Introduction Directory UMM :Data Elmu:jurnal:A:Agriculture, Ecosystems and Environment:Vol78.Issue3.May2000:

Agriculture, Ecosystems and Environment 78 2000 139–147 Estimating legume N 2 fixation in grass-clover mixtures of a grazed organic cropping system using two 15 N methods Finn P. Vinther a,∗ , Erik S. Jensen 1 b a Danish Institute of Agricultural Sciences, Department of Soil Science and Plant Physiology, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark b Plant Ecosystems and Nutrient Cycling, Plant Biology and Biogeochemistry, Risø National Laboratory, DK-4000 Roskilde, Denmark Received 14 January 1999; received in revised form 3 June 1999; accepted 17 August 1999 Abstract The input of Nitrogen N through symbiotic N 2 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 15 N isotope dilution techniques and two labelling procedures. Using either pre-labelling of the soil with immobilisation of the 15 N by addition of a carbon source before establishment of the grass-clover mixture or adding the 15 N 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 N 2 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. Keywords: Dinitrogen fixation; 15 N isotope dilution; Ryegrass-clover mixtures; 15 N immobilisation method; Denmark

1. Introduction

Symbiotic N 2 fixation SNF in legumes is a fun- damental process for maintaining soil fertility and the ∗ Corresponding author. Tel.: +45-89991861; fax: +45-89991619. E-mail address: finn.vintheragrsci.dk F.P. Vinther. 1 Present address. Agroecology, Department of Agricultural Sci- ences, Royal Veterinary and Agricultural University, DK-2630 Taastrup, Denmark continued productivity of organic cropping systems. The amount of N 2 fixed and the N contribution from leguminous crops are influenced by a number of envi- ronmental factors, including soil type, nutritional sta- tus of soil, species and varieties, water availability and temperature as well as soil and crop management Ledgard and Steele, 1992; Jensen et al., 1997. The complex interactions among these factors need further study to achieve predictable and sustainable outcomes from the crops in organic farming systems. 0167-880900 – see front matter ©2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 8 0 9 9 9 0 0 1 2 4 - 3 140 F.P. Vinther, E.S. Jensen Agriculture, Ecosystems and Environment 78 2000 139–147 Estimates of SNF are influenced by the method of measurement used. The soil is typically labelled with 15 N by adding labelled nitrate or ammonium to the standing crop grass-clover crop. Inaccuracies in 15 N isotope dilution estimates arise from poor matching of reference and legume crop, coupled with the decline in 15 N enrichments of plant available soil N Witty, 1983; Witty and Ritz, 1984; Giller and Witty, 1987. Stabilising the 15 N enrichment of the plant available N over time could potentially contribute to improved estimates of fixation. The primary objective of this work was to esti- mate the input of symbiotically fixed N 2 into an or- ganic cropping system via a grass-clover mixture and to evaluate its importance for the N supply of crops in the rotation. The study included an evaluation of 15 N isotope dilution methods for estimating SNF in grass-clover mixtures.

2. Materials and methods