Impacts of Cover Crops on Phosphorus and Nitrogen Loss with Surface Runoff

Date: 
Sep 2017

Issue

Cover crops are a recognized conservation practice to reduce soil erosion, and Iowa research has shown a winter cereal rye cover crop greatly reduces nitrate loss with subsurface drainage. However, little research has evaluated the impact of cover crops on total and dissolved nitrogen (N) and phosphorus (P) loss with surface runoff.

Objective

This project will continue an existing study for two more years, conducted under natural rainfall, to evaluate impacts of a winter cereal rye cover crop on soil, N, and P loss with surface runoff in a field testing high in P, managed with a corn-soybean rotation.

Approach

Systems evaluated in 12 areas ranging from one to three acres are chisel- plow/disk tillage and no-tillage with or without a cereal rye cover crop seeding each year. Soil samples for the late-spring nitrate test are collected when corn is 6 to 12 inches tall. Dry matter yield and total P and N are measured in rye cover crop samples collected each spring and fall to study nutrient uptake and recycling into the soil. Grain yield, grain N and P concentrations, and removal are measured each year. Runoff from the test areas is analyzed for total solids and several P and N forms.

Project Updates

May 2020

FINAL REPORT:

This report is for the funding period that encompassed runoff collection in 2017 and 2018. Four-year results summarized by the following points will be useful for reducing freshwater quality impairment and N and P exports to the Gulf of Mexico and provide useful information of stacked conservation management practices impacts on crop yield. (Note: A report for the period 2015-2016 summarized preliminary results from 2015 and 2016. More recent funding by the INRC supported work during 2019 and part of 2020.. A small amount of that funding is intended to complete ongoing evaluations to finish the study with six years encompassing three corn-soybean rotation cycles. This will strengthen the results, especially given extreme weather conditions that affected results from the first four years.)

1. No-till management and use of a rye cover crop slightly reduced corn and soybean yield. No-till reduced corn yield (compared with tillage) by 5 to 6% with or without a cover crop, and use of a rye cover crop reduced corn yield by 12% with or without tillage. No-till reduced soybean yield (compared with tillage) by 4 and 9% with and without a cover crop, respectively, and use of a rye cover crop reduced soybean yield by 12% with tillage and 7% without tillage.

2. The tillage system did not significantly affect cereal rye biomass yield or amounts of N and P recycled from above-ground plants parts. 

3. The results confirmed the value of no-till management compared with tillage to reduce soil erosion. On average across the four years, no-till (compared with tillage) reduced soil loss by 53% with the rye cover crop and by 75% without the cover crop. Use of a rye cover crop (compared with no cover crop) reduced soil loss by 32% with no-till and by 64% with tillage. The stacking of no-till and cover crops conservation practices reduced soil loss by 83% compared with tillage without a cover crop.

4. Results proved expectations of significant sediment-bound P loss reduction by no-till and cover crops, quantifying these effects which have not been quantified before in the north-central region. No-till (compared with tillage) reduced the total P loss by 21 and 43% with or without rye cover crop, respectively, and a rye cover crop (compared with no cover crop) reduced the total P loss by 42 and 20% with tillage and no-till, respectively. Stacking no-till and cover crop practices reduced total P loss by 55% compared with tillage without a cover crop.

5 No-till (compared with tillage) reduced the loss of dissolved P by 28 and 41% with or without a rye cover crop, respectively. This result is especially important for Iowa because surveys and research in Ohio and Ontario have suggested that no-till management can increase  dissolved P loss from fields. Use of a rye cover crop (compared with no cover crop) reduced the total P loss by 42 and 20% with tillage and no-till, respectively, but reduced dissolved P loss only with tillage (by 18%) and not with no-till. The results for cover crop effects on dissolved P loss with no-till contrast with results of an ongoing Kansas study, which showed that cover crops increased dissolved P loss in two of three years. Stacking both conservation practices (no-till and cover crop) reduced dissolved P loss by 41% compared with tillage without a cover crop.

6. The evaluation by this study of tillage system and cover crop effects on total and dissolved N loss with surface runoff is unique and greatly contributes to the knowledge of impairment of surface water resources. These combinations of conservation practices have not been evaluated for nitrogen loss in the US. No-till (compared with tillage) reduced total N loss by 65 and 54% with or without a rye cover crop, respectively, and reduced dissolved nitrate loss by 73 and 15% with or without the cover crop. Use of a rye cover crop (compared with no cover crop) reduced total N losses by 27% with tillage but only by 4% with no-till. Moreover, the cover crop reduced dissolved nitrate loss with tillage by 47% but increased the loss with no-till by 66%. Still, stacking the no-till and cover crops conservation practices reduced total N loss by 67% and reduced dissolved nitrate loss by 55% compared with tillage without a cover crop.

Other activities and accomplishments:

- 2 field days, 2 presentations, 1 workshop