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RESEARCH ARTICLE

Seasonal variation in the suitability of different methods for estimating biological nitrogen fixation by grain legumes under rainfed conditions

AM McNeill, CJ Pilbeam, HC Harris and RS Swift

Australian Journal of Agricultural Research 47(7) 1061 - 1073
Published: 1996

Abstract

Biological nitrogen fixation (BNF) by legume crops was estimated in a Mediterranean environment at ICARDA in northern Syria for 3 consecutive seasons beginning in 1991-92. Using the classical isotope dilution technique (NID), estimates ranged from 32 to 82 kg N/ha for chickpea and from 18 to 82 kg N/ha for lentil. In comparison the simple N-difference method gave lower, sometimes negative, estimates for BNF by both crops in the latter 2 seasons but a higher estimate for chickpea in the first year. Discrepancies in the estimates from N-difference were correlated with differences in the amount of soil N taken up by the legume and the non-fixing wheat reference crop. Since soil N uptake by lentil in the first year was similar to wheat, the estimates of BNF from the 2 methods for that season were similar. The indirect effects of an interaction of added N fertiliser on N derived from the soil and thus on N uptake and estimated BNF are discussed in relation to the use of the isotope dilution method with A-value modification (NAV). Despite some significant differences in A-value for soils receiving different amounts of fertiliser it is demonstrated that the A-value method used in this study, with fertiliser rates of 10 kg N/ha to the legume and 30 kg N/ha to the non-legume, resulted in BNF estimates for lentil similar to those obtained using classical isotope dilution. However, this was not the case for chickpea where a direct inhibitory effect of fertiliser N at 30 kg N/ha resulted in lower estimates of BNF from NID than NAV. Since the reference crops derived as much as 90% of their N from the soil, it is recommended that future BNF studies using isotope dilution techniques for lentil and chickpea crops at ICARDA use a fertiliser rate lower than that used in this study. An isotope dilution method utilising a slow-release source of 15N or the natural abundance technique for estimating BNF are suggested as potentially useful alternatives. The need for a basic understanding of the soil N dynamics pertinent to each site as a prerequisite for choosing an appropriate method for estimating BNF is highlighted.

Keywords: BNF; N difference; N isotope dilution; A-value

https://doi.org/10.1071/AR9961061

© CSIRO 1996

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