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

Date: 
Feb 2013

Issue

The Iowa Science Assessment of Nonpoint Source Practices included cover crops as a nutrient reduction strategy for producers to consider. This also was identified as an area where research is lacking, and where larger scale studies are needed to better assess water quality impacts.

Objective

Determine more predictably the impact of a rye annual cover crop on sediment, phosphorus and nitrogen loss with surface runoff for corn and soybean production systems managed with no-till or tillage. Although results will be obtained at one site, existing knowledge about Iowa soils, climate and processes involved will allow for acceptable extrapolations to other regions in Iowa.

Approach

A long-term experiment site will be established at an Iowa State University research farm. Cropping systems, tillage systems and runoff monitoring systems will be established during 2014, and the first water quality evaluations will be in 2015. The experiment will evaluate selected combinations of harvest management systems for corn and soybean with or without a rye cover crop and with or without tillage. The P fertilizer will be broadcast uniformly across all plots to maintain an optimum soiltest P level. This placement method is used by most farmers in the state, and is the one for which cover crops would be the most beneficial. The N for corn will be injected into the soil at a uniform rate across the plots. Runoff will be analyzed for total solids, dissolved P, total P, dissolved ammonium-N, dissolved nitrate-N and total N. Soil of all plots will be sampled before applying all treatments. Total P and N will be measured in all plant parts harvested to estimate removal.

Project Updates

March 2018

FINAL REPORT

IMPACTS OF COVER CROPS ON PHOSPHORUS AND NITROGEN LOSS WITH SURFACE RUNOFF: This multi-year project is designed to assess the value of no-till management of cover crops to reduce N and P loss with surface runoff from Iowa fields. The first phase from fall 2013 to spring 2015 involved establishing the tillage and crop management systems, land preparation and purchase and equipment installation. The second phase was to collect crop, soil and runoff for the 2015 and 2016 crop years, which is summarized here. The cereal rye cover crop spring growth was excellent both years. Corn was planted in 2015. Statistical analysis showed yield was the highest for tillage without a rye cover crop, intermediate for no-till without a cover crop and the lowest for both tillage with a cover crop and no-till with a cover crop. Therefore, the rye cover crop significantly reduced corn yield managed with or without tillage. Soybean was planted in 2016, and the statistical analysis showed yield was the highest for no-till or tillage without a rye cover crop, and the lowest for no-till or tillage with a rye cover crop. Thus, the tillage systems had no effect on soybean yield, but the cover crop reduced yield with or without tillage. The results of this study should be considered with caution because in both years there was lower-than-normal rainfall and runoff from early spring to early summer. Without a cover crop, no-till management slightly reduced total and dissolved P losses, greatly reduced total N loss and slightly reduced nitrate loss compared with tillage. With tillage, use of a cover crop greatly reduced the total N and P loss, slightly reduced dissolved P loss and slightly increased nitrate loss. With no-till, the cover crop slightly reduced total N and P loss, but slightly increased dissolved N and P forms. Additional data will be collected in the 2017 and 2018 crop years. 

March 2017

There was no surface runoff at the site from January through March. Precipitation as rainfall or snow melted to its liquid equivalent for the period was only 6 inches, which occurred in small events that did not cause runoff. Work until the middle of March consisted of continuing data management for field and lab information, analyzing grain samples collected to measure N and P removal, and analyzing soil samples collected after harvest and before the fall P application to measure soil-test P. During late March, fieldwork began by surveying rye cover crop regrowth, which was excellent, and setting up the runoff monitors and samplers for the new growing season.

December 2016

Work continued with data collected from the 12 small watersheds during the second crop-year of this project. There was no measurable runoff during the fall months. Fieldwork consisted of harvesting soybean, taking soil samples, applying fertilizers, and doing site maintenance. Soybean grain was harvested with a combine equipped with yield monitors and GPS. Preliminary yield data showed soybean yield was lower and approximately similar for systems managed with a cereal rye cover crop with no-till or tillage (51 and 50 bu/acre, respectively) than for systems managed without cover crop with no-till or tillage (56 and 59 bu/acre, respectively). Observations during the season had shown smaller plant and less inter-row canopy cover in watersheds with a cover crop. Fertilizer P was applied to bring soil-test P back to high or very-high levels, and was the first time P was applied since spring 2014.

September 2016

The cereal rye cover crop was over-seeded on September 21 before harvesting soybean. Work continued by analyzing samples from the few runoff events of the previous quarter and collecting runoff from five new events. In contrast to other areas of Iowa, rainfall at the site was much less, with a 3-month total of 11.2 inches. Therefore, runoff occurred in only three to five of the 12 watersheds, with the exception of the day with the highest rainfall. Seventy-six samples were collected and analyzed for dissolved reactive P, total P, nitrate-N, ammonium-N, total N and sediment. The largest loss of P, nitrate-N, and ammonium-N were for no-till with the rye cover crop, and the smallest was for chisel-plow tillage with the cover crop. 

June 2016

During this quarter, analysis of samples and data for nitrogen and sediment content in runoff for the 2015 crop year was completed, and spring field work got underway. The runoff monitoring units were reinstalled to the 12 H-flumes in late March and became fully functional in early April. The cereal rye cover crop was chemically terminated April 26. There were no large rainfall events in the area of the site during this quarter. There were three events with measurable runoff in only from three of the 12 plots.

March 2016

Work this quarter focused on detailed study of yield monitor data from the fall of 2015, and continuing chemical analyses of collected grain samples and runoff samples. Study of corn grain yield data from the flow areas of each of the 12 watersheds showed yields ranging from 132 to 194 bushels per acre and statistically higher yields with tillage and without a rye cover crop. Analysis of total nitrogen, phosphorus and solids continues. Dissolved reactive phosphorus (DRP) concentrations tended to be higher with a cover crop with or without tillage. Total P concentrations were higher with a cover crop only with no-till.

December 2015

Collection and analysis of surface runoff samples from the research site continued for concentrations of sediment and various fractions of phosphorus and nitrogen. The crop harvest of the 12 small watersheds was done in late September. Rye cover crop dry-matter yield was measured just before the first hard freeze. A total of 4.73 inches of rainfall occurred in this quarter and 140 runoff samples were collected. Analysis of total nitrogen, phosphorus and solids is underway. Partial results indicate higher dissolved reactive phosphorus (DRP) concentrations for no-till with cover crop, intermediate concentrations for no-till and tillage without cover crop, and the lowest concentration for tillage with cover crop. 

September 2015

Yield of the rye cover crop dry matter was measured just before its termination in the middle of April. Nitrogen and phosphorus content in above-ground rye dry matter was analyzed. Tillage (chisel-plow and disking) was done for six of the watersheds about one week after the rye cover crop was killed. Corn was planted in early May with 30 lb N/acre applied with planter attachments. After planting, residue cover was determined. Lower cover values were observed for systems managed with tillage with or without cover crop than for no-tillage with or without a cover crop. At the V4 corn growth stage, plant height was measured to determine treatment effects on corn growth. The corn height was greater without a previous cover crop than with one. Before N side-dressing in early June, one-foot soil samples were taken. Soil nitrate differences between systems were small, but were statistically greater for watersheds without a cover crop than with one.

June 2015

Installation of the 12 H-flumes began last fall and was completed by the third week of March. Solar panels and monitoring equipment were installed the last week in March, with earthen berms completed the first week of April. A rye cover crop seeded to six of the watersheds last fall established well, and by March 31 was green, uniform and provided approximately 90 percent ground cover. Researchers met several times with the ISU farms personnel to develop detailed plans and procedures for managing corn planting and rye cover crop termination. Runoff monitoring is underway for the 2015 crop.

March 2015

This project is designed to determine the impact of a rye annual cover crop on sediment, phosphorus and nitrogen loss. Twelve areas ranging from 1 to 2 acres accommodate four systems replicated three times, all based on corn-soybean rotation. Since runoff will not be collected until the 2015 crop year, the 2014 soybean crop was harvested across all 12 small watersheds. A uniform and non-limiting potassium fertilizer rate was applied across all the watersheds, the rye cover crop biomass yield was measured, and installation of the H-flumes continued. The rye cover crop seeded to six of the watersheds in the middle of September before soybean matured, established very well. Rye dry matter yield was measured just before the first killing freeze.

December 2014

One project is designed to determine the impact of a rye annual cover crop on sediment, phosphorus and nitrogen loss. Twelve areas ranging from 1 to 2 acres accommodate four systems replicated three times, all based on corn-soybean rotation. All plots were planted to soybean in the spring, and were developing well by September. The cereal rye cover crop was seeded in mid-September, and was well-established by October when soybeans were harvested. Installation of H-flumes and runoff autosamplers for data collection is underway.

September 2014

One project is designed to determine the impact of a rye annual cover crop on sediment, phosphorus and nitrogen loss. Twelve areas ranging from 1 to 2 acres accommodate four systems replicated three times, all based on corn-soybean rotation. The systems are: (1) no-till without cover crop, (2) no-till with a cereal rye cover crop each year, (3) chisel-plow/disk tillage without a cover crop and (4) chisel-plow/disk tillage with a cereal rye cover crop each year. In March, soil samples were taken and analyzed. Different P fertilizer rates were applied to about two-thirds of the areas so the site has the uniform soil-test P level needed. Then researchers planted soybean no-till in six of the areas and planted it after doing tillage to the other six areas. In 2015, corn will be planted to all areas. The work continues, including the purchase H-flumes and runoff autosamplers.

June 2014

One project is designed to determine the impact of a rye annual cover crop on sediment, phosphorus and nitrogen loss. A site for the project was chosen at an Iowa State University research farm south of Ames. Twelve areas ranging from 1 to 2 acres will accommodate four systems replicated three times, all based on corn-soybean rotation. The systems will be (1) no-till without cover crop, (2) no-till with a cereal rye cover crop each year, (3) chisel-plow/disk tillage without a cover crop, and (4) chisel-plow/disk tillage with a cereal rye cover crop each year. During March, soil samples were taken and analyzed. Different P fertilizer rates will be applied to about two-thirds of the areas so the site has the uniform soil-test P level needed. Tillage of six of the areas as called for by the field layout will be done, so the site is ready to implement all systems in the fall. Needed berms will be established, and equipment purchased and installed to begin runoff measurements after crop harvest and cover crops seeding.

March 2014

Two potential sites for a second project on cover crops were visited last October. Since then, the landscape, slopes and other available GIS-based information have been studied to determine the best site for this study. Tillage and crop rotation treatments for the systems to be evaluated will be established this spring, plus runoff-monitoring systems installed. This project is designed to determine the impact of a rye annual cover crop on sediment, phosphorus and nitrogen loss.