How Cover Crops Reduce Nitrogen Input and Improve Soil Health

Kritsanee Iamjud, Fultz, Lisa

Kritsanee Iamjud and Lisa M. Fultz

The cover cropping system is an effective tool and a best management practice for agricultural sustainability and can help farmers reduce the amount of nitrogen fertilizer that they have to apply to fields, thus saving them money and increasing profitability.

In an initial study, winter cover crops and a fallow control were examined with four different nitrogen fertilizer application rates (zero, 80, 160, 240 pounds of nitrogen per acre) in small-scale corn production as part of a no-till system at the LSU AgCenter Macon Ridge Research Station in Winnsboro, Louisiana. Cover crops were planted in October of 2016 and 2017 and terminated in early February of 2017 and 2018. Corn was planted around March 15 and harvested in August each year. Soil samples from 0 to 3 inches in depth were collected in March after cover crop termination and in October before cover crop seeding. Soils analysis included soil nutrients, organic matter, inorganic nitrogen, microbial community composition and soil enzymes (contribute to biomass breakdown and nutrient release).

The results showed that legume cover crops (hairy vetch, berseem clover, Austrian winter pea and crimson clover) increased corn grain yield overall and maximized yield at 80 pounds of nitrogen per acre compared to grass (cereal rye), brassica (tillage radish) and mixed grass and brassica (cereal rye and tillage radish), which maximized corn grain yield at 160 pounds of nitrogen per acre (Figure 1).

After corn harvesting in the fall, soil nitrate-nitrogen concentrations increased with higher nitrogen fertilizer application rates. However, after cover crop growth and termination in the spring, the soil nitrate-nitrogen concentrations decreased with no difference between fertilizer rates, demonstrating that cover crops used the excess soil nitrate-nitrogen to be released back to the soil after termination. Of the cover crop types, leguminous cover crops contributed more soil nitrate-nitrogen than the grass and brassica treatment. Cover crop types did not affect soil organic matter, but nitrogen applications increased soil organic matter by 8% compared to fallow treatment. Moreover, among the extractable soil macronutrients, different cover crop types impacted soil phosphorus and potassium concentrations. The legume treatment had 19% higher concentrations of phosphorus than grass and brassica. In contrast, the grass and brassica treatment had greater potassium concentrations than legume species.

The establishment of cover crops and nitrogen applications benefited soil health parameters, including soil enzymes for carbon and nitrogen cycles. The microbial community biomass was higher in spring after termination of cover crops and lower in fall after corn harvesting. With different nitrogen rates, the total microbial abundance was great at 80 to 160 pounds nitrogen per acre. The grass and brassica treatment increased arbuscular mycorrhizal fungi (a group of fungi that can improve nutrient and water uptake in plants) compared to leguminous species.

On-farm demonstrations of three sites in northeast Louisiana were investigated for three years in soybean-corn rotation systems. The objective of this study was to demonstrate the impact of fallow season cover crops on soil health in row-crop production systems. Multispecies mixtures of cover crops were established in the fall following harvest. Soil samples were collected to a 6-inch depth following cash crop harvesting and before cash crop planting from 2017 to 2020. Soil health parameters included soil organic matter, soil enzymes, soil respiration, soil active carbon, soil protein-nitrogen and soil microbial community composition.

The data revealed that multispecies cover crop mixtures did significantly affect soil organic matter (for sites 1 and 3), soil carbon cycle enzyme and soil protein-nitrogen (for site 3) concentrations, with mixtures of cover crops being higher than fallow. Soil enzymes, protein-nitrogen (plant available nitrogen) and active carbon (easily broken down) all increased over time in all cover-cropped fields.

Overall, the incorporation of cover crops for crop rotation was able to reduce nitrogen fertilizer needs, sustain corn grain production and improve soil health in the small scale. The legume cover crop treatment maximized yield at 80 pounds of nitrogen per acre, while grass and brassica maximized corn grain yield at 160 pounds of nitrogen per acre. Of on-farm demonstrations, multispecies cover crops improved soil organic matter, soil protein-nitrogen and soil carbon cycle enzymes. Moreover, the indicators of soil health were increased over time.

Kritsanee Iamjud is a graduate student, and Lisa M. Fultz is an associate professor, both in the School of Plant, Environmental, and Soil Sciences.

(This article appears in the fall 2020 issue of Louisiana Agriculture.)

Crimson clover, black oat and tillage radish standing in the field.

Cover crop mixture of legume, grass and brassica species established at the Macon Ridge Research Station, Winnsboro, Louisiana. Photo by Kritsanee Iamjud

The field on the left is green with cover crops, and the field on the right is brown soil with no cover crop.

An on-farm demonstration of a cover crop treatment to the field on the left and no cover crop on the field at right during the fallow season at the farm of Macon LaFoe, Monroe, Louisiana. Photo by Lisa M. Fultz

Lamjud line chart.

Figure 1. Cover crop and nitrogen rate interaction on corn grain yield. The legume cover crops were hairy vetch, berseem clover, Austrian winter pea and crimson clover, and the nonlegumes were cereal rye and tillage radish and mixed cereal rye and tillage radish.

1/4/2021 10:24:01 PM
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