How do cover crops impact field Nitrogen and Phosphorous Losses?

field of cereal rye cover crop

Cover crops are used for a variety of reasons in a crop rotation. Adding cover crops can improve soil structure, retain nutrients, suppress weeds, reduce water and wind erosion, increase soil organic matter and provide forage. The presence of a cover crop can also influence the amount of nitrogen, phosphorus  and sediment in surface and subsurface water leaving at the edge of field. A summary of cover crops water quality impacts from Ohio field comparisons was recently published.

The data comes from a project measuring nutrient losses from production fields led by the USDA-ARS Soil Drainage Unit housed at The Ohio State University. This group has been monitoring water quality impacts in farmer production field since 2011, with a network of 40 monitored field sites. Data from 2012 to 2019 was evaluated for cover crop comparisons. If a growing crop was present from November-April period, that field was considered to have a cover crop. A variety of species were included as cover crops: alfalfa, clover, hay, oats, mustard, radish, rye, wheat and multi species mixes.

The most consistent and significant impacts were reductions of Nitrate-N loss. When cover crops were grown the average monthly tile Nitrate-N loads were reduced by 50%. The amount of monthly nitrate-N load reduction was 0.9 to 2.3 pounds per acre with an annual reduction of 18 pounds.

Figure 1 shows the monthly nitrate loads.  The reduction occurred primarily in the January to June period which are the months with the highest losses. Without a growing crop, nitrates in the soils are subject to leaching through the tile system.

Figure 1. Monthly tile nitrate loads from Ohio field sites comparing cover crops versus no cover crop during 2012 to 2019 water years.
Total Phosphorous (TP) and Dissolved Reactive Phosphorous (DRP) tile loading did increase with a cover crop. The losses of TP and DRP from surface runoff show decrease. For both tile and surface runoff, the amount of change measured as pounds per acre is small and not statistically different based on annual average monthly losses. The lack of a statistical difference indicates a high amount of variability in the P data.

Overall P losses are influenced by more factors than nitrogen. Characteristics of the cover crop species including the amount of P uptake and response to freeze-thaw cycles plus severity of temperature changes along with climate, soil characteristics and fertilizer management make P loss site specific.

Figure 2 shows the monthly average losses of DRP. The monthly quantity of DRP loss is a fraction of a pound with the highest losses occurring in November through June.

Figure 2. Monthly tile DRP loads from Ohio field sites comparing cover crops versus no cover crop during 2012 to 2019 water years.
A summary of all variables compared in the comparison of cover versus no cover crop are shown in Table 1.

Summary messages from Table 1:  

    • Cover crops are effective to reduce N losses with reductions of 18 to 19 pounds per acre (52-55% decrease)
    • Reductions in tile nitrate were much greater than the numeric increases in tile DRP and TP and the P losses were not statistically different.
    • Reductions in surface DRP and TP were greater than the increases in tile DRP and TP.

Table 1. Summary of all water impacts measured comparing cover crops to no cover crop during 2012 to 2019. Arrow pointing up indicate an increase. Arrow pointing down indicate a decrease.
The take home message is cover crops are a very effective tool for reducing nitrate nitrogen losses. Reducing nitrogen losses is a goal from both the Ohio River and Lake Erie watersheds. Phosphorus impacts of cover crop are variable but trend toward being neutral when both surface and tile losses are considered. More work looking a cover crop species and site interactions is needed to assure unintended consequences on P loss are not experienced.

You can view a presentation on this data given by lead author Dr Brittany Hanrahan from 2021 Conservation Tillage and Technology Conference at https://go.osu.edu/ctccovercrop

For more general information about cover crops and water quality see https://agbmps.osu.edu/bmp/cover-crops-nrcs-340

Source:
Hanrahan, B.R., K.W. King, E.W. Duncan and V.S. Shedekar. (2021). Cover crops differentially influenced nitrogen and phosphorus loss in tile drainage and surface runoff from agricultural fields in Ohio, USA. Journal of Environmental Management 293.

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