Aerial interseeding and planting green to enhance nitrogen capture and cover crop biomass carbon

Abstract

The US state of Maryland incentivizes farmers to plant cover crops to reduce nitrogen (N) loading to the Chesapeake Bay and to sequester carbon (C). Maryland has a greater percentage of agricultural land in cover crops than any other state, but Maryland farmers typically plant cover crops in October, after harvest and terminate them early in spring, thus severely limiting the cover crop growing time with sufficient temperatures. We hypothesized that extending the cover crop growing season, by interseeding cover crops earlier in fall and terminating them later in the spring, would increase both fall and spring cover crop biomass and N content, reduce nitrate (NO3) leached during winter through early spring, increase soil mineral N, and increase soil moisture in early summer. We tested this hypothesis across 18 site-years by partnering with commercial farmers on the Eastern Shore of Maryland. The farmers managed a brassica-legume-cereal cover crop on their corn (Zea mays L.) or soybean (Glycine max [L.] Merr.) fields according to three treatments: (1) aerial interseed cover crop prior to cash crop harvest and terminate it at or after cash crop planting (Extended); (2) drill cover crop after cash crop harvest and terminate it several weeks before cash crop planting (Standard); and (3) a no-cover crop control in 2018 and 2019 (No Cover). For each treatment, we measured cover crop biomass + N content (fall and spring), NO3 in 70 or 100 cm deep drainage water (fall-winter), as well as soil mineral N and moisture (in June). The Extended treatment exhibited higher fall biomass (1,700 versus 294 kg ha−1) and total N content (65.3 versus 9.6 kg N ha−1) only in a wet year, but produced greater spring cover crop biomass and N content than the Standard treatment every year. In the year with a very wet fall, NO3-N leaching loss was reduced by 84% for Extended and by 45% for Standard compared to No Cover. We found no difference in NO3 leaching between Extended and Standard in years with a dry fall (2017 and 2019). Averaged over all three years, Extended and Standard did not differ in June soil NO3 concentration. Greater reductions in NO3 leaching may make early aerial interseeding preferable to post-harvest drilling, while increased biomass produced in spring with later termination made Extended desirable for increased C inputs. Hence, extending the cover-cropping season can be beneficial to the farmer and to the environment due to increased fall and spring cover crop C inputs to the soil and reduced NO3 leaching in wet years, reducing potential eutrophication of nearby waterways.