Evaluation of reference plants by labelling with sulfur-35 in the estimation of nitrogen fixation by nitrogen-15 isotope dilution

Hamilton, S.D.; Chalk, P.M.; Smith, C.J.; Hopmans, P.

Plant and Soil 145(2): 177-185

1992


ISSN/ISBN: 0032-079X
Accession: 008636302

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Abstract
Lupins, canola, ryegrass and wheat fertilized with Na-2-35SO-4 and either 15NH-4Cl or K-15NO-3 (N:S = 10:1), were grown in the field in unconfined microplots, and the sources of N and S (fertilizer, soil, atmosphere, seed) inplant tops during crop development were estimated. Modelled estimates of the proportion of lupin N derived from the atmosphere, which were obtained independently of reference plants, were used to calculate the proportion of lupin N derived from the soil. Total uptake of N and S and uptake of labelled N and S increased during crop development. Total of uptake of S by canola was higher than lupins, but labelled S uptake by lupins exceeded uptake by canola. The form of N applied had no effect on uptake of labelled and unlabelled forms of N or S. Ratios of labelled to unlabelled S and ratios of labelled to unlabelled N derived from soil sources decreased during growth, and were less than S than for N for each crop at each smpling time. Although ratios of labelled to unlabelled soil-derived N were similar between crops at 155, 176 and 190 days after sowing, ratios of labelled to unlabelled S for lupins were higher than for the reference crops and declined during this period. The ratios of labelled to unlabelled S in lupins and the reference plants therefore bore no relationship either to ratios of labelled to unlabelled soil-derived N in the plants, or total S uptake by the plants. Therefore the hypothesis that equal ratios of labelled N to unlabelled soil-derived N in legumes (R-leg) and reference plants (R-ref) would be indicated by equal ratios of labelled to unlabelled S was not supported by the data. The results therefore show that the accuracy of reference plant-derived values of R-leg cannot be evaluated by labelling with 35S.