Use of the15N natural abundance technique for the quantification of the contribution of N2 fixation to sugar cane and other grasses

This paper originates from an address at the 8th International Symposium on Nitrogen Fixation with Non-Legumes, Sydney, NSW, December 2000 The use of the 15N natural abundance technique to quantify contribution of biological nitrogen fixation (BNF) to any plant is based on the observation that N derived from soil is generally slightly different [usually higher in 15N abundance (δ15N‰)] than that of the air. Plants or micro-organisms growing solely on BNF generally accumulate N with 15N isotopic abundance lower than that of the air (i.e. δ15N‰ is negative), while plants obtaining all N from the soil generally show a positive δ15N‰ signal. The technique is applied by estimating the 15N abundance of the putative ‘N2-fixing’ crop and analysing the 15N abundance of neighbouring non-N2-fixing reference plants. However, often there are such large variations in the N derived from the soil by different non-N2-fixing plants that in natural ecosystems it is often impossible to even distinguish plants that are benefiting form BNF, let alone quantity this contribution. The reasons why soil derived N can vary so widely, especially in natural ecosystems, are briefly discussed and a sampling strategy is described to assess possible BNF inputs to sugar cane plants in commercial plantations in Brazil. The results suggest that in nine of the 11 sites studied, BNF inputs were significant ranging from 25 to 60% of N assimilated.