15N methodologies for quantifying the response of N2-fixing associations to elevated [CO2]: A review.

Methodologies based on 15 N enrichment ( E ) and 15 N natural abundance ( NA ) have been used to obtain quantitative estimates of the response of biological N 2 fixation (BNF) of legumes (woody, grain and forage) and actinorhizal plants grown in artificial media or in soil exposed to elevated atmospheric concentrations of carbon dioxide e[CO 2 ] for extended periods of time, in growth rooms, greenhouses, open top chambers or free-air CO 2 enrichment (FACE) facilities. 15 N 2 has also been used to quantify the response of endophytic and free-living diazotrophs to e[CO 2 ]. The primary criterion of response was the proportional dependence of the N 2 -fixing system on the atmosphere as a source of N. i.e. the symbiotic dependence ( P atm ). The unique feature of 15 N-based methods is their ability to provide time-integrated and yield-independent estimates of P atm . In studies conducted in artificial media or in soil using the E methodology there was either no response or a positive response of P atm to e[CO 2 ]. The interpretation of results obtained in artificial media or with 15 N 2 is straight forward, not being subject to the assumptions on which the E and NA soil-cultured methods are based. A variety of methods have been used to estimate isotopic fractionation attendant on the NA technique, the so-called ‘B value’, which attaches a degree of uncertainty to the results obtained. Using the NA technique, a suite of responses of P atm to e[CO 2 ] has been published, from positive to neutral to sometimes negative effects. Several factors which interact with the response of N 2 -fixing species to e[CO 2 ] were identified. [ABSTRACT FROM AUTHOR]