Am. Mcneill et al., DIRECT MEASUREMENT OF NITROGEN-FIXATION BY TRIFOLIUM-REPENS L AND ALNUS-GLUTINOSA L USING N-15(2), Journal of Experimental Botany, 45(275), 1994, pp. 749-755
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.