Mitigating nitrogen pollution with under-sown legume–grass cover crop mixtures in winter cereals

Nitrogen (N) pollution from N inputs to agricultural soils contributes to widespread eutrophication and global climate change. One period susceptible to N losses is between winter grain harvest in summer and corn planting in spring in a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.]–winter grain rotation. Cover crops used to immobilize N during this period often depend on tillage, which can exacerbate N losses. Therefore, we evaluated whether reduced-till cover crops could decrease nitrate (NO₃^–) leaching and nitrous oxide (N₂O) emissions during this period. We tested this strategy in a cropping systems experiment on a 4-ha plot in central Pennsylvania over 2 yr. This experiment compared a clover (Trifolium pratense L.)–timothy (Phleum pratense L.) cover crop no-till underseeded into a standing spelt crop with a vetch (Vicia villosa Roth)–triticale (× Triticosecale Wittm. ex A. Camus) cover crop established with tillage after spelt harvest. These systems were compared based on fortnightly N₂O emissions using static chambers (n = 4 per six sample dates) and potential NO₃^– leaching using anion resin bags (n = 4 per system per year). Reduced-till cover crops minimized peak N₂O emissions during the fall compared with tilled cover crops. However, reduced-till cover crops did not decrease potentially leachable NO₃^– relative to tilled cover crops despite decreases in soil inorganic N. Cover crop N isotopes revealed that clover N may have mineralized and leached over the winter. Our results suggest that reduced-till cover crops can decrease N₂O emissions to mitigate the climate impact of agriculture but that winter-hardy cover crops should be chosen to mitigate leaching.