Impacts of Cover Crops on Phosphorus and Nitrogen Loss with Surface Runoff
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.
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.
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.
The last reports shared 2018 results for soil, dissolved-reactive P (DRP) and total P losses. This semester’s work focused on analyzing runoff for total N, dissolved ammonium and nitrate, and on field work. The N loss was much smaller for NT than for TILL, but the cover crop effect was small and inconsistent. Total N losses were 9 and 8 kg N/ha for NT and NTCC, but were 27 and 39 kg N/ha for TILL and TILLCC. Most of the N lost was organic N (87 to 95% of the total). Dissolved ammonium and nitrate losses were very small and contributed little to the total N loss. Ammonium losses were 0.32, 0.16, 0.71, and 0.83 kg N/ha for NT, NTCC, TILL, and TILLCC. Nitrate losses were 0.54, 0.86, 2.3, and 1.1 kg N/ha for NT, NTCC, TILL and TILLCC.
Soybean yield in 2018 was greatly affected by excess late spring to early summer rainfall. Harvest was done with a combine equipped with a yield monitor using RTK, and ArcGIS was used to determine yield for each of the 12 areas. Yield was slightly lower with NT than with TILL (as was for soybean in 2016). Yield for NT was similar with or without cover crop (32.1 and 32.0 bu/acre) but yield for TILLCC was slightly lower (35.5 bu/acre) than for TILL (39.9 bu/acre). No obvious rye growth differences existed by the time of soybean harvest. Chemical analyses of post-harvest soil samples were completed. System effects were similar for Bray-1, Mehlich-3, and Olsen methods. There was the expected P stratification within the top-6-inch soil layer, and use of the cover crop slightly reduced soil P. The Bray-1 results for a 0-2 inches depth were 51, 41, 56 and 51 ppm for NT, NTCC, TILL, and TILLCC, whereas for 2-6 inches were 28, 19, 25, and 18 ppm, respectively. A 6-inch sampling depth is recommended for both tillage systems. Bray-1 results were 36, 28, 35 and 31 ppm for NT, NTCC, TILL, and TILLCC, respectively. The P stratification was expected but differences due to the cover crop were not observed in the past, which could have resulted from a random year effect or cumulative effects over the four years of the study.
The rye cover crop biomass was sampled April 15, and yield was 720 kg DM/acre, which was approximately similar for both tillage systems. Corn was planted on June 4 (later than normal due to frequent rain). March-June rainfall was 17 inches, distributed in frequent events of low intensity. Of eight runoff events, only one event caused runoff from all areas. A total 296 runoff samples were collected, with a subsample filtered for analyses of DRP, ammonium and nitrate. Total P, organic N and total solids are being measured on unfiltered runoff. Runoff will continue to be collected and analyzed until the fall corn harvest.
Outreach this period included a field day scheduled for Aug. 19.
As was the case for the late spring and summer months, rainfall and surface runoff during the fall 2018 quarter at the research field were higher than for the previous years. Rainfall during this quarter was 168 mm and there were five runoff events. A total of 310 runoff samples were collected. These samples have been analyzed for dissolved reactive P, total P and total solids (soil) concentrations. In contrast to results for late spring and summer, the amount of runoff and losses of soil, runoff dissolved reactive P and total P were the greatest for the system managed with tillage without cover crop, intermediate for no-till without a cereal rye cover crop or tillage with a cover crop, and the smallest for no-till with a cover crop. In the previous quarter that was the ranking for soil and total P loss. For runoff dissolved reactive P, however, the order of loss from the greatest to lowest was tillage with a cover crop, tillage without a cover crop, no-till with a cover crop, and no-till without a cover crop. These results are from preliminary raw data, at this time we are putting together and studying in detail the data collected during the entire year.
Analysis of the runoff samples for ammonium, nitrate and total N continues. The cereal rye cover crop was over-seeded on September 12 when most soybean leaves were yellow. In spite of this early rye seeding date, the excess soil moisture and earlier than normal fall cold temperatures resulted in uneven germination and growth even with a seeding rate of 2.5 bu/acre. Soybeans were harvested October 27. Soybean growth was greatly affected by excess late spring and early summer rainfall and poor herbicide effectiveness, which resulted in low and extremely variable yield across and within the 12 small watersheds. At this time, the team is carefully working with the field observations, the yield monitor file and GIS methods to obtain the best possible yield estimates for each watershed. Post-harvest soil samples were taken October 30 and are being analyzed.
Excessive rainfall and runoff in late June, much higher than normal, did extensive damage to berms and terraces of the study site. Therefore, during this quarter, extensive repair work was done by project personnel and specialists contracted by Committee for Agricultural Development (an affiliate organization of College of Ag and Life Sciences that owns the field). There was no direct damage to the soybean crop. Rainfall and runoff during the quarter also was higher than normal, but there was no additional damage. Rainfall during this quarter was 20 inches, and it produced four runoff events that resulted in many runoff samples. We have completed the phosphorus (P) analyses of runoff collected from May until the July 6 runoff event. Presently we are conducting the analyses of runoff for total solids and nitrogen (N) for that period and processing the more recent runoff samples for analysis.
The preliminary results show that dissolved reactive phosphorus (DRP) and total P concentrations were highest for the system managed with tillage and a rye cover crop, and the lowest with small differences for the other three systems (tillage without cover crop and no-till with or without cover crop). The loss (which considered runoff volume) of both runoff P fractions showed different results. For total P, the ranking, from higher to lower loss, was tillage without a cover crop, tillage with cover crop, no-till without a cover crop, and no-till with cover crop. On the other hand, the ranking for DRP loss, from higher to lower loss, was tillage with cover crop, tillage without a cover crop, no-till with cover crop, and no-till without a cover crop.
During this quarter, the P chemical analyses of runoff from the rainfall simulations that were conducted during the dry early spring period were completed. In these simulations, the soil and total P losses were the highest for the system managed with tillage without a cover crop, the lowest for no-till with cover crop, and intermediate for the other two systems. In contrast, the DRP losses were the lowest for the system managed with tillage without a cover crop and higher for the other three systems (with small differences among them).
No strong conclusions are possible because significant runoff has been occurring during September and October. However, the partial results for the first half of 2018 are that no-till and a cover crop clearly reduce total P loss, which is consistent with results from previous years. However, the effects of these two management practices on dissolved P loss interact in a complex and inconsistent way over time.
There was no surface runoff from any of the 12 small watersheds of the research site this quarter. Laboratory and data management work consisted of continuing analyses of corn grain, soil and runoff samples collected in previous periods. The grain analyses and calculated N and P removed with harvest were completed. Grain yield was the highest for tillage without cover crop, intermediate for no-till without a cover crop and tillage with a cover crop and the lowest for no-till with cover crop. The N removed with grain was the highest for tillage without cover crop, intermediate for no-till without a cover crop and tillage with a cover crop, and the lowest for no-till with cover crop. The P removed with grain showed very small differences for tillage with or without cover crop and no-till without a cover crop, but much lower removal for no-till with cover crop.
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.