake data to evaluate crop N use efficiencies in relation to test failure rate. Chlorophyll meter measurements (SPAD-502) taken over the course of the growing season were most effective in identification of plant N deficiency after the V10 stage of growth. Sites where soil NO₃-N tests predict a high probability of N response but SPAD measurements indicate no N deficiency are most likely sites where in-season soil N mineralization and /or crop N-use efficiency is high. A significant proportion of sites with a high PSNT failure rate (12%) were those where a low harvest index indicated grain development was impaired. Although the chlorophyll meter provided excellent assessments of the N status of corn plants, the required timing of these measurements is later than the practical time for N additions in much of the North Central region.

Currently NC-218 is the only multistate (regional) project addressing the important issues of nitrogen mineralization, nitrogen availability and the impact of these parameters on water resources. A search of the entire CRIS database identifies 72 projects with some activity related to nitrogen mineralization and availability. NC-218 members or affiliates are participants in 19 of these 72 projects, indicating that the participants in the proposed project represent a substantial portion of the national effort on these subjects.

The members of the NC-218 committee have a long history of successful cooperation and collaboration through a core experiment approach to accomplish project objectives. Many of the NC-218 members participated in the previous NC-201 project that was also focused on broad participation in a core experiment. The core experiment approach consists of a common protocol designed to address project objectives in which members from various states within the region conduct identical experiments and pool the data across states and years. A centralized analysis and joint interpretation of the data results in conclusions and recommendations with true regional application and value. In addition, a centralized analytical approach was used to analyze soil samples from all of the core experiment locations by uniform methods in a single laboratory supervised by one of the committee members. The success of this approach has lead to the realization that the coordinated efforts would be useful to examine cross-regional core experiments to better understand cropping system response as influenced by climate and soil characteristics. This cross-regional activity is made possible by inclusion of project participants from outside the North Central region. This approach will be used in our proposed new project to gain a cross-regional appreciation of N diagnostic test and the effects of C sequestration on soil N availability.

Objectives

1. Develop and evaluate rapid tests for soil N mineralization capacity across the various soils and climatic regimes of the region and determine the feasibility and most appropriate conditions for use of these tests.
   
2. Conduct fundamental work to enhance current understanding of the role of active C and N pools in cropping systems and to predict net N mineralization as influenced by C sequestration management.
   
3. Develop a guidance document for agricultural professionals focusing on N best management practices and optimum rate determinations for the region.
   
4. 
   

Methods

Objective 1: Develop and evaluate rapid tests for soil N mineralization capacity across the various soils and climatic regimes of the region and determine the feasibility and most appropriate conditions for use of these tests.

A common field experimental design and a centralized analytical laboratory approach will be used to accomplish the stated objectives. In general, the field component of the project will involve installation of N response experiments at sites representing the soils and cropping systems used for corn production in states represented by committee members. Project participants will select new or appropriate existing experimental locations within their states. Each site will have at least five N rates (including a no N control) bracketing the range of the anticipated optimum N rate for the experimental location and will contain at least four replications. Corn grain yield will be measured in each plot at the end of the growing season. This will allow identification of the economic optimum N rate and construction of a model describing N response at each site. One approach for optimum N rate determination and model selection has been described by Schmitt and Randall (1992) and utilized by Bundy and Andraski (1995). These response functions can be used to project the consequences of applying reduced N rates (below economic optimum) on crop production levels and the risk of N loss to the environment. The amount of excess N applied at rates above the observed optimum can be used to evaluate the relationships between excess N and potential nitrate losses. This will allow estimation of the consequences of reducing N inputs on the potential for N loss and on crop productivity.

At each site, initial soil samples will be taken from control plots (no added N) before corn planting in the spring. Samples will be collected to a depth of 90 cm in 30-cm increments (120-cm sampling depth preferred where soil conditions permit). These samples will be analyzed for inorganic N to quantify soil residual N contributions to the crop N supply. Separate 0-15 cm soil samples will be collected from the same plots and shipped in field moist condition to selected laboratories for performance of existing and new tests for N mineralization. Specifically, Dr. Richard Mulvaney at the University of Illinois will perform tests for N mineralization potential by amino sugar analysis of the samples. Dr. Mulvaney has developed this promising new procedure for assessing N mineralization capacity by analysis of acid hydrolysates of soils for total hydrolysable N, ammonium N and amino sugar N using diffusion methods (Mulvaney and Khan, 200_). Preliminary comparisons of soil amino sugar levels with field data on corn N response show a promising relationship between amino sugar levels and the occurrence of yield response to added N (Mulvaney et al., 2000). Dr. Mulvaney is currently working on a quick test procedure for estimating amino sugar N that would eliminate the need for the acid hydrolysis step in the current method (R. Mulvaney, personal communication, 2000). If available, use of this procedure on samples from the core experiment would directly address the objective of identifying a rapid test procedure to assess the N mineralization of soils. Arrangements for the analyses in Dr. Mulvaneys laboratory will be coordinated through Dr. Robert Hoeft, who is the official NC-218 representative from the University of Illinois.

Similarly, samples will be provided to Dr. M.A. Tabatabai, official NC-218 representative from Iowa State University, for determination of arylamidase activities in soils and characterization of organic N pools by determination of potential N mineralization, and microbial biomass N and C. Recent work in Dr. Tabatabais laboratory shows that soil arylamidase activity may be a predictor of N mineralization (Acosta-Martinez and Tabatabai, 2000) and that N mineralization potential and microbial biomass N and C measurements can provide an assessment of active N pools in soils under various management practices (Deng et al., 2000). Additional tests found to be useful predictors of soil N mineralization and availability, after complete analysis of the NC-218 project data, will also be performed on these samples.

Based on the optimum N rates identified in each experiment and the N rate treatments applied, the amount of excess N applied (+ or -) will be calculated and the relationship between excess N applications and nitrate leaching potential will be determined. In each experiment, nitrate leaching potential will be assessed by one or more of the following approaches.

1. Soil profile nitrate content at the end of the corn growing season measured to a depth of 120 cm in treatments representing the range of N rates applied at each core experiment site. Useful predictive relationships have been found between end-of-season soil nitrate and soil water nitrate concentrations in leachates (Jemison and Fox, 1994; Andraski et al., 2000).
2. Soil water nitrate-N concentrations obtained with porous cup samplers installed at a 120-cm depth in treatments representing the range of N rates applied at each core experiment site. The design and installation of porous cup samplers has been described by Linden (1977), and this methodology was critically reviewed by Litaor (1988). Recent applications of the technique to corn production systems (Andraski et al., 2000), showed close correlations between soil water nitrate-N concentrations, end-of season soil nitrate measurements, and excess fertilizer N applied to corn.
3. Nitrate-N concentrations and quantity of N leached in soil drainage water captured by zero tension (Jemison and Fox, 1994) or equilibrium tension lysimeters (Brye et al., 1999) installed in appropriate treatments.
4. Nitrate-N concentrations and loads in tile drainage water measured in experiments with subsurface tile drainage systems designed to intercept and collect drainage from designated treatments in the core experiment (Gast et al., 1978; Randall et al., 1997).

Data from the core experiments conducted throughout the region will be pooled and subjected to uniform statistical analysis (SAS, 1992) and interpretation by the committee. The minimum analysis will include initial soil nitrate measurements, results of all chemical and biological tests performed to assess N availability, corn yield, and results from the method(s) used to assess nitrate leaching potential. Relationships among diagnostic tests, optimum N rates, yields, and potential for nitrate leaching will be studied using regression techniques.

Objective 2: Conduct fundamental work to enhance current understanding of the role of active C and N pools in cropping systems and to predict net N mineralization as influenced by C sequestration management.

Samples from field experiments described under objective 1 having a range of C accumulation/depletion characteristics will be used to assess the influence of C dynamics on N mineralization and availability. Laboratory incubations (25^oC) will be performed with these soils using periodic leaching to quantify inorganic N mineralized and gas chromatographic analysis of CO₂ in the headspace of the incubation containers to determine C released (Cabrera and Kissel, 1988; Garcia, 1992). Potential C and N mineralization assays on soils collected from the field plots will be used to determine the size and turnover rate of active and more resistant soil C and N pools (Bonde and Rosswall, 1987). The potential C and N mineralization data (cumulative mineralization curves) will be fit to the following additive exponential model to describe meaningful soil organic matter (SOM) pools and associated turnover rates for C and N (Bonde and Rosswall, 1987). The kinetic analysis of incubation data will be used to relate to mineralizable soil N pools under field conditions. To assess the impact of soil C sequestration management on soil N availability characteristics the recently deposited plant residue organic matter fraction will be examined. Physical fractionation of the soil will be carried out by separating the soil in free and intraaggregate soil organic matter pools (iPOM). Intraagregate soil organic matter is part of the macroaggregates where it is incorporated and physically protected (Cambardella and Elliot, 1992). Therefore, free and occluded particulate soil organic matter have different characteristics and play different roles in supplying nutrients. For these reasons, the use of soil physical fractionation can be used to define SOM with greater biological significance, especially for N availability (Oades and Ladd, 1977). The feasibility of the above analysis to correlate with other potential N mineralization assays will be explored to determine the effects of soil C sequestration management on soil N availability. Dr. W. Horwath, Univ. of California-Davis, and Dr. C. Rice, Kansas State Univ. will be responsible for coordinating the above analyses for the core experiment.

Objective 3: Develop a guidance document for agricultural professionals focusing on N best management practices and optimum rate determinations for the region.

Data from the proposed project and from previous projects conducted by the NC-201 and NC-218 committees will form the database for the guidance document to be prepared as a committee activity. If data from the new project is combined with core experiment results from the previous projects conducted by NC-201 and NC-218, information from about 500 experiments conducted throughout the region will be available to form the research base for the guidance document. A summary of current approaches to N best management practices for corn in the North Central region will be included in the document.

The combined research database will allow a comprehensive assessment of soil nitrate testing as a tool for improving N management in the region. Guidance on the optimum use of preplant and presidedress testing including sampling depths, soils and identification of cropping systems most likely to benefit from use of soil nitrate tests.

The influence of temperature measured in terms of air temperatures, growing degree days or as soil heat units on adjustments or interpretation of soil nitrate test results or other measures of N availability will be determined for the region. Use of plant measures of N sufficiency such as the chlorophyll (SPAD) meter for assessing the N status of the crop and for predicting the need for additional N will be developed for various crop production systems in the region.

Optimum N rates for corn will be determined for the region from those experiments with an adequate number of N rates to establish a N response model. Techniques for establishing optimum N rates are the same as those described under objective 1 of this proposal. Relationships of these optimum N rates to soil characteristics, cropping systems, yield levels, and climatic conditions will be evaluated. Guidance on use of chemical and biological N availability tests, results of long-term soil incubations, and existing or proposed quick tests for N mineralization for improved identification of optimum N rates for corn will be provided.

The consensus opinion in relatively recent reviews on prediction of N mineralization and availability (Jarvis et al., 1996; Whitmore, 1999) is that a single technique is not likely to provide accurate prediction of crop N needs across a broad range of soils, climate and cropping systems such as exist in the North Central Region. Instead, a combination of methods based on different principles is viewed as more likely to provide an accurate assessment of N availability with application to a broad range of production conditions (Meisinger, 1984; Whitmore, 1999; Schroder et al., 2000). For example, a combination of a soil nitrate measurement with a chemical extraction procedure and a biological method for N mineralization could provide a more reliable assessment of the available N supply for crops than any of these methods alone. Information available from the regional database provides an ideal opportunity to investigate combinations of the exhaustive range of tests and procedures evaluated by the committee to construct a process involving the most effective procedures for predicting N availability across the diverse conditions of the region or in sub-regions having more similar soil, climatic, and crop production conditions. This would address the identified information need to explore the possibility that a combination of tests has greater and more broadly applicable potential for predicting N availability than any single method. The outcome of this activity will be featured in the N management guidance document.

Measurement of Progress and Results

Outputs

• Evaluation of rapid tests to predict soil N mineralization in the region including the promising new soil amino sugar assay and arylamidase activity procedures.
• Enhanced understanding of active C and N pools in cropping systems and their influence on N mineralization under management resulting in soil C sequestration or depletion.
• Corn response to N rates and its relationship to various diagnostic tests for N availability and mineralization.
• Establishment of the relationship between excess N use in cropping systems and nitrate loss by leaching throughout the region.
• Quantification of the relationships between applied N rates, corn yield, and potential for nitrate loss by leaching.
• Output 6. A comprehensive assessment of combining the most promising diagnostic tests into a multiple procedure approach for assessing N mineralization and availability.<br> <br> Output 7. An N management guidance document for the North Central region containing the results, products and recommendations emerging from the proposed project.

Outcomes or Projected Impacts

• Increased scientific knowledge based on results from this project. Through publications in refereed journals and other scientific outlets, information from this project will enhance the knowledge base on use of diagnostic tests to predict N mineralization and availability, the role of soil C and N pools in determining N mineralization, and relationships between rates of applied N and nitrate losses to the environment.
• Increased client knowledge based on use of the N management guidance document resulting from this project. Agricultural professionals will use the improved N management techniques described in this publication to promote and implement improved N management practices for their clients.
• Substantial savings in N fertilizer costs to farmers. Using the assumption that about 10 million metric tons of fertilizer N are applied annually in the United States and about 43% of this N is applied to corn in the Corn Belt states (Keeney and Follet, 1991), the value of this fertilizer N is about $19 billion at an average retail cost of $0.20 per lb of N. If improved N management practices emerging from this project can achieve a 10% reduction in N applications in the Corn Belt by avoiding excess N applications and losses of applied N, savings in N costs would be about $1.9 billion. This would be in addition to the undocumented costs of water quality problems associated with nitrate losses from cropping systems.
• Improved water quality. Implementation of N management practices developed in this project will facilitate lower losses of nitrate to ground water and a reduced nitrate contribution from the region to Gulf of Mexico hypoxia.

Milestones

(0):0

Projected Participation

View Appendix E: Participation

Outreach Plan

The main client groups for information emerging from this project are agricultural and environmental scientists and practicing agricultural professionals including consultants, county and regional Extension staff, and industrial agronomists. Information transfer to the scientific clientele will be through refereed journal publications, presentations at scientific meetings and conferences, and published articles in conference proceedings. The major outreach product for the practicing professional clients will be the N management guidance document that will be prepared based on the project results. This publication will provide essential background information on N management principles as well as a research-based assessment of the performance of various management options in the North Central Region and other regions where committee members reside. The N management document can be used by the practicing professionals to conduct educational programs for farmers who are the ultimate clients for the research results from this project. Several of the committee members have major Extension responsibilities at their respective universities. These members will utilize results from this project to enhance their educational programs on N management by using the regional guidance document as a teaching reference and by incorporating results of the research completed in this project to prepare state-specific Extension materials.

Organization/Governance

The Technical Committee will consist of the project leaders for each of the cooperating agencies, the Administrative Advisor and Cooperative States Research Service consultant. The Technical Committee shall elect for two years a chair, a secretary, and an executive committee member-at-large. At the end of the 2-year term, a new member-at-large will be elected, the secretary will become chair, and the member-at-large will become secretary. The chair will appoint working committees as needed in the conduct of the project and insofar as possible will preside at meetings of the Technical Committee. The secretary will keep the official minutes of all meetings of the Technical Committee and Executive Committee and distribute copies of such minutes to all Technical Committee members. The secretary will preside at any meetings of the Technical Committee and the Executive Committee which 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 Technical Committee will have an Executive Committee consisting of the current chair, the secretary and the member-at-large and, at the discretion of the chair, the Administrative Advisor and such additional members as may be appointed by the chair in consultation with the Administrative Advisor can be designated as ex-officio members of the Executive Committee. 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 as needed in the conduct of the project upon approval of the Executive Committee and the Administrative Advisor.

Literature Cited

Acosta-Martinez, V. and M.A. Tabatabai. 2000. Arylamidase activity of soils. Soil Sci. Soc. Am. J. 64:215-221.

Andraski, T.W., L.G. Bundy, and K.R. Brye. 2000. Crop management and corn nitrogen rate effects on nitrate leaching. J. Environ. Qual. 29:1095-1103.

Bonde, T. A., and T. Rosswall. 1987. Seasonal variations of potentially mineralizable nitrogen in four cropping systems. Soil Sci. Soc. Am. J. 51:1508-1514.

Bratkovich, A., S.P. Dinnel, and D.A. Goolsby. 1994. Variability and prediction of freshwater and nitrate fluxes for the Louisiana-Texas shelf: Mississippi and Atchafalaya river source functions. Estuaries. 17:766-778.

Brye, K.R., J.M. Norman, L.G. Bundy, and S.T. Gower. 1999. An equilibrium tension lysimeter for measuring drainage through soil. Soil Sci. Soc. Am. J. 63:536-543.

Bundy, L.G., D.T. Walters, and A.E. Olness. 1999. Evaluation of soil nitrate tests for predicting corn nitrogen response in the North Central Region. NC Regional Res. Pub. No. 342. Univ. of Wis., Madison, WI.

Bundy, L.G., and T.W. Andraski. 1995. Soil yield potential effects on performance of soil nitrate tests. J. Prod. Agric. 8:561-568.

Burkart, M.R., and D.E. James. 1999. Agricultural nitrogen contributions to hypoxia in the Gulf of Mexico. J. Environ. Qual. 28:850-859.

Cabrera, M.L., and D.E. Kissel. 1988. Potential mineralizable nitrogen in disturbed and undisturbed soil samples. Soil Sci. Soc. Am. J. 52:1010-1015.

Cambardella, C.A., and E.T. Elliott. 1992. Particulate soil organic matter across a grassland cultivation sequence. Soil Sci. Soc. Am. J. 56:777-783.

Council on Agr. Sci. and Technol. (CAST). 2000. Storing carbon in agricultural soils to help mitigate global warming. Issue Paper, no. 14. CAST, Ames, IA.

Deng, S.P., J. M. Moore, and M.A. Tabatabai. 2000. Characterization of active nitrogen pools in soils under different cropping systems. Biol. Fertil. Soils 31:000-000. (In press).

Garcia, F.O. 1992. Carbon and nitrogen dynamics and microbial ecology in tallgrass prairie. Ph.D. Diss. Kansas State Univ., Manhattan, (Diss. Abstr. 92-35625).

Gast, R.G., W.W. Nelson, and G.W. Randall. 1978. Nitrate accumulation in soils and loss in tile drainage following nitrogen applications to continuous corn. J. Environ. Qual. 7:258-261.

Jarvis, S.C., E.A. Stockdale, M.A. Shepherd, and D.S. Powlson. 1996. Nitrogen mineralization in temperate agricultural soil: processes and measurement. Adv. Agron. 57:187-235.

Jemison, J.M., and R.H. Fox. 1994. Nitrate leaching from nitrogen-fertilized and manured corn measured with zero-tension pan lysimeters. J. Environ. Qual. 23:337-343.

Keeney, D.R., and R.F. Follet. 1991. Managing nitrogen for groundwater quality and farm profitability: Overview and introduction. pp. 1-7. In R.F. Follet et al. (ed.) Managing nitrogen for groundwater quality and farm profitability. SSSA, Madison, WI.

Linden, D.R. 1977. Design, installation and use of porous ceramic samplers for monitoring soil-water quality. U.S. Dept. Agric. Tech. Bull., 1562.

Litaor, M.I. 1988. Review of soil solution samplers. Water Resour. Res., 24:727-733.

Mulvaney, R.L., and S.A. Khan. 200_. Diffusion methods to determine different forms of nitrogen in soil hydrolysates. Soil Sci Soc. Am. J. (In press).

Mulvaney, R.L., S.A. Khan, and R.G. Hoeft. 2000. A soil organic fraction that reduces the need for nitrogen fertilization. In R.G. Hoeft (ed.) Proc. Ill. Fert. Conf. p. 21-33.

Oades, J.M., and J.N. Ladd. 1977. Biochemical properties: carbon and nitrogen metabolism. In: Soil factors in crop production in a semi-arid environment (J.S. Russel and E.L. Gracen Ed.), pp. 127-160.

Rabalais, N.N., W.J. Wiseman, R.E. Turner, B.K. Sen Gupta, and Q. Dortch. 1995. Nutrient changes in the Mississippi river and system responses on the adjacent continental shelf. Estuaries, 19:386-407.

Randall, G.W., D.R. Huggins, M.P. Russelle, D.J. Fuchs, W.W. Nelson, and J.L. Anderson. 1997. Nitrate losses through subsurface tile drainage in CRP, alfalfa, and row crop systems. J. Environ. Qual. 26:1240-1247.

Rosenberg, N.J., R.C. Izaurralde, and E.L. Malone (eds.). 1999. Carbon sequestration in soils: Science, Monitoring, and Beyond. Proc. of the Ste. Michaels Workshop, Dec. 1998. Battelle Press, Columbus, OH.

SAS Institute, Inc. 1992. SAS/STAT users guide. Release 6.03/4^th ed. SAS Inst., Cary, NC.

Schmitt, M.A. and G.W. Randall. 1992. Calibration and correlation of soil N tests for improved N recommendations. p.181-187. In Field research in soil science. Minnesota Agric. Exp. Stn. Misc. Publ. 75-1992.

Schroder, J.J., J.J. Neeteson, O. Oenema, and P.C. Struik. 2000. Does the crop or the soil indicate how to save nitrogen in maize production? Reviewing the state of the art. Field Crops Res. 66:151-163.

Shepard, R. 2000. Nitrogen and phosphorus management on Wisconsin farms: Lessons learned for agricultural water quality programs. J. Soil Water Conserv. 55(1):63-68.

Whitmore, A.P. 1999. Evaluating the nitrogen supplying power of soils in field cropping systems. AB-Note 186. Research Institute for Agro-biology and Soil Fertility, Wageningen, 23 pp.

Wisconsin Dept. of Natural Resources (WDNR). 2000. Nonpoint source control plan for the Lake Mendota priority watershed project. Pub. WT-536-00 REV, June 2000. WDNR, Madison, WI.

Attachments

Land Grant Participating States/Institutions

CA, IA, IL, IN, KS, MI, MN, MO, NE, OH, OR, SD, WI

Non Land Grant Participating States/Institutions

USDA-ARS/Iowa