Assessment of the relative uptake of added and indigenous soil nitrogen by nodulated legumes and reference plants in the nitrogen 15 dilution measurement of nitrogen 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
Accession: 004788653

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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 2 pot experiments. In the 1st experiment, 15N-labeled 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 2nd experiment using a soil of higher total N, sodium nitrate or ammonium sulfate were surface-applied at a rate equivalent to 1 kg Na 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. When ammonium sulfate was added to label to 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 on N fixed by the legume when compared with that for the control or sodium nitrate treatments.