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: glasshouse application of method

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

Soil Biology and Biochemistry 17(3): 323-328


ISSN/ISBN: 0038-0717
DOI: 10.1016/0038-0717(85)90069-0
Accession: 001304140

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A method for calculating the relative uptake (R) of added N and indigenous soil N by a legume (Trifolium subterraneum) and non-legume (Lolium rigidum) growing together was investigated in two pot experiments. In the first experiment, 15N-labelled sodium nitrate was applied to the soil surface at rates equivalent to 0.3 or 1.0 kg N ha-1. Twenty one days later, the legume had fixed about 95% of its total N and this was unaffected by N addition. There was no difference in R values between legume and non-legume at both N rates. In the second experiment using a soil of higher total N content, sodium nitrate or ammonium sulphate were surface-applied at a rate equivalent to 1 kg N ha-1 and harvests were made at 3, 6, 12 and 27 days after N addition. Fixation of atmospheric N2 by the legume did not begin until day 12 but accounted for about 40% of the total N assimilated by the legume by day 27. There was no difference in R values between legume and non-legume throughout the growth period when sodium nitrate was applied. However, when ammonium sulphate was added to label the soil N, the uptake of added N relative to indigenous soil N was greater for the non-legume than the legume. This caused an overestimation (51 vs 43%) of the proportion of N fixed by the legume when compared with that for the control or sodium nitrate treatments.