Growth, composition, biological N₂ fixation and nutrient uptake of a leguminous cover crop mixture and the effect of their removal on field nitrogen balances and nitrate leaching risk

Cover crops (CC) are an important source of nitrogen (N) in organic farming systems. Only few data are available about the effect of management activities (liquid slurry amendments, crop residue management) on growth, nutrient uptake and biological N₂ fixation (BNF) of a CC mixture. Furthermore, little information is available about the effect of CC harvesting on nutrient flows, nitrate leaching risk and soil mineral N supply of the succeeding main crop. The objectives of the presented field trials were (1) to measure the impact of organic manuring (straw residues and liquid slurry applications) on growth, composition, and BNF of a CC mixture with legumes and oil radish as components; (2) to determine the effect of CC species composition on nutrient content and uptake (N, P, K, Mg); and (3) to evaluate the effect of CC removal on field N balances and nitrate leaching risk. A CC mixture with legumes and non-legumes was able to compensate for many environmental and cultivation effects by influencing the competitive ability of the partners. For example, an increase of soil N supply due to additions of slurry or removal of cereal straw promoted growth of non-legumes at the expense of the legumes, resulting in N shortage at the end of the growing period, as shown by lower N contents and a wider C/N ratio of the non-legume partner. Low N availability at the beginning of the CC growth enhanced legume growth and/or reduced non-legume growth, resulting in a higher N supply in later periods of CC growth. A high legume percent composition within a CC mixture increases overall N content in the aboveground biomass and the N content of non-legumes within the mixture, and decreases the C/N ratio. Large amounts of nutrients were removed from the field by the harvesting of the CC aboveground biomass, significantly reducing the nitrate leaching risk. However, a reduction of the nitrate leaching risk was found only on fields where the green manure was incorporated in autumn.