Use of the N-15 natural abundance technique to quantify biological nitrogen fixation by woody perennials

Biological nitrogen fixation (BNF) associated with trees and shrubs plays a major role in the functioning of many ecosystems, from natural woodlands t o plantations and agroforestry systems, but it is surprisingly difficult to quantify the amounts of N-2 fixed. Some of the problems involved in measur ing N-2 fixation by woody perennials include: (a) diversity in occurrence, and large plant-to-plant variation in growth and nodulation status of N-2-f ixing species, especially in natural ecosystems; (b) long-term, perennial n ature of growth and the seasonal or year-to-year changes in patterns of N a ssimilation; and (c) logistical limitations of working with mature trees wh ich are generally impossible to harvest in their entirety. The methodology which holds most promise to quantify the contributions of N-2 fixation to t rees is the so-called 'N-15 natural abundance' technique which exploits nat urally occurring differences in N-15 composition between plant-available N sources in the soil and that of atmospheric N-2. In this review we discuss probable explanations for the origin of the small differences in N-15 abund ance found in different N pools in both natural and man-made ecosystems and utilise previously published information and unpublished data to examine t he potential advantages and limitations inherent in the application of the technique to study N-2 fixation by woody perennials. Calculation of the pro portion of the plant N derived from atmospheric N-2 (%Ndfa) using the natur al abundance procedure requires that both the N-15 natural abundance of the N derived from BNF and that derived from the soil by the target N-2-fixing species be determined. It is then assumed that the N-15 abundance of the N -2-fixing species reflects the relative contributions of the N derived from these two sources. The N-15 abundance of the N derived from BNF (B) can va ry with micro-symbiont, plant species/provenance and growth stage, all of w hich create considerable difficulties for its precise evaluation. If the %N dfa is large and the N-15 abundance of the N acquired from other sources is not several delta(15)N units higher or lower than B, then this can be a ma jor source of error. Further difficulties can arise in determining the N-15 abundance of the N derived from soil (and plant litter, etc.) by the targe t plant as it is usually impossible to predict which, if any, non-N-2-fixin g reference species will obtain N from the same N sources in the same propo rtions with the same temporal and spatial patterns as the N-2-fixing perenn ial. The compromise solution is to evaluate the N-15 abundance of a diverse range of neighbouring non-N-2-fixing plants and to compare these values wi th that of the N-2-fixing species and the estimate of B. Only then can it b e determined whether the contribution of BNF to the target species can be q uantified with any degree of confidence. This review of the literature sugg ests that while the natural abundance technique appears to provide quantita tive measures of BNF in tree plantation and agroforestry systems, particula r difficulties may arise which can often limit its application in natural e cosystems.