Non‐Legume Cover Crops Can Increase Non‐Growing Season Nitrous Oxide Emissions

Core Ideas Nitrous oxide emissions were greater in winter than spring or fall. Tillage radish increased over‐winter N₂O fluxes. Non‐legume cover crops increased N₂O fluxes under apparent NO₃ limiting conditions. Cover crops retain post‐harvest nutrients but how they impact non‐growing season nitrous oxide (N₂O) emissions is unclear. Therefore, we quantified how cover crop type (fall rye [Secale cereale L.] or oilseed radish [Raphanus sativus L.]) and fertilizer source (compost or inorganic fertilizer) affected N₂O emissions, soil water‐extractable organic C (WEOC) and nitrate (NO₃) dynamics over two non‐growing seasons. A treatment with no fertilizer or cover crop was also included. Weekly, N₂O fluxes were determined using vented static chambers; soil WEOC and NO₃–N concentrations were measured monthly. Each non‐growing season, mean N₂O fluxes were 74 to 450% greater in the winter (21 December–20 March) than spring (21 March–20 June) or fall (22 September–20 December). In winter 2014–2015, oilseed radish increased the mean N₂O flux by 39 and 323% compared with fall rye and no cover crop, respectively, while the mean N₂O fluxes were strongly correlated to the pre‐winter (16 Dec. 2014) NO₃ concentrations (r = 0.96; P < 0.001), indicating NO₃ levels < 6 mg NO₃–N kg–¹ limited N₂O fluxes. In 2014–2015, fall rye and oilseed radish had 76 and 154% greater cumulative N₂O emissions than amended soils with no cover crop, respectively. Across both winters, an exponential model explained 67% of variability between the pre‐winter WEOC to NO₃ ratio and N₂O fluxes, indicating that organic C and NO₃ controlled over‐winter N₂O fluxes. Non‐legume cover crops increased non‐growing season N₂O emissions, suggesting that cover crops concentrate denitrification substrates in root‐associated soil to enhance N₂O fluxes.