Nitrogen and phosphorus cycling in relation to stand age of Eucalyptus regnans F. Muell |
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Authors: | P. J. Polglase P. M. Attiwill M. A. Adams |
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Affiliation: | (1) School of Agriculture and Forestry, University of Melbourne, 3052 Parkville, Victoria, Australia;(2) CSIRO Division of Soils, G.P.O. Box 639, 2601 Canberra, A.C.T., Australia;(3) Natural Resources Division, Department of Conservation and Environment, P.O. Box 406, 3101 Kew, Victoria, Australia |
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Abstract: | Lupins, canola, ryegrass and wheat fertilized with Na2 35SO4 and either 15NH4Cl or K15NO3(N:S=10:1), were grown in the field in unconfined microplots, and the sources of N and S (fertilizer, soil, atmosphere, seed) in plant 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 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 for S than for N for each crop at each sampling 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 to total S uptake by the plants. Therefore the hypothesis that equal ratios of labelled N to unlabelled soil-derived N in legumes (Rleg) and reference plants (Rref) 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 Rleg cannot be evaluated by labelling with 35S. |
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Keywords: | isotope dilution lupins 15N 35S symbiotic N2 fixation |
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