Impacts of Cover Crops on Phosphorus and Nitrogen Loss with Surface Runoff
Surface runoff accounts for most of the phosphorus (P) delivered to streams from Iowa fields. However, important fractions of runoff P are dissolved reactive P (orthophosphate P) and particulate (sediment-bound) P. Dissolved P is readily available to aquatic organisms and a large proportion of particulate P becomes available over time, depending on properties of the receiving water body. Recent surveys of Iowa streams and in the Lake Erie watershed suggest the amount of dissolved P loss from fields and its impact on water quality is greater than often assumed. Iowa research has shown higher orthophosphate P loss with fertilizer than with manure, and some conservation practices that reduce erosion and particulate P loss may not reduce -- or may even increase -- dissolved P loss. Assessment of dissolved P loss has become especially important because it is the most active runoff P fraction impairing water quality.
This research continues evaluations already underway for one more year. This is needed because weather at the research site (mainly rainfall) did not provide consistent runoff during the last four years and no reliable conclusions can yet be drawn.
- To continue assessing for one more year (2019 crop year) the impacts of a winter cereal rye cover crop on soil, P, and nitrogen (N) loss with surface runoff in a field testing very high in P, managed with a corn-soybean rotation and no-till or chisel-plow/disk tillage.
- To complete data management, statistical analyses and final reporting of soil-test measurements, crop yields, soil loss and several N and P fractions for five crop years (2015 through 2019).
The project has been developed at a 42-acre field at the Hermann Iowa State University farm located south of Ames where 12 areas ranging from 1 to 3 acres accommodate four systems replicated three times. Evaluations include, as in previous years: (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.
This one-year project is a continuation for a fifth year of an INRC study that has been evaluating soil, P, and N loss on a field scale and under natural rainfall in corn-soybean rotations from soils testing high in P, as affected by tillage and cover crops. Four systems replicated three times have been evaluated in 12 small watersheds. The systems are no-tillage; tillage by spring chisel-plowing/disking of corn residue; disking of soybean residue with a cereal rye cover crop; and disking of soybean residue without a cereal rye cover crop.
During this period, analysis was completed of rye cover crop biomass sampled on April 15 for dry matter and N and P concentrations. Dry matter yield was 762 and 678 kg/ha for no-till or tillage, lower than last year due to adverse weather after overseeding it on Sept, 5, 2018. N and P recycled were 29 and 27 kg N/ha with no-till, and 3.4 and 2.6 kg P/ha, respectively.
Rainfall from July to December was 16.3 inches in 20 events from July until the corn harvest and one-inch total after that. Given mostly low intensity rainfall and excellent corn soil cover, there were only two runoff events during this period, resulting in collection of 57 runoff samples. Preliminary results for average runoff concentrations for all events of 2019 were summarized.
The DRP concentration was slightly higher for no-till with or without cover crop (1.4 and 1.5 mg P/L), than for tillage with or without cover crop (1.1 and 0.9 mg P/L). Total P, which includes both dissolved and particulate P, was higher for tillage with or without cover crop (2.8 and 4.7 mg P/L) than for no-till with or without cover crop (1.8 and 2.0 mg P/L). The cover crop had little effect on DRP with no-till or tillage and on total P with no-till. However, the cover crop with tillage reduced by one half total P.
Nitrate and ammonium concentrations were higher with tillage with or without cover crop (3.5 and 3.3 mg/L nitrate-N, and 2.0 and 2.3 mg/L ammonium N, respectively), than for no-till (1.2 and 2.5 mg/L nitrate-N, and 1.0 and 1.9 mg/L ammonium N, respectively). The cover crop significantly reduced concentrations of both dissolved N forms only with no-till. As expected, runoff sediment concentrations were much greater with tillage with or without cover crop (1.6 and 4.0 g/L) than for no-till with or without cover crop (0.33 and 0.35g/L). The cover crop significantly reduced sediment concentrations with tillage but not with no-till.
Work continues with data management. The rye cover crop was overseeded Sept. 5. Corn grain was harvested Nov. 4, with a combine equipped with a yield monitor and using RTK GPS, and data management continues. Soil samples were taken from all systems from depths of 0-2 and 2-6 inches after corn harvest.