Plant and soil N of different winter cover crops as green manure for subsequent organic white cabbage

Leguminous cover crops used as green manures can reduce fertilizer inputs by supplying nitrogen (N) via mineralization of incorporated N-rich biomass derived from biological N-2 fixation. In a multi-year trial at three locations in Germany, the effects of leguminous, non-leguminous and mixed green manure crops on the yield of the subsequent cash crop white cabbage (Brassica oleracea convar. capitata var. alba) were investigated. The winter cover crop treatments were forage rye (Secale cereale L.), a mixture of forage rye with winter Hungarian vetch (Vicia pannonica Crantz), sole-cropped winter Hungarian vetch, winter pea (Pisum sativum L.), and winter faba bean (Vicia faba L.) with bare soil as a control. Sole-cropped legumes showed higher marketable cabbage head yields (head weight > 1.0 kg) compared to the other cover crop treatments, with 25.5, 25.9 and 28.1 Mg ha(- 1) for vetch, pea and faba bean, respectively. The aboveground biomass of the legume winter cover crop treatments had higher N offtakes with 185, 177 and 159 kg N ha(- 1) for vetch, pea and faba bean, respectively, with significantly lower carbon (C)/N ratios compared to rye and rye with vetch. The constant C/N ratio of the aboveground biomass of leguminous cover crops throughout the growing period indicates that the optimum incorporation date to achieve high N mineralization rates is less time dependent in leguminous compared to non-leguminous cover crops. The results of the present study show that leguminous winter cover crops do not reduce the soil N availability for a succeeding high N demanding cabbage crop resulting in yields comparable to agricultural practice without winter cover crops.