Impacts of Cover Crops on Phosphorus and Nitrogen Loss with Surface Runoff
Research has shown the agronomic benefits of using annual rye cover crops in corn and soybean production systems to reduce nitrate loss with subsurface drainage. Although reduced soil loss through erosion also should reduce loss of sediment-bound phosphorus from fields, little or no research has been conducted under natural rainfall conditions to determine the impact of cover crops on total or dissolved nitrogen and phosphorus loss with surface runoff.
This project will 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.
Previous funding made it possible to establish a long-term experiment site on an Iowa State University research farm. Cropping systems, tillage systems and runoff monitoring systems were established in 2014, and water quality evaluations began in 2015. The project site involves 12 areas ranging from 1 to 2 acres with four systems replicated three times, all based on corn-soybean rotation. The four systems are no-till without cover crop, no-till with a cereal rye cover crop each year, chisel-plow/disk tillage without a cover crop and chisel-plow/disk tillage with a cereal rye cover crop each year. Runoff is analyzed for total solids, dissolved P, total P, dissolved ammonium-N, dissolved nitrate-N and total N. Total P and N is measured in rye cover crop samples collected in fall and spring to study nutrient uptake and recycling into the soil. These nutrients also are measured in harvested corn and soybeans.
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.
There was no measurable surface runoff this quarter. Fieldwork consisted of harvesting corn, sampling soil of all 12 watersheds from depths of 0-2 and 2-6 inches, and doing site maintenance. Corn grain was harvested Oct. 18 with a combine equipped with a yield monitor and using RTK to have the most accurate georeference possible. At harvest time the cereal rye cover crop showed an uneven stand and little growth, because although it was over-seeded at the ideal time, there had been little rainfall until harvest. ArcGIS was used to determine grain yield for the area of each watershed from where runoff flow was collected. Preliminary yield data, without study of outliers or statistical analyses, showed yield was lower with no-till than with tillage, and also lower with cover crop than without cover crop. Analyses of dissolved ammonium and nitrate in runoff was completed this quarter, while analyses of total N in runoff continues.
Rainfall at the study site during this quarter was slightly below normal and without long-lasting heavy rainfall events. Small, numerous rain events together with a good corn soil cover resulted in exceptionally low soil erosion and runoff. Corn-soybean rotation systems being evaluated with three replications (corn is grown this year) are tillage with or without cereal rye cover crop (Till, Till-CC) and no-till with or without cover crop (NT, NT-CC). There was runoff July 21 from five of the 12 watersheds and on August 21 from three watersheds. The spring quarter also was dry, with runoff occurring from only one watershed May 2 and two watersheds June 14. Runoff samples were analyzed for dissolved reactive P (DRP), total P (TP), and total solid (TS). The largest observed runoff and soil losses were 3.1 mm and 30 kg soil/ha, respectively, which occurred for a watershed managed with tillage and without a cover crop. In spite of very low runoff and P losses, the results showed no-till reduced both dissolved and total P loss compared with tillage, and the rye cover crop reduced both DRP and TP losses for both tillage systems. A rainfall simulation study also showed no-till reduced DRP and TP losses compared with tillage, and that including a rye cover crop in a system managed with tillage decreased the loss of both P forms.
Fieldwork got under way in April. Measurements of the rye cover crop dry matter yield were taken before its termination with herbicide. The above-ground rye biomass was cut randomly from three, one-square-foot areas of each of the six watersheds that included systems with a cover crop. Tillage was done April 25 for six of the watersheds — the other six are managed with no-till. Corn was planted May 8 with 50 pounds N/acre banded with planter starter attachments to 2-inch depth bands placed at each side of the row. This quarter was drier than normal at the site. The three-month rainfall was 9.9 inches distributed in 12 events. This rainfall amount was good for the corn, but there was little runoff. Given the dry weather and a dry forecast, field rainfall simulations were done immediately following planting to compare the systems during the period of the year that normally means the highest surface runoff and erosion risk. Runoff samples from the simulations are being analyzed.
There was no surface runoff at the site 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.
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.
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.
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.
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.
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 of DRP for tillage with cover crop.