Use of the N-15 natural abundance technique for the quantification of the contribution of N-2 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 observatio n 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-orga nisms growing solely on BNF generally accumulate N with N-15 isotopic abund ance lower than that of the air (i.e. delta N-15% is negative), while plant s obtaining all N from the soil generally show a positive delta N-15% signa l. The technique is applied by estimating the N-15 abundance of the putativ e '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 i n the N derived from the soil by different non-N-2-fixing plants that in na tural 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 b riefly discussed and a sampling strategy is described to assess possible BN F inputs to sugar cane plants in commercial plantations in Brazil. The resu lts suggest that in nine of the 11 sites studied, BNF inputs were significa nt ranging from 25 to 60% of N assimilated.