N[sub2](C[sub2]H[sub2])-fixation in early stages of a primary succession on a reclaimed salt marsh.

Nitrogen fixation activity was determined for Lotus tenuis, Medicago lupulina and Trifolium pratense. The three species grew in clones in grassland in an area reclaimed from brackish water in the 1940s. The N2[C2H2]-fixation was measured in soil cores throughout 1974 and 1975. From cores taken in dense and uniform stands of the species, the yearly N2[C2H2]-fixation at maximum cover was estimated. L. tenuis fixed about 4 g N m-2 yr-1 (area with max. cover 130%). i.e. 30-56% of its requirement. Both M. lupulina and T. pratense fixed about 7 g N m-2 yr -1 (maximum cover 37% and 80%) i.e. 67% of their N-requirement. Average N2[C2H2]-fixation for the whole area was 0.4 g N m-2 yr-1, considerably less than the N-addition through rainfall. [ABSTRACT FROM AUTHOR]