Estimating legume N2 fixation in grass-clover mixtures of a grazed organic cropping system using two 15N methods

The input of Nitrogen (N) through symbiotic N2 fixation (SNF) in grass-clover mixtures was determined in an organic cropping system for grazing during 3 years. The mixture of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) was established by undersowing in spring barley (Hordeum vulgare L.) and maintained subsequently for two production years. Dinitrogen fixation was determined using the 15N isotope dilution techniques and two labelling procedures. Using either pre-labelling of the soil with immobilisation of the 15N by addition of a carbon source before establishment of the grass-clover mixture or adding the 15N labelled ammonium to 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 between 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 per year. The total input from N2 fixation in the rotation was between 30 and 50 g N m−2 per year (above-ground plant parts only). This variation could be ascribed to differences in climatic conditions during the 3 years. The amount of N in grass-clover residues incorporated in the soil after the second production year ranged between 19.6 and 29.6 g N m−2, of which 52–66% was estimated to be derived from fixation. The amount of atmospherically derived N actually entering the soil in the grazed cropping system is discussed. ©2000 Published by Elsevier Science B.V. All rights reserved.