Isotopic studies on the uptake of sulphur by pasture plants

Abstract

On two contrasting soils, Kowai sandy loam (recent soil) and Gorge silt loam (lowland yellow-brown earth) in inland Canterbury, New Zealand, plant uptake of ³⁵S-sulphur placed at various soil depths and also the uptake of associated soil sulphur (³²S) present at the same depths were investigated in developed pastures under field conditions. Results from the first harvest in spring 1972 showed that on both soils, white clover and perennial ryegrass plants used sulphur from all four depths studied (i.e., 7.5, 22.5, 37.5, and 52.5 cm). The relative contribution of roots, at each soil depth, to the ³²S uptake was also assessed. With respect to the four depths studied, approximately 50% of the ³²S uptake occurred at the 7.5 cm depth on both soils. Inclusion of the uptake from the 22.5 cm depth accounted for 76% and 90% of total ³²S uptake in the Kowai and Gorge soils respectively. In the autumn 1973 trials, sulphur uptake (³²S and ³⁵S) from the 22, 52, 75, and 100 cm depths was investigated. On the Kowai soil uptake by both grass and clover plants occurred from all four depths whereas on the Gorge soil uptake occurred only in the upper two depths Possible reasons for this difference are discussed. A comparison of spring and autumn uptake from common depths (22 and 52 cm) indicated that in the autumn greater relative uptake of ³²S occurred from the 52 cm depth. The results indicate that despite variability in plant ³²S uptake data, soil sampling below 7.5 cm is necessary at these sites to characterise adequately the sulphur status of the soil with respect to levels of available sulphur present.