Because of lower nitrogen (N) input it is assumed that organically cultivated legumes
fix more N2 than legumes grown under conventional cropping. Using the natural 15N
abundance method, we assessed symbiotic N2 fixation by white and red clover. The
clover was growing in a grass-clover meadow installed as part of the crop rotation of a
long term (30 years) field experiment. Dry matter yields were similar for organic and
conventional meadows on plots fertilized at levels typical for the respective system,
but organic meadows tended to higher clover yields. The proportion of N derived from
atmosphere (Ndfa) in both clover species was on average 84%. It was not significantly
affected by the cropping system. Because of more legumes, amounts of fixed N2 (Nfix)
were higher in organic than conventional meadows. Under low fertilization intensity,
low available potassium and phosphorus contents limited Nfix through lower legume
dry matter production while Ndfa remained high
