Direct measurement of nitrogen fixation by Trifolium repens L. and Alnus glutinosa L. using 15N2 |
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Authors: | McNeill, A.M. Hood, R.C. Wood, M. |
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Abstract: | A closed-system flow-through enclosure apparatus was used tomeasure symbiotic nitrogen fixation directly. A legume-basedsystem comprising 6-week-old Trifolium repens L. (white clovercv. Blanca) growing with Lolium perenne L. (perennial ryegrasscv. Trani) in an agricultural soil was incubated for 19 d ina 15N-enriched atmosphere (mean value 3.663 atom%). An actinorhizal-basedsystem comprising 1 -year-old Alnus glutinosa L. (alder) saplingsgrowing with Festuca rubra L. (red fescue) in open-cast coalspoil was incubated for 21 d in a 15N-enriched atmosphere (meanvalue 3.265 atom%). Indirect estimates of N2 fixation were carriedout concurrently using N difference and 15N isotope dilutiontechniques. The theory underlying the three techniques and modificationswhich were adopted for comparative purposes are discussed. Thedirect measurements of N2 fixation were then compared with theindirect estimates using Pinc, the proportion of the N incrementduring 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 isotopemethodologies, both indirect (0.91) and direct (0.90). Valuesfor alder were far more variable, ranging from 0.16 to 0.92;this was due largely to variability within the trees and a verysmall N increment during the measurement period. Key words: N2 fixation, 15N2, white clover, alder, enclosure apparatus |
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