N-fixation of selected green manure plants in an organic crop rotation

The aim of this study was to investigate the N-fixation potential of differ ent leguminous green manure plants grown in the autumn after harvest of a b arley main crop. Fixed above ground N derived from the atmosphere (N-dfa) w as estimated both by the N-15 isotope dilution method and by the total-N di fference method. Winter rape (Brassica napus), winter rye (Secale cereale) and Italian ryegrass (Lolium multiflorum) were grown as non-fixing control plants for the estimation of N-fixation of leguminous green manure plants. It was concluded that Italian ryegrass was the most suitable control plant, and that the choice of control plant can be important for the results. Whe n using Italian ryegrass as control plant, no significant difference was ob served between the total N difference method and the N-15 isotope dilution method. N-fixation varied strongly among the plant species and differed between the two years. Hairy vetch (Vieia villosa), crimson clover (Trifolium incarnat um) and Persian clover (Trifolium respinatum) fixed more than 100 kg N ha(- 1) at least in one of the two investigated years. The highest estimated N w as 149 kg N ha(-1) in the above ground plant material of hairy vetch in 199 7. N-dfa of common vetch (Vicia sativa) and Egyptian clover (Trifolium alex andrinum) was below 60 or 100 kg N ha(-1) in 1996 and 1997, respectively. W eather conditions and soil moisture, which are crucial during germination a nd early plant development, may be an explanation for lower N-fixation in 1 996 than in 1997. With respect to N, the results indicate that extended use of green manure could strongly reduce the need for full year green manure crops in stockless organic crop rotations.