Completing Three Corn-Soybean Rotation Cycles for Ongoing Research on Impacts of Cover Crops on Phosphorus and Nitrogen Loss with Surface Runoff

Aug 2020


This project will continue developing a field-scale study under natural rainfall that has been evaluating impacts of cover crops on soil, nitrogen and phosphorus losses with surface runoff for corn-soybean rotations managed with tillage and no tillage. The assessment of dissolved N and P loss with surface runoff -- in addition to evaluating the influence of conservation practices to reduce soil erosion and particulate (sediment bound P) N and P loss -- has become especially important because dissolved N and P are the most biologically active runoff fractions for impairing water quality.


This project will continue to evaluate the impacts of cereal rye cover crops with tillage and no-till systems for corn-soybean rotations on soil, P and N loss with surface runoff, with an emphasis on evaluating potential synergistic, unintended consequences of nutrient reduction practices.


Researchers will complete three corn-soybean rotation cycles of an ongoing INRC project, to assess impacts from a winter cereal rye cover crop with tillage or no-till management on losses with surface runoff of soil, sediment-bound N and P, and dissolved N and P, from a 42-acre field testing high to very high in soil-test P and managed with recommended N management. Results will be shared with stakeholders through outreach activities during and after the funding period.

Award Number: 

Project Updates

Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.

January 2023


Key Research Questions

No-till management and cover crops are recognized nutrient management practices that are considered by the Iowa Nutrient Reduction Strategy and both state and federal agencies. Research in Iowa or the north-central region has demonstrated the effectiveness of no-till management at reducing soil-bound P loss from fields and that cover crops significantly reduce the amount of nitrate lost with subsurface drainage. However, the impacts of no-till management on N and dissolved P loss from corn and soybean fields and of cover crops at reducing loss of N or P with surface runoff have not been sufficiently studied. Therefore, the main objectives of a six-year study were to assess at a field scale and natural rainfall effects of no-till and cover crops (compared with tillage and no cover crop) for corn-soybean rotations at reducing soil, nitrogen (N) and phosphorus (P) loss with surface runoff in fields with high to very high soil-test P levels. The field data collection was conducted from 2015 to 2020 with funding provided by Iowa Nutrient Research Center (INRC). The funding was provided in successive two-year grants, each with preliminary final reports to the INRC, until results from 2015 through 2018 were summarized in a report submitted to the INRC in 2019. Thereafter, two additional one-year grants allowed for continuing field data collection until the end of 2020 to complete six years of the corn-soybean rotation and for finishing laboratory analyses and data management during 2021. This final comprehensive report summarizes results for the six years of the study.

Research Findings

Work for the project began early in summer 2013 to locate an appropriate field for the study. By the fall of that year an appropriate 17-ha field was identified at the Iowa State University (ISU) Hermann Farm located in southern Boone County which had been managed for bulk production of corn and soybean with no-till management for about ten years. The dominant soil type was Clarion loam with very small areas of Nicollet loam and Webster clay loam (less than 0.2 ha each). A preliminary dense grid soil sampling in fall 2013 showed that soil-test P to a 15-cm depth ranged from the Optimum to Very High ISU soil-test P interpretation categories. Therefore, in early spring 2014, P fertilizer was applied using variable-rate technology to uniformize as much as possible soil-test P levels across the field. The systems we wanted to evaluate were chisel-plow/disk tillage or no-tillage with or without a cereal rye cover crop. The four management systems with three replications were established on 12 small watersheds ranging from 0.61 to 1.25 ha in size delimited by existing terraces and newly constructed berms (Fig. 1). Slopes ranged from 1.51 to 3.11% and the systems were assigned to the three replications (blocks) according to slope. The tillage treatments (chisel/plow-disk and no-till) were first established in spring 2014, and soybean was planted following the previous rotation. Crop measurements were cereal rye cover crop aboveground dry matter yield and both N and P recycled at its termination in the spring, corn and soybean grain yield, and N and P removed with grain harvest. Runoff from each water shed was analyzed for total solids, total P, dissolved reactive P (DRP), total N, dissolved ammonium, and dissolved nitrate. The runoff assessment equipment included 0.46-m H-flumes to receive runoff and ISCO 6712 autosamplers with appropriate electronic hardware and software. The equipment installation was finished in March 2015 when soils thawed. Therefore, runoff collection began in spring 2015 before the first-year rye cover crop was terminated, the tillage systems had been in place for one year, and before corn was planted. The automatic runoff sampler was equipped with one liter 24 bottles programed to take a 300-ml sample to each bottle after every one-cubic meter of runoff (Fig. 2). To assess statistical differences between the runoff measurements we conducted analyses of covariance to determine whether geometric shape differences among the 12 watersheds improved the significance of system differences each year and on average across years. We used the log-transformed runoff values and the covariables mean slope, length from the highest elevation point to the H-flume, mean and maximum flow length, a form factor (area divided by the square of the length), elongation ratio (diameter of a circle with the same area as the watershed divided by the length), and a compactness coefficient (perimeter divided by the circumference of an equivalent circular area). These covariables often improved the significance of treatment effects. Results from this six-year field-scale study are very relevant for Iowa and states of the north-central region because of the lack of previous studies evaluating the effects of the four systems on soil, N, and P losses with surface runoff. Results summarized by the following points will be useful for reducing freshwater quality impairment and both N and P exports to the Gulf of Mexico and provide useful information of stacked conservation management practices impacts on corn and soybean yield in addition to the runoff losses information. 1. Soil test P was purposely maintained at levels higher than optimum for corn and soybean crops, were very variable across both systems and years. On average, soil-test P was higher with no-till than with tillage only without the cereal rye cover crop, and was also higher without the cover crop for both tillage systems but the difference was proportionally larger for no-till than for tillage. 2. The tillage system did not affect significantly the cover crop biomass yield or amounts of N and P recycled from aboveground plants parts, and were 2100 kg dry mater/ha, 53 kg N/ha, and 17 kg P/ha. 3. No-till management and use of a rye cover crop slightly reduced both corn and soybean yield. Corn yield was the highest with tillage without the cover crop, intermediate for no-till without the cover crop (a 4% reduction), and the lowest for both tillage systems with the cover crop (a 10% further reduction). Soybean yield also was the highest for tillage without a rye cover crop and the lowest for the other three systems (a 10% reduction). 4. Results confirmed the value of no-till and cover crops to reduce soil erosion. Soil losses were highest for tillage without the rye cover crop and the systems tillage with the cover crop and no-till without or with the cover crop reduced soil losses by 69, 77, and 84 percent, respectively. 5. Results proved expectations of significant sediment-bound P loss reduction by no-till and cover crops but these effects for a first time in Iowa at a field scale. Total P losses were highest for tillage without the rye cover crop and the systems tillage with the cover crop and no-till without or with the cover crop reduced P losses by 51, 38, and 51 percent, respectively. 6. Dissolved reactive P losses were not statistically different for the systems in this study but numerically losses were the highest for tillage without the cover crop and the other systems reduced losses by 12 to 26 percent. These results are very important for Iowa because surveys and research in Ohio and Ontario have suggested that no-till management increases the dissolved P loss from fields and a Kansas study with no-till showed that cover crops increase DRP losses. This study and others in Iowa demonstrated that no-till and cover crops often increase the proportion of dissolved P loss of the total runoff P but no necessarily the amount of dissolved P lost. 7. Losses of total N and both dissolved ammonium and nitrate were the highest for tillage without the cover crop. The systems tillage with the cover crop and no-till without or with the cover crop reduced ammonium losses by 44, 17, and 35 percent, respectively, total N losses by 40, 61, and 62 percent, respectively, and nitrate losses by 47 to 55 percent. The results for N losses are very relevant because this is the only study that has evaluated the combined effects of tillage systems and cover crops on loss of total N and dissolved N forms with runoff in Iowa and the north central region. 8. All runoff loss reductions by the cover crop, including the dissolved N and P fractions, were proportionally much higher for tillage than for no-till. Therefore, use of cover crops is more beneficial with tillage than with no-till management. In conclusion, stacking the no-till and cover crops conservation practices on average reduced corn and soybean grain yield by 11 and 13 percent compared with tillage without a cover crop. However, no-till combined with a cover crop reduced soil, total P, DRP, total N, ammonium, and nitrate by 84, 51, 21, 63, and 47 percent, respectively, compared with tillage without a cover crop.

Other activities and accomplishments

- 2 field days, 12 presentations, 2 workshop


Mallarino, A.P., M.U. Haq, J.D. Jones, and M.J. Helmers. 2021. Management of fertilizer, manure, tillage, cover crop, and alum or gypsum soil amendments to minimize dissolved P loss from corn and soybean fields. p. 53-59. In 32nd Integrated Crop Management Conf. Proceedings. 1-2 Dec. Iowa State Univ. Extension. Mallarino, A.P., M.J. Helmers, P.A. Barbieri, C.H. Pederson, R.D. Vandepol, R.M. Cruse, J.E. Sawyer, and D. Jaynes. 2015. Impacts of cover crops on phosphorus and nitrogen loss with surface runoff. Agric. Eng., Agronomy, and Central Iowa Research Farms Annual Reports. ISRF14-16,30; RFR-A14117. Iowa State Univ., Ames. Gomez-Botero, M., A.P. Mallarino, P.A. Barbieri, M.U. Haq, M.J. Helmers, M.L. Thompson, C. Pederson, J.E. Sawyer, and R.M. Cruse. 2020. Impacts of cereal rye cover crop and tillage systems on phosphorus loss with runoff from fields managed with corn-soybean rotations managed with corn-soybean rotations. ASA, CSSA, SSSA Annual Meetings. Nov. 9-13