Use of the 15N natural abundance technique for the quantification of the contribution of N2 fixation to sugar cane and other grasses

The use of the N-15 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 N-15 abundance (delta N-15%)] than that of the air. Plants or micro-organisms growing solely on BNF generally accumulate N with N-15 isotopic abundance lower than that of the air (i.e. delta N-15% is negative), while plants obtaining all N from the soil generally show a positive delta N-15% signal. The technique is applied by estimating the N-15 abundance of the putative 'N-2-fixing' crop and analysing the N-15 abundance of neighbouring non-N-2-fixing reference plants. However, often there are such large variations in the N derived from the soil by different non-N-2-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.