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

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.