Cover crop composition mediates the constraints and benefits of roller-crimping and incorporation in organic white cabbage production

Roller-crimping of cover crops grown during winter may facilitate reduced tillage efforts in organic vegetable production. A one-year field experiment was conducted twice (autumn 2015-2017) in sandy loam soil in Denmark testing two cover crop termination systems, roller-crimping (RC) without tillage before crop planting, and full incorporation by tillage (FI) of mowed cover crops in organic white cabbage (Brassica oleracea L. convar. capitata var. capitata f. alba) production. Three legume species were investigated: winter faba bean (Vicia faba L.), winter pea (Pisum sativum L.), hairy vetch (Vicia villosa Roth), in pure stands or in a mixture with winter rye (Secale cereale L.). Roller-crimping reduced total white cabbage biomass by 31% (2016) and 19% (2017). Marketable white cabbage yield was 100% (2016) or 24% (2017) lower under RC than FI, likely caused by delayed N release from cover crops and reduced soil dehydrogenase activity. White cabbage root growth was reduced by RC following pea/rye, where N availability was low. Despite reduced root growth and yield, RC still had the advantage of reducing weed growth by 63% compared with FI three weeks after cover crop termination in 2017 and of decreasing N leaching risk, indicated by reduced soil mineral N content in 0.5-1.5 m depth in autumn. Marketable yield was 35% higher following legumes compared with legume/rye mixtures in both termination systems in 2017, due to 105 kg N ha(-1) higher mineral soil N content in 0-2.5 m depth in the spring and faster N mineralisation from plant material with a lower C/N ratio. This yield increase corresponded with increased root growth following RC legumes. In contrast, legume/rye mixtures had the advantage of decreasing weed growth by 50-68% and N leaching risk by 22 kg ha(-1) in 0.5 - 2 m depth. Although RC demonstrated ecological benefits such as weed suppression and indicated reduced N leaching risk, the trade-off with yield losses could create a barrier to adoption. This trade-off could be mitigated by using pure legume cover crops and by improving management of supplemental N fertility.