The goal of the proposed research is to provide data on how a range of cover crop practices following both corn and soybean crops impact the loss of phosphorus by surface runoff. We will investigate several mechanisms by which cover crops could affect the loss of phosphorus, including: 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 concentration of P-carrying sediment in runoff water or 4. Increase the concentration P dissolved in runoff water. Phosphorus reduction might occur by plant uptake and phosphorus increase might occur by freezing injury that releases soluble phosphorus from cover crop tissues. Research has already been published that compares the solubility of phosphorus in live and dead tissues from a wide range of cover crop species. What is lacking, and our research will provide, is data that shows the actual runoff volume and P concentration from single species or multi-species cover crops. We will generate this data from research plots and farm fields using simulated and natural rain events during the cover crop season. 2022-2023 will be the third and final year of this P runoff project.

1. Determine effect of individual species and mixed cover crop on:
a. Runoff volume generated as percent of rainfall.
b. Time and rain volume required to cause runoff to begin.
c. Concentration of total and dissolved phosphorus in runoff water.
d. Total P load lost to runoff during a single storm and all storm in a whole season.
2. Determine effect of early interseeding establishment of multispecies cover crop on runoff volume and P content, as compared to cover crop drilled after crop harvest and no cover crop.
3. Compare effect of multispecies cover crop on runoff at different times of year:
a. Fall
b. Winter
c. Spring
d. Early summer

We propose to use two main tools to measure cover crop impacts on phosphorus runoff from no-till fields. The two tools are namely the portable Cornell rainfall simulator and semi permanently installed mini runoff weir. Both are small-scale instruments that measure runoff as affected by field conditions. The runoff weirs will be installed after the cover crop emerges in non-wheel tracked areas of representative cover crop growth since research (Kaspar et al., 2001) has shown that compaction due to wheel traffic can have a greater effect on runoff than cover crops. The big advantage of such small scale measurements is that they can be replicated on a number of sites and treatments. The disadvantage is that they represent only the crop-soil conditions and not the whole field watershed properties. The cost to instrument a whole field water for runoff is prohibitive for this program (>$20,000 for a single watershed treatment). We propose to bridge this gap by installing replicated miniweirs within one or two existing large, established instrumented watersheds so that results can be compared and correlated for several storms with regard to P concentrations and volumes of runoff.

The Cornell rainfall simulator can be moved from plot to plot and is not permanently installed in the field. It does not depend on natural rainfall events but provides its own simulated rain at a set intensity using deionized water. This apparatus was developed at Cornell University and involves about 100 small tubes that provide droplets that simulate the impact of rainfall at a controlled rate. All of the rain is confined so that the runoff has to leave the soil surface through a tube that leads to a collection bottle at a lower elevation. Using a constant rainfall rate, the simulator can determine hydrologic parameters such as time after rain initiation when runoff begins and soil infiltration capacity. It also allows for collection of the runoff water to measure its volume and analyze its contents.

The PI’s lab currently has three of these Cornell rainfall simulators, two of them just purchased with current MSB funds from this project. They can be most efficiently used two at a time in tandem. A single operator can set up two Cornell rainfall simulators with the start of rainfall offset by 15 minutes. The rainfall simulators will be used where a large number of treatments are involved or where the travel time to sample after each natural rain event is prohibitive.

The mini erosion weirs are 75 cm long and 40 cm wide. They are installed facing downslope, 5 cm below the ground with 10 cm above the ground. They are designed to collect the runoff from a 0.31 m2 area. They will be installed immediately after the last cover crop planting for an experiment in the fall, generally in October. They will be left in the ground until spring planting in late April or early May. In some cases they will be removed and reinstalled after planting to measure runoff from early-season (May-June) storms when the summer crop has not created a full canopy. A set of 12 of these weirs were used successfully to collect preliminary runoff data from three rain storms during an earlier spring period (see Figure 3). The PI currently is installing 24 of these mini erosion weirs in cover crop treatment plots in anticipation of
continuation of this project.

Analysis of samples.
We propose to analyze runoff water samples from both types of apparatus for the following parameters.
1. Volume of runoff, expressed as millimeters (or inches) as well as percent of rainfall.
2. Amount of sediment in runoff, expressed as grams per square meter or pounds per acre.
3. Concentration of total phosphorus as ppm or milligrams per liter
4. Concentration of dissolved reactive phosphorus as milligrams per liter
5. Concentration of dissolved organic phosphorus as milligrams per liter

Prior to determination of dissolved phosphorus the runoff water samples will be vacuum filtered through a 0.45 micron polycarbonate membrane. Organic phosphorus will be determined as the difference in dissolved reactive phosphorus before and after persulfate digestion (Johnes and Heathwaite, 1992). All phosphorus analyses will be run on a Lachat flow auto analyzer using an orthophosphate manifold and a modification of the ascorbic acid method (Watanabe and Olsen, 1965). Loading of the various forms of phosphorus will be calculated as P concentration x runoff volume and expressed as milligrams per square meter and pounds per acre.

Results will be reported for individual rainfall events for both the simulated events using the Cornell rainfall simulator and for natural rainfall events (great enough to generate runoff) using the mini erosion weirs. For the experiments with mini erosion weirs installed, it will also be possible to sum up the total phosphorus loss for the season.

Cover crop treatments for 2020-2023.
Using the Cornell rainfall simulator, various cover crop species will be tested for impact on P in runoff from no-till fields. These species will include the following (including a no-cover crop control):
1. Cereal Rye
2. 3-species mix (Radish + rye + Crimson Clover)
3. No cover control

Two of these monitored sites are on coastal plain soils at the Central Maryland research and education facility at Beltsville Maryland. However two commercial fields with medium to high phosphorus risk soils (average soil phosphorus Fertility Index Value of 200-300) on the lower Eastern Shore will also be investigated using the portable Cornell rainfall simulator. The same cover crops will be established by interseeding in early September / late August or drilled after crop harvest in late September or October on these commercial fields. One of these fields will be in soybean and the other in corn in sprng 2022. A control strip will also be left without cover crop for comparison. The rainfall simulation and runoff collection will take place on at least two occasions (November and April) with 4 replicated rain simulations comparing the cover cropped area to the no cover control in each field.

Update:

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Update:

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Update:

View uploaded report

Nutrient Loss in Runoff - Do Cover Crops Make It Better or Worse?
Lay Language Summary of Work April 2022 – March 2023
Ray Weil
Environmental Science and Technology Department
University of Maryland

The goal of the proposed research is to provide data on how a range of cover crop practices following both corn and soybean crops impact the loss of phosphorus (P) and nitrogen (N) in surface runoff. Both P and N cause water quality deterioration by eutrophication.

We are investigating several mechanisms by which cover crops could increase or decrease the loss of P. Most runoff research has been conducted on plowed soils where P attached to sediment is the main pathway of P loss. However, most Maryland farms use some version of no-till soil management which leaves the soil surface undisturbed and mulched with crop residues during the off-season. No-till soil management tends to greatly reduce the amount of soil that erodes and carries away P during intense rain events, but no-till, especially combined with cover cropping, is thought to stratify P near the soil surface, increasing the concentration of P that come into contact with rainwater and can be dissolved in the runoff. Phosphorus in runoff might be reduced by cover crop P uptake but it might be increased by freezing injury that releases soluble P from cover crop tissues. Research has already been published that compares the solubility of phosphorus in live and dead tissues from a wide range of cover crop species. What is lacking, and our research will provide, is data that shows the actual runoff volume and P concentration from single-species or multi-species cover crops in differing soil in a no-till system.

We are generating this data from field plots that represent Maryland’s typical long-term no-till crop production with retention of all crop residues and the use of some kind of cover crop during the winter season. The field used to collect runoff from natural rain events has silt loam topsoil with clayey subsoil and somewhat impaired drainage that limits infiltration and favors the production of runoff during heavy rain events. To study runoff from controlled simulated rainfall, this silty field and a field with a similar history but very sandy soils were both used. Both fields have been managed with no-till techniques for most of the years since 1993, with the Maryland nutrient management program since 2000, and have had some kind of cover crop in most years since 2006. The current cover crop treatments were imposed in 2020, so the year covered by this report was the third year of the cover crop treatments. In 2020 a no-cover crop control treatment (No cover) was established with only weeds growing between crop harvest in fall and crop planting in spring. This control treatment essentially represents the withdrawal of cover cropping from a system that had cover crops for 15 years, while allowing any winter weeds to grow. The other two cover crop treatments represent the enhancement of the previous system by the extension of the cover crop growing season with earlier planting in fall and later termination in spring. These two cover crop treatments are interseeded several weeks prior to corn or soybean harvest using a high clearance air-seeder with drop tubes that spread the seed on the ground under the senescing crop canopy. The two cover crops treatments intersown were a single-species cover crop of cereal rye (Rye) and a three-species cover crop mixture of forage radish, cereal rye (Rye) and crimson clover (Clover).

The project is measuring the effect of the three cover crop treatments (No cover, Rye, and 3-Way) on the amount of runoff generated and the hydraulic properties of the soils as well as the concentrations of the nutrients, nitrogen and phosphorus, in the runoff water. The project is studying the runoff from both natural and simulated rain events during the cover crop growing season. The runoff from natural rainfall events is captured using mini-erosion weirs that channel the runoff from a 0.3 m2 area into a buried 5-liter jug. Because of the current and historical no-till practices and crop residue cover, the soi is protected from direct raindrop impact and surface sealing. Rather intense rainfall is therefore required to generate any runoff, resulting in sporadic and uneven opportunities to collect runoff from natural rain events. The rainfall simulations can apply water at a controlled rate that is high enough to ensure the production of runoff. Our rain simulations are performed with distilled water in a Cornell sprinkler infiltrometer that provides a controlled drip rate from hundreds of capillary tubes and collects the ponded water through a tube leading to a 1-liter bottle buried downslope that is replaced repeatedly until 5 liters of runoff have been collected.

Detecting the effects of cover crops, positive or negative, on runoff volumes and nutrient concentrations was difficult because we were interested in the long-term, no-till cropping situations managed under conservative nutrient management plans as is typical of farming in Maryland. The differences between two soils of contrasting textures, on the other hand, were obvious and highly significant, especially with regard to the rates of infiltration and runoff. Somewhat surprisingly, in a corn-soybean rotation, the previous crop going into the winter had a greater effect on some runoff and nutrient loss parameters than did the presence of a cover crop. Generally, in the systems studied, the nutrient concentrations in the runoff were quite moderate, and dissolved N and P were both present mainly in the rarely measured organic forms.

During the remaining year of this project, we will plan to analyze both inorganic and organic forms of the dissolved nitrogen and phosphorus associated with the runoff from the natural and simulated rain events sampled in the winter-spring of 2023, and assess the long-term impact of enhanced cover cropping on nutrient loss potential.

While cover crops can provide many benefits to the farmer, the Maryland cover crop program is primarily focused on the reduction of nitrogen loading to the Chesapeake Bay. The main pathway for nitrogen losses from farm fields is via groundwater contaminated soluble nitrogen by leaching. 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, 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 rain storms or heavy snow melt. A secondary pathway in areas of poorly drained sandy soils is leaching of phosphorus to drainage ditches. There is little research on how cover crops impact phosphorus losses. Some studies that suggests that cover crops might increase soluble phosphorus at the soil surface where it would be susceptible to becoming dissolved in runoff water. In fact, 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 the proposed research is to provide data on how a range of cover crop practices impact the loss of phosphorus by surface runoff.