Symbiotic nitrogen fixation of grass-clover leys under organic and conventional cropping

Symbiotic di-nitrogen (N-2) fixation of legumes is a greenhouse gas-neutral source of N for agriculture and is the most important external source of N in organic farming. In an experiment, in which organic and conventional cropping systems have been compared at two fertilizer input levels since 1978, we tested (i) if organically cropped clover has a higher yield and fixes more N-2 because of lower N fertilizer input than under conventional cropping, and (ii) if low phosphorus (P) and/or potassium (K) input limit clover growth and N-2 fixation under organic cropping. To compare performance of N-2 fixation, we determined the proportion of N derived from the atmosphere (P-Ndfa; %; N-15 natural abundance method) in clover. To compare clover growth performance we determined the yield of Trifolium pratense and T. repens. The amount of N-dfa (A(Ndfa); g m(-2)) reflects total sward N-2 fixation and depends on clover growth and its P-Ndfa. P-Ndfa was high for all cropping systems and fertilizer levels, indicating that symbiotic N-2 fixation process in clover plants was not limited in any of the treatments. A(Ndfa) was comparable in all the treatments except the unfertilized control, where it was decreased due to low yields of the total sward and its clover component. In the organic treatments decreased yield of total sward had no effect on A(Ndfa) because the proportion of clover was increased. These results demonstrate that both organically and conventionally cropped grass-clover leys fully profited from symbiotic N-2 fixation.