Changyoon Jeong, James Hendrix, Josh Copes and Lisa M. Fultz

Cover cropping is the most common practice to reduce nitrate leaching and surface runoff during winter fallow periods when no crops are planted and the following main crop growing season. The use of legume cover crops, either single or combined with others, provides a supplementary nitrogen-rich green manure for the following main crop. Legume cover crops can alter the availability of mineral nitrogen in the field, either decreasing the availability during the cover crop season or increasing the availability after residue is incorporated into the field. Additionally, the cover crop also improves water holding capacity compared to uncovered soil.

Many studies highlight the efficiency of cover crops as a best management practice, promoting higher crop yields and reducing nitrogen loss to the environment. The net effect of cover crops on emission of nitrous oxide, a greenhouse gas, is essentially dependent on the cover crop carbon-to-nitrogen ratio, weather conditions and the management of cover crop residue for the following main crop. Incorporating cover crop residues into the soil often results in a short-term increase in nitrous oxide emissions, particularly with legume cover crops. Some data show higher precipitation also increased nitrous oxide emissions from cover crop residue-incorporated fields. However, less information has been reported on how residue management after the termination of cover crops influences the composition of greenhouse gases emitted, especially nitrous oxide. Residue management practices affect soil mineral nitrogen and soluble carbon availability, water and oxygen content, temperature, pH and soil texture, and are thus expected to have a significant effect on nitrous oxide emissions.

Nitrous oxide, a greenhouse gas more potent than carbon dioxide, is emitted through nitrification, nitrifier-denitrification and denitrification processes. It is responsible for the destruction of the ozone layer and exacerbating climate change. Nitrous oxide emissions are always observed after the application of mineral and organic fertilizers into the soil. The objective of this study was to evaluate nitrous oxide emissions from fields under residue management, either incorporated or surface-applied, after the termination of cover crops. Furthermore, incorporating legume residue potentially reduces the need for applied synthetic nitrogen fertilizer, which is an efficient way to decrease nitrate leaching and recycle nutrients.

The field experiment was carried out at the Red River Research Station in Bossier City, Louisiana, and Macon Ridge Research Station in Winnsboro, Louisiana, with treatments of cowpea (a legume), sorghum-sudangrass and a mixed application of cowpea and sorghum-sudangrass as cover crops. Cover crops were planted in June 2018 and terminated in November 2018. Two weeks after termination, cover crop residues were either incorporated into the soil or left on the surface. The experimental design consisted of two types of residue management and four treatments, including a control. Greenhouse gases, including nitrous oxide, were measured from December 2018 to April 2019. Gas samplers and temperature sensors were installed, and air samples were collected and analyzed for nitrous oxide, methane and carbon dioxide.

Cover crop residue, either on the soil surface or incorporated into the soil, mitigated nitrous oxide emissions compared with nontreated fields in silty loam and sandy loam soils. In addition, the measured nitrous oxide gas emission showed significant differences between treatments (Figures 1, 2 and 3). Cowpea residue incorporated into the soil decreased nitrous oxide emissions compared with the residue left on the soil surface. However, sorghum-sudangrass residue incorporated into the soil showed elevated nitrous oxide emissions. Other scientists have confirmed a minimal effect on nitrous oxide emissions with cover crops when measured for time periods of one year or greater. This short-term study observed some increase in nitrous oxide changes after the cover crop residue was incorporated into soils. Such an increase in nitrous oxide emissions is consistent with results suggested from other studies on the effect of mulches to stimulate nitrous oxide emissions.

Studies on the impact of nitrous oxide emissions associated with residue management after cover crop termination and integration of legume crops are somewhat limited. The adoption of such practices needs to be explored as a best management practice that may benefit crop yields, reduce inorganic nitrogen fertilizer use and address other environmental issues such as nitrous oxide emissions.

Changyoon Jeong is an associate professor of soil and water quality at the Red River Research Station, Bossier City, Louisiana; James Hendrix is an area water resources agent in northeast Louisiana; Josh Copes is an assistant professor at the Northeast Research Station, St. Joseph, Louisiana; and Lisa M. Fultz is an associate professor in the School of Plant, Environmental and Soil Sciences.

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

Jessie Heflin, a student worker, collects gas samples from the stationary chamber in a cover crop field at the Red River Research Station in Bossier City, Louisiana. Photo by Changyoon Jeong

These instruments measure greenhouse gas emissions at a cover crop field at the Red River Research Station, Bossier City. Sensor “pups” are connected to soil moisture, temperature and electrical conductivity sensors. The retriever receives real-time data from the sensor pups, and data saves to the USB memory (Spectrum Technologies Inc., Aurora, Illinois). Photo by Changyoon Jeong

Figure 1. Nitrous oxide emissions from field plots of cowpea under surface residue (CP_S) and incorporated residue (CP_T) compared with control without residue.

Figure 2. Nitrous oxide emissions from field plots of sorghum-sudangrass under surface residue (SS_S) and incorporated residue (SS_T) compared with control without residue.

Figure 3. Nitrous oxide emissions from field plots of a mixture of cowpeas and sorghum-sudangrass under surface residue (CP+SS_S) and incorporated residue (CP+SS_T) compared with control without residue.