Site variability effect on field measurement of symbiotic nitrogen fixation using the nitrogen 15 isotope dilution method

Reichardt, K.; Hardarson, G.; Zapata, F.; Kirda, C.; Danso, S.K.A.

Soil Biology and Biochemistry 19(4): 405-410


ISSN/ISBN: 0038-0717
Accession: 006423821

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The sensitivity of the isotope dilution method involving the application of 15N-enriched fertilizers to estimate symbiotic N2 fixation in legumes under field conditions is analysed with respect to the variability of atom% 15N excess in the fixing and non-fixing (reference)crops. Field data collected along a 96 m transect with 63 plots split into two strips, one sown with ryegrass and the other with alfalfa, showed that the sensitivity of the methodology increased with the level of fixation. The results further showed that the sensitivity of the 15N isotope dilution method for estimating N2 fixed can be greatly improved by selecting fairly homogeneous sites, although use of a heterogenous site is quite practicable if the non-fixing reference crop is sited reasonably close to the test plots. Errors associated with mismatch between reference and fixing crops or heterogeneity of site become smaller as the amount of fixation increases. The data also revealed that relative comparisons of N2 fixing abilities of legume treatments based on their 15N enrichments may not be valid for soils with highly variable N content. Under such circumstances, the use of a reference crop sited close to each legume treatment is necessary.