DIRECT MEASUREMENT OF NITROGEN-FIXATION BY TRIFOLIUM-REPENS L AND ALNUS-GLUTINOSA L USING N-15(2)

A closed-system flow-through enclosure apparatus was used to measure s ymbiotic 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 so il was incubated for 19 d in a N-15-enriched atmosphere (mean value 3. 663 atom%). An actinorhizal-based system comprising 1-year-old Alnus g lutinosa L. (alder) saplings growing with Festuca rubra L. (red fescue ) in open-cast coal spoil was incubated for 21 d in a N-15-enriched at mosphere (mean value 3.265 atom%). Indirect estimates of N-2 fixation were carried out concurrently using N difference and N-15 isotope dilu tion techniques. The theory underlying the three techniques and modifi cations which were adopted for comparative purposes are discussed. The direct measurements of N-2 fixation were then compared with the indir ect estimates using P-inc, the proportion of the N increment during th e measurement period that was derived from fixation. The simple N diff erence method gave similar values for P-inc (0.94 and 0.97) as those d erived from more complicated isotope methodologies, both indirect (0.9 1) and direct (0.90). Values for alder were far more variable, ranging from 0.16 to 0.99; this was due largely to variability within the tre es and a very small N increment during the measurement period.