Estimating legume N-2 fixation in grass-clover mixtures of a grazed organic cropping system using two N-15 methods

The input of Nitrogen (N) through symbiotic N-2 fixation (SNF) in grass-clo ver mixtures was determined in an organic cropping. system for grazing duri ng 3 years. The mixture of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) was established by undersowing in spring barl ey (Hordeum vulgare L.) and maintained subsequently for two production year s. Dinitrogen fixation was determined using the N-15 isotope dilution techn iques and two labelling procedures. Using either pre-labelling of the soil with immobilisation of the N-15 by addition of a carbon source before estab lishment of the grass-clover mixture or adding the N-15 labelled ammonium t o the established crop, resulted in similar estimates of the proportion of N derived from the atmosphere (pNdfa). However, the average coefficient of variance of the pNdfa values was significantly lower with the pre-labelling immobilisation method. The proportion of fixed N in the clover varied betw een 0.70 and 0.99 with the lowest values in the beginning and at the end of the growing season. The total amount of N fixed in the above-ground plant parts of the grass-clover mixture varied between 10.0 and 23.5 g N m(-2) pe r year. The total input from N-2 fixation in the rotation was between 30 an d 50 g N m(-2) per year (above-ground plant parts only). This variation cou ld be ascribed to differences in climatic conditions during the 3 years. Th e amount of N in grass-clover residues incorporated in the soil after the s econd production year ranged between 19.6 and 29.6 g N m(-2), of which 52-6 6% was estimated to be derived from fixation. The amount of atmospherically derived N actually entering the soil in the grazed cropping system is disc ussed. (C)2000 Published by Elsevier Science B.V. All rights reserved.