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. Our experiments include
cover crops growing in both corn and soybean residues. If funded, 2023-2024 will be the final year of this P runoff
project during which we will complete runoff measurements in April and May, analyze runoff samples accumulated
from 2022-2023 runoff events. Since preliminary results and literature evidence suggests that a significant portion of
nutriets in runoff may be organic rather than phosphate or nitrate, the lab analyses of these samples will include
sediment, sediment-associated N and P, and dissolved inorganic and organic forms of P (and N). These results will be
statistically analyzed and final reports, publications and recommendation created.

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 reactive phosphorus in runoff water.
d. Total P load lost to runoff during a single storm and all storms in a whole season.

2. Determine the total phosphorus in the runoff and the proportion that is in organic forms as affected
by
a. Crop residue (corn compared to soybean).
b. Cover crop treatment (No cover, rye or rye-radish-clover mix).
c. Time after crop harvest (seasonal changes)

3. 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.

4. 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.

Our lab currently has three of these Cornell rainfall simulators, two of them purchased with previous MSB funds. 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. We have installed
24 of these mini erosion weirs in cover crop treatment plots in anticipation of continuation of this project.

Update:
Progress Report for Maryland Soybean Board Project
"Phosphorus runoff from no-till soils – do cover crops make it better or worse?"
Ray Weil – University of Maryland

In this final year of the runoff project, we continue to work with two contrasting soils (one sandy and one silty clay) with plots of three basic cover crop treatments: no cover control (weeds only), rye cover cop, and a 3-way mix of radish, rye and crimson clover. We collected runoff samples from natural rainfall during April and from simulated rainfall during April and part of May 2023. Also, during April and May 2023, we sampled the near-surface soil around each of the erosion weirs and separated the samples into the upper 2.5 cm (or 1 inch) and the next 12.5 cm (or 5 inches). The upper 2.5 cm of soil represents the layer most likely to interact with rainfall. This soil is now in the process of being extracted to determine the easily soluble phosphorus that is likely to desorb from the soil into runoff water.

We conducted simulated rainfall on two fields. Each run produced five runoff samples taken sequentially plus a sample of the distilled water used to produce the rain to check for any contamination. We must have many hundreds of samples to analyze and have been working on the nitrogen and phosphorus analysis during the months of May, June, and July. We are analyzing dissolved inorganic nitrogen in the ammonia nitrate form, as well as reactive phosphate for phosphorus. We are then “cooking” the samples under high pressure in an autoclave using an alkaline persulfate digestion mixture to convert all organic nitrogen and phosphorus to nitrate or phosphate ions. These digested samples then need to be analyzed again for nitrate and phosphate to obtain the total dissolved nitrogen and phosphorus. Then by subtraction, we calculate the amount of nitrogen and phosphorus in organic forms.

The data on nutrients carried in runoff water will comprise part of the thesis for a master’s graduate student who has been working on this project for several years and expects to complete her degree in 2024.

Update:

View uploaded report

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 studies, 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 (Sedghi and Weil, 2022; Sedghi et al., 2022).

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 the 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 (Hallama et al., 2019). 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. Cover crops can affect the loss of phosphorus by several, somewhat contradictory, mechanisms.

Cover crops might reduce P losses in runoff by:
• Reducing the volume of runoff water from a storm.
• Increasing the amount of rain required to start runoff from fields.
• Protecting soil from erosion thus reducing the P-carrying sediment in runoff water.
• Reducing phosphorus in surface soil during winter because of cover crop P uptake.
• Reducing the need for applying phosphorus fertilizer.

Cover crops might increase P losses in runoff by:
• Increasing the concentration of P in the upper few cm of soil.
• Leaving high-P plant residue on the soil surface.
• Increasing concentrations of P dissolved in runoff water due to rain interacting with above.
• Releasing P from cover crop tissues during freezing injury or frost-kill of the cover crop.

Research has already been published that compares the solubility of phosphorus in live and dead tissues from a wide range of cover crop species (Cober et al., 2018; Miller et al., 1994). Winter-killed brassica cover crops have been shown to concentrate soil test extractable PO4-P at the soil surface in spring (White and Weil, 2011). 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 (Grove et al., 2007).

A few studies around the world have investigated cover crop effects of P runoff, but we found none in Maryland and none using multi-species cover crops. A perennial forage vegetative cover during winter in Manitoba, Canada, resulted in more than double the soluble P and total P loads in runoff from snow melt as compared to dead annual crop residue cover (Liu et al., 2014). The increase was attributed to P dissolving out of the frost injured green plant tissue. A study on soybeans in
Missouri (Zhu et al., 1989) reported that runoff volume from erosion plots was reduced by 44 to 53% by the presence of three grass cover crops, but soluble P concentration in the runoff was increased by 161 to 286%, resulting in less runoff water but more soluble P loading from the cover cropped plots. A recent study in Iowa (on a no-till soils) reported that a rye cover crop, despite having only modest biomass and being planted up and down slope, reduced both runoff volume and P concentration in the runoff from a 65 mm simulated rain. The runoff was 27 mm with bare soil between corn stover rows and only 9.5 mm with a cereal rye cover crop. The total dissolved P concentration in the runoff water was reduced from 21 mg/L to 9.3 mg/L, thus reducing the total soluble P loss from almost 6 to less than 1 kg P/ha (Korucu et al., 2018).
These values should be viewed in the context of the 0.05 mg/L dissolved P environmental limit for streams flowing onto lakes. Our preliminary data from the winter of 2022 show that cover crops in corn and soybean residue may significantly reduce runoff volume by increasing infiltration rate.