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Phosphorus runoff from no-till soils - do cover crops make it better or worse?
LAY SUMMARY RESEARCH CONDUCTED APRIL 2020 TO MARCH 2021

Ray Weil
Dept. of Environmental Science and Technology
University of Maryland, College Park, Md

While cover crops can provide many benefits to the farmer, the Maryland cover crop program is primarily focused on the reduction of nitrogen (N) loading to the Chesapeake Bay. The main pathway for nitrogen losses from farm fields is via leaching that contaminates groundwater with soluble nitrogen (mainly nitrate). Research, including our work sponsored by the Maryland Soybean Board, have clearly shown that cover crops can be very effective in reducing such nitrogen leaching and that their effectiveness is dependent on early cover crop establishment in fall.

Water quality troubles in the Chesapeake Bay are related to both nitrogen and phosphorus (P), but much less is known about the impacts of cover crops on phosphorus losses than on nitrogen losses. The main pathway for phosphorus transport from croplands to bodies of water is via surface runoff during intense rainstorms or heavy snowmelt. A secondary pathway in areas of poorly drained sandy soils is the leaching of phosphorus to drainage ditches. There is little research on how cover crops impact phosphorus losses. Some studies suggest that cover crops might increase soluble phosphorus at the soil surface where it would be susceptible to becoming dissolved in runoff water.

Cover crops can be an important tool for increasing P availability and crop yields in the phosphorus-deficient soils found in many parts of the world where there has been little application of P. Cover crop mechanisms that cycle P and make soil P more soluble and plant–available may also allow high productivity on Maryland farms with lower levels P fertilization. This could be part of a long-term strategy to make farming more sustainable both economically and environmentally. The goal of this research project is to provide data on how selected cover crop practices impact the loss of phosphorus by surface runoff in a corn-soybean production system. Cover crops can affect the loss of phosphorus by several, somewhat contradictory, mechanisms. Cover crops might:

1. Reduce the volume of runoff water from a storm.
2. Increase the amount of rain required to start runoff from fields.
3. Reduce the amount of P-carrying sediment lost in runoff water.
4. Increase the concentration P dissolved in surface soil and runoff water by P cycling.
5. Reduce phosphorus in surface soil and runoff water because of plant P uptake.
6. Release of soluble phosphorus from cover crop tissues during frost injury.

Research has already been published that compares the solubility of phosphorus in live and dead tissues from a wide range of cover crop species. Winter-killed brassica cover crops have been shown to leak nutrients including phosphorus when they are damaged or killed by cold temperatures. As a result, brassica cover crops have increased soil test extractable P at the soil surface in spring. Other cover crops, such as cereal rye, also have been shown to increase soil test P near the soil surface in the absence of P applications, if to a lesser extent than brassicas.

We used two main tools to measure cover crop impacts on phosphorus runoff from no-till fields, namely the portable Cornell rainfall simulator and the semi-permanently installed mini runoff weir. Both are small-scale instruments that measure runoff as affected by field conditions. The runoff weirs are installed in non-wheel tracked areas of representative cover crop growth after the cover crop emerges since research has shown that compaction due to wheel traffic can have a greater effect on runoff than cover crops. The weirs have an area of 0.33 sq. m (0.4 sq. yard) and are installed with 4 cm (1.5 inch) below ground and 10 cm (4 inch) aboveground to prevent water from outside the weir from being collected in the buried jugs attached to the weir by a large black vinyl hose.

The cover crop treatments that were tested in 2020-2021 for impact on P (and nitrogen) in runoff from no-till fields were 1) Cereal Rye, 2) three-species mix of Radish, Rye and Crimson Clover, and 3) no cover crop control. Because of Covid-19 restrictions, we used a single moderately high soil phosphorus site on coastal plain soils (silt loam underlain by silty clay loam) at the University of Maryland research farm at Beltsville Maryland. We also used a sandy site, but no runoff was every produced from that site. The slope of the land where runoff was collected varied from 5 to 6%. In May 2020 we established excellent stands of both corn and full-season soybean. Rye and a mixed species cover crop were drill-interseeded into the corn in June. The cover rye and mixed cover crops were air-seeded into soybeans at early leaf drop or the cover crops was drilled immediately after harvest. We began installing metal erosion weirs and buried runoff collection jugs shortly before harvest of the cash crops for 18 weirs on the fine-textured soil and six on sandy soil (none of which ever collected any runoff water).

Runoff was collected from 15 runoff-producing storms from 13 October 2020 through 30 March 2021 The runoff volume and concentration data were log-normally distributed (many very low values but a few much higher values), and we therefore, we ran the analysis of variance on log-transformed the data.

The nitrate-N concentrations were about 100 times as great as the phosphate-P concentrations in the same samples. The most notable trend was a reduction in both N and P concentrations over time. This trend was similar to what we found in the 2019-2020 cover crop season and suggests that the greatest nutrient losses occur with the first few storm events after crop harvest in fall. Cover crops had little to no effect on the phosphate-P concentrations in the runoff water or on the volume of runoff. However, the runoff from the no-cover plots was higher in nitrate-N than the runoff from either of the cover crop treatments. The runoff from harvested soybean plots was significantly higher in both phosphorus and nitrogen than in runoff from harvested corn plots.

Soil cores were taken around each erosion weir, divided into increments from 0-2.5 cm (0-1 inch) and 2.5-15 cm (1-6 inch), and then composted for each weir. These samples were air-dried and extracted by a very weak solution of a neutral Calcium salt is similar to the soil solution and is considered equivalent to water soluble. The filtrate was then analyzed on the Lachat® autoanalyzer for both nitrate-N and phosphate-P. The trend for both nitrate-N and phosphate-P to be higher in runoff from soybean compared to corn residue plots is further supported by the soil extraction data. There were no effects of cover crop treatments, but there was more soluble phosphate and nitrate in the surface soil layer from plots with soybean residue than in plots with corn residue. This result suggests that the soybean crop and residue concentrated more nutrients on the soil surface than did corn, or that microbes decaying the corn residues immobilized (tied up) more of both nutrients than did the microbes decaying the soybean residue. Finally, as with the 2019-2020 data, it is also notable that we were not able to detect any increase in phosphate-P in the three-species mix cover crop that included forage radish, even after the radish was severely damaged and some plants killed by cold temperatures in January-February 2021.