Reduced nitrate leaching in a perennial grain crop compared to maize in the Upper Midwest, USA

Abstract:

Global expansion of high-input annual grain crops and associated nitrogen (N) fertilizer use can have negative consequences for the environment and human health. Nitrate nitrogen (NO₃-N) leaching from fertilized annual crops can contaminate groundwater and pollute natural aquatic ecosystems and rural drinking water sources. Intermediate wheatgrass (IWG; Thinopyrum intermedium) is a perennial grass being domesticated to serve as the world’s first widely available perennial grain crop. Our objective was to measure water quality variables and crop yields in order to model NO₃-N leaching beneath IWG, maize, and switchgrass under three N fertilizer treatments; low N (0 kg N ha⁻¹), medium N (maize = 80, switchgrass and IWG = 40 kg N ha⁻¹), and high N (maize = 160, switchgrass and IWG = 120 or 160 kg N ha⁻¹). The switchgrass and IWG medium N treatments also included alfalfa as an intercrop. The NO₃-N concentration in soil solution 50 cm below the surface was one and two orders of magnitude lower in high N fertilized IWG compared to switchgrass and maize, respectively. Soil solution NO₃-N increased with N fertilizer in all crops. Soil water content was less at 50 and 100 cm depths in IWG compared to switchgrass and maize but was unaffected by N fertilizer treatment. Using the Denitrification and Decomposition (DNDC) model, average annual NO₃-N leaching estimates in the high N treatments were 21.7 kg N ha⁻¹ for maize and 3.7 and 0.2 kg N ha⁻¹ for switchgrass and IWG, respectively. Despite consistent biomass yields through time, IWG grain yields decreased with stand age at all N fertilizer treatments. Alfalfa did not persist in the IWG and switchgrass medium N treatments, thus other legumes should be tested for intercropping with perennial grain crops. Intermediate wheatgrass has great potential to provide food-quality grain and biomass while preventing NO₃-N leaching.

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