Dinitrogen fixation and residue nitrogen of different managed legumes and nitrogen uptake of subsequent winter wheat.

Fixed nitrogen accumulated by legumes is the main nitrogen source for organic farming systems. Knowledge about the amount of fixed nitrogen, its pathways into forage yield, crop residues, soil-N and yield formation of the following crop is needed for designing crop rotations. Two field experiments were conducted in Northern Germany with differently managed (cut, mulched) legumes (red clover, alfalfa, white clover) in pure stands and various mixtures with two companion grasses (Lolium multiflorum, Lolium perenne) have been grown to determine N2-fixation, residue nitrogen and N-uptake of subsequent crops. Cropped grass/legume reached higher N2-fixation than mulched. While green manure grass/legume left up to 280 kg ha of N in mulch, stubble and roots on the field, most cropped grass/legume mixtures left less than 110 kg N ha in crop residues. Pure legume swards or legume rich mixtures showed higher N2-fixation than grass rich mixtures. N-uptake in late autumn and at maturity of the subsequent wheat was strongly correlated to the legume content in DM-yield.