Kritsanee Iamjud and Lisa M. Fultz

One of the benefits of growing cover crops is nutrient cycling and availability for the subsequent cash crop, which in Louisiana includes soybeans, corn, wheat and cotton. Cover crops scavenge nutrients left over after cash crop removal, and they release nutrients back to the soil after termination, potentially benefiting the following cash crop. In Louisiana, cover cropping systems often are grown through the winter and terminated in spring. Several studies regarding cover cropping systems have been done in recent years. However, knowledge about the impacts of cover crop mixtures on nitrogen degradation and availability after termination is still lacking in the Midsouth.

For this study, the aim was to examine cover crop biomass degradation and the release of nitrogen and other nutrients in cover crop biomass. The goal was to identify the optimum time of inorganic nitrogen availability for the subsequent cash crop. At the same time, the essential nutrients in biomass and soil were measured.

From a two-year study, 16 treatments at the LSU AgCenter Macon Ridge Research Station in Winnsboro, Louisiana, were conducted in a six-year no-till system. Samples were collected from eight cover crop treatments (seven monoculture and mixes and one control) at two nitrogen fertilizer application rates (zero and 160 pounds of nitrogen per acre). This research was replicated at the LSU AgCenter Dean Lee Research and Extension Center in Alexandria, Louisiana, with the number of cover crop treatments reduced to four. Cover crop biomass production was assessed at termination in mid-February, and biomass degradation was tracked at intervals of one, two, three, four, six and eight weeks after termination without incorporation into the soil.

For both sites, mixed cover crops were well-established and produced excellent vegetative growth with and without nitrogen application. At Dean Lee, the mixed cover crops produced greater amounts of biomass than fallow. The range was 1,362 to 1,747 pounds per acre, while the fallow field produced 633 pounds per acre. Cover crop production at the Macon Ridge trial ranged from 1,329 to 3,728 pounds per acre, while the fallow field produced 554 pounds per acre. Additionally, weed production was low in every cover crop treatment.

At Dean Lee, mixed cover crop treatments had a greater potential to scavenge nitrogen compared to fallow, and leguminous species (crimson clover, Austrian winter pea and berseem clover) assimilated more nitrogen than grasses (cereal rye, wheat and black oat). Total nitrogen assimilated in cover crop biomass ranged from 38 to 66 pounds of nitrogen per acre. For Macon Ridge, total nitrogen assimilated in cover crop biomass ranged from 25 to 88 pounds per acre. Moreover, the three species mixed treatment (a mix of crimson clover, hairy vetch and radish and a mix of black oats, crimson clover and radish) tended to have more total nitrogen per area at both sites.

After the cover crops’ chemical termination, the majority of degradation occurred within the first four weeks. Up to eight weeks after termination, cover crop residue in the field ranged from 13% to 48%. Soil nitrate-nitrogen availability for both sites reached a peak at six weeks after cover crop termination before decreasing at week eight. Overall, soil inorganic nitrogen concentrations (the amount potentially contributed for the following crop) were 6.2 pounds per acre at week 6 on average for the Dean Lee site and 12.3 pounds per acre at the Macon Ridge site.

Soil protein nitrogen measures the potentially mineralizable nitrogen in the soil that can be mineralized by microorganisms and become plant available nitrogen. The Macon Ridge site provided two times more protein nitrogen than the Dean Lee site. The content of soil protein nitrogen sharply increased at week 3 and fluctuated after that for the Dean Lee site and the Macon Ridge site. Plus, the protein nitrogen at both sites remained higher even eight weeks after termination, indicating that inorganic nitrogen was still continually being released from the cover crop residues.

The mixture of cover crops produced more biomass and scavenged and assimilated more nitrogen than fallow weeds. Two months after termination, degradation of biomass contributed to cycling nutrients to the soil, with the greatest availability of inorganic nitrogen at six weeks after termination. These findings indicate that the optimum timing of nitrogen degradation and availability to the subsequent cash crop is six weeks after cover crop termination.

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.)

Mix of forage radish, hairy vetch and crimson clover at the Macon Ridge Research Station site before termination. Photo by Kritsanee Iamjud

Black oat and cereal rye mixture showing excellent growth before termination in February at the Macon Ridge Research Station site. Photo by Kritsanee Iamjud

Legume, grass and brassica residues at the Macon Ridge Research Station site at week 8 after termination. Photo by Kritsanee Iamjud