Applications of 15N methods for measuring nitrogen fixation in trees

Trees are important components of several agricultural systems, and play a significant role in restoring or increasing soil fertility and in decreasing soil erosion. Of particular interest are nitrogen fixing trees (NFTs), which are able to grow well on nitrogen deficient soils. Methods that can accurately measure the individual contributions of soil N and fixed N2 to the growth of NFTs under different management conditions are of great importance in devising strategies for maximizing/optimizing nitrogen fixation. Techniques based on 15N for measuring N2 fixed in NFTs are still in the developmental stages but most of the initial results indicate their great potential for measuring N2 fixed and in research aimed at increasing nitrogen fixation in NFTs. Experiments carried out in the International Atomic Energy Agency's laboratory using 15N techniques have identified large genotypic differences in N2 fixation among various NFT species and provenances, clones or isolines. Most of the 15N approaches used to measure N2 fixation in trees have been adapted from experience gained with grain and pasture legumes. Given the massive sizes and perennial growth habits of most NFTs, there is considerable scope to modify these techniques. Some of the factors that need to be seriously addressed are: the choice of suitable reference crops, sampling techniques, appropriateness of various methods of labelling soil with 15N, frequency of 15N addition, and assessment of potential errors associated with the retranslocation of stored N to aerial parts when assessing N2 fixed during regrowth. 45 refs