Assessment of the relative uptake of added and indigenous soil nitrogen by nodulated legumes and reference plants in the 15N dilution measurement of N2 fixation: derivation of method

Ledgard, S.F.; Morton, R.; Freney, J.R.; Bergersen, F.J.; Simpson, J.R.

Soil Biology and Biochemistry 17(3): 317-321


ISSN/ISBN: 0038-0717
DOI: 10.1016/0038-0717(85)90068-9
Accession: 001304139

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Current methods for measuring N2 fixation by nodulated legumes involve the addition of small amounts of 15N-labelled plant-available N compounds to soil so that plant N derived from the soil may be identified. All such methods assume that the proportions of added N and indigenous soil N assimilated by N2-fixing and non-fixing plants grown in the same soil are the same, irrespective of the amount of soil N assimilated. The development of a method for assessing these proportions is described. Nodulated legumes and reference plants are grown in soils receiving none or one rate of addition of labelled N compound containing two or more levels of enrichment of 15N. The proportions of added and indigenous N assimilated are determined from the intercepts and slopes of regression lines relating isotopic composition of plant N to that of added N, together with some other readily-obtainable plant N measurements.