Comparison of cover crop monocultures and mixtures for suppressing nitrogen leaching losses

Authors: GAIMARO, JOSH - Us Army Corp Of Engineers (USACE); Timlin, Dennis; TULLY, KATHERINE - University Of Maryland

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/8/2021
Publication Date: 11/30/2021
Citation: Gaimaro, J., Timlin, D.J., Tully, K. 2021. Comparison of cover crop monocultures and mixtures for suppressing nitrogen leaching losses. Agricultural Water Management. 261:107348. https://doi.org/10.1016/j.agwat.2021.107348.
DOI: https://doi.org/10.1016/j.agwat.2021.107348

Interpretive Summary: The 2025 goal of the Chesapeake Bay Program is to significantly reduce the amount of nitrogen (N) lost from agricultural land compared to values recorded in 2014. One way to reduce N loss from agricultural fields is to plant a rapidly growing grass or other cover crop such as forage radish in the fall, after the main crop is harvested. Along with water the cover crop will also take up N and store it in the leafy material thus protecting it from being lost from the soil from where it may enter water tributaries that empty into the Bay. Selection of the proper cover crop can be critical to successfully conserving N in the soil. We compared N losses among forage radish, cereal rye and a forage radish+cereal rye mixture. We found that N in the soil water was always higher (by 5x) in the soil with no cover crop compared to cover cropped soils. The majority of N loss from the soil planted to rye occurred during the fall while most of the N losses in forage radish occurred during the winter and spring (after they winter-killed). We concluded the radish+rye mixtures provided the best protection from N losses indicating that they are a flexible management tool to reduce leaching losses from cash crop systems. Application of this research on how cover crop species affect N leaching losses can help agricultural managers and growers design cropping systems to minimize N losses to the Chesapeake Bay.

Technical Abstract: The 2025 goal of the Chesapeake Bay Program is to reduce agriculture's nitrogen (N) loading by 20% from 2014 values. Cover cropping is an important best management practice for recycling and conserving N in cropping systems that might otherwise be lost to groundwater via leaching and runoff. The reduction of N leaching by winter cover crops depends largely on precipitation, timing of planting, and the selection of the appropriate crop species. We compared nitrate-nitrogen (NO3--N) leaching losses among forage radish (Raphanus sativus L.), cereal rye (Secale cereal L.), a forage radish+cereal rye mixture, and no a cover control. Replicated field trials were conducted at the University of Maryland Central Maryland Research and Education Center over 2016-2018. We collected porewater from 60 cm below the ground surface using porous cup lysimeters following rainfall events and used NO3 --N concentrations paired with the HYDRUS 1-D soil moisture model to compare N leaching losses (in kg N ha-1) among cover crop treatments. We show that soil porewater NO3 --N concentrations were always higher (by 5x) in the no cover control compared to rye and radish+rye treatments (P<0.0001). Overall, N leaching losses (kg N ha-1) were highest in the no cover control plots (P<0.0001), but the majority of N leaching losses in rye plots occurred during the fall while the majority of radish N leaching losses occurred during the winter and spring (after they winter-killed). The radish+rye mixtures reduced N leaching losses by 80% in both years regardless 27 of radish planting date indicating that they are a flexible management tool to reduce leaching losses from cash crop systems. Understanding how different cover crop species affect N leaching losses, can help us design cropping systems to minimize N losses to the Chesapeake Bay.