Enhanced nitrogen mineralization in mowed or glyphosate treated cover crops compared to direct incorporation

Information on how management by mowing and herbicide alter residue quality and nitrogen (N) inputs would be valuable to improve prediction of N availability. Mowing and glyphosate application are widely used by growers to limit cover crop growth and facilitate incorporation. A mixture of cover crops, hairy vetch (Vicia villosa L.), oriental mustard (Brassica juncea L.) and cereal rye (Secale cereale L.), was investigated as a means to improve soil quality and optimize N availability. There is limited information on how mowing or glyphosate application influence cover crop decomposition and N mineralization from these heterogeneous residues. A rye cover crop was grown in the field over the winter and transferred to containers as an intact soil profile to conduct a greenhouse study. Management treatments (mowing and glyphosate) were imposed eight days before incorporation. Plant and soil N dynamics were monitored. The experiment was repeated with the addition of a tri-mixture cover crop. Inorganic $\mathrm{N}{\mathrm{O}}_{3}^{-}$ in bare soil ranged from 6 to 10 μg N g soil-1 over 39 days. Similar or lower levels of soil $\mathrm{N}{\mathrm{O}}_{3}^{-}$ were observed after rye residue incorporation, from 2 to 6 μg N g-1; consistent with N-immobilization. Application of untreated, mixed cover crop residues generally was associated with higher levels of soil inorganic $\mathrm{N}{\mathrm{O}}_{3}^{-}$, from 3 to 11 μg N g-1. For both rye and mixed residues, management by mowing or glyphosate enhanced N mineralization by 10 to 100%, compared to untreated residues. At the same time, application of mowing or glyphosate 8 days before cover crop incorporation seemed to reduce the amount of residues by about half compared to untreated controls. Belowground biomass was reduced more than aboveground, although recovery of senescent roots may have been incomplete. Management by glyphosate or mowing enhanced soil inorganic N availability in the short-term while simultaneously reducing carbon and N inputs.