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

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-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 mean soil porewater NO3-–N concentrations were 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 rye and radish+rye mixtures reduced N leaching losses by 80% in both years. Understanding how different cover crop species affect N leaching losses, can help us design cropping systems to minimize N losses to the Chesapeake Bay especially as climate change alters rainfall patterns across the region.