Non-legume cover crops can increase non-growing season nitrous oxide emissions

Abstract

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.