Early sowing dates and pre-plant nitrogen affect autumn weed control and nitrogen content of winter cover crops in rotation with spring crops

Agroecological services of cover crops depend mostly on their biomass accumulation, which in turn depends on growing season weather and on nitrogen (N) availability. We hypothesised that cover crop growth and weed control can be increased with an early sowing date and under high residual soil inorganic N left after the previous cash crop harvest. This hypothesis was tested in Northern Italy, with a two-year field experiment in two locations with five cover crops, comparing two cover crop sowing dates (beginning of September, SD1, and mid September, SD2), and evaluating the effects of pre-plant soil mineral N addition (N0 and N1). The cover crops, grown between two maize crops, were terminated in March. Weather conditions in both years were drier than normal, reducing on average cover crop growth. In November, cover crop aboveground biomass and N content ranged between 0.5 and 3.3 t DM ha−1 and between 18 and 70 kg N ha−1, respectively, with significantly lower values for SD2 compared to SD1 for most species. In most cases high pre-plant soil mineral N significantly increased cover crop biomass and N content, suggesting that cover crop growth was N-limited. Weed growth was higher in Egyptian clover and hairy vetch (0.7 t DM ha−1 on average) than in white mustard and black oat (0.2 t DM ha−1 on average), due to limited competitive ability of legumes. Cover crop biomass accumulation and weed control were enhanced by early sowing (in particular for legumes) and in conditions of high mineral N residue in the soil (in particular for non-legumes).