Direct measurement of nitrogen fixation byTrifolium repensL. andAlnus glutinosaL. using15N2

A closed-system flow-through enclosure apparatus was used to measure symbiotic nitrogen fixation directly. A legume-based system comprising 6-week-old Trifolium repens L. (white clover cv. Blanca) growing with Lolium perenne L. (perennial ryegrass cv. Trani) in an agricultural soil was incubated for 19 d in a 15N-enriched atmosphere (mean value 3.663 atom%). An actinorhizal-based system comprising 1 -year-old Alnus glutinosa L. (alder) saplings growing with Festuca rubra L. (red fescue) in open-cast coal spoil was incubated for 21 d in a 15N-enriched atmosphere (mean value 3.265 atom%). Indirect estimates of N2 fixation were carried out concurrently using N difference and 15N isotope dilution techniques. The theory underlying the three techniques and modifications which were adopted for comparative purposes are discussed. The direct measurements of N2 fixation were then compared with the indirect estimates using Pinc, the proportion of the N increment during the measurement period that was derived from fixation. The simple N difference method gave similar values for Pinc (0.94 and 0.97) as those derived from more complicated isotope methodologies, both indirect (0.91) and direct (0.90). Values for alder were far more variable, ranging from 0.16 to 0.92; this was due largely to variability within the trees and a very small N increment during the measurement period.