Delaying sowing of cover crops decreases the ability to reduce nitrate leaching

Under favorable growth conditions cover crops are efficient tools to scavenge available nitrogen in the soil after the main crop and reduce nitrate leaching. However, their ability to reduce nitrate leaching is influenced by the available growth conditions after sowing and their characteristics to assimilate biomass and nitrogen (N). A flexible sowing time of cover crops without compromising their benefits to the environment implies knowledge of how the growth conditions after sowing of the cover crop affect leaching. Fodder radish, phacelia, and oats have different growth and N uptake characteristics, and their ability to reduce nitrate leaching depending on growth conditions after sowing may therefore differ. In this study, these three cover crops were evaluated, and their ability for taking up soil N and reducing nitrate leaching was quantified based on four sowing times from early August to early September with intervals of 7–11 days in 2019, 2020, and 2021 in Denmark. N uptake was measured in autumn each year and nitrate leaching measurements took place during the drainage period until the following spring. The N uptake decreased with subsequent sowing times across the cover crops and years. However, the decrease was different in different cover crops and different years. Nitrate leaching reduction also decreased with subsequent sowing times. On average, fodder radish reduced nitrate leaching by 64 kg N ha^-1, significantly more than phacelia and oats with 47 and 44 kg N ha^-1, respectively, across all sowing times and drainage periods. With an average reduction of nitrate leaching of 0.8 kg N ha^-1 per day and no interaction of N uptake and cover crops, the ability of fodder radish, phacelia, and oats to reduce nitrate leaching was not significantly different. Fodder radish and early sowing was the most effective combination to obtain the maximum benefits of nitrate leaching reduction by a cover crop.