Comparison of three different methods of determining soil sulphur mineralization in relation to plant sulphur availability in soils

Abstract

S mineralization in soils with different fertilizer histories [0, 188 and 376 kg superphosphate (SP) ha/year applied since 1952] and animal camping treatments (camp and non-camp soils) was determined simultaneously under the same glasshouse conditions using three different methods (i.e. open incubation, pots with and without plants). Results obtained showed that amounts and patterns of soil S mineralized differed from one system to the other. In the open incubation system, the pattern of S mineralization showed a rapid release of S in the first 4 weeks followed by no substantial release during the remaining 20 weeks of incubation. In both pot systems, S mineralization was slow initially and increased significantly with time. Total amounts of 32S mineralized in the open incubation system ranged from 7.5 to 11.9 µg S/g soil, while corresponding values for pot systems were 2.3–3.7 and 2.3–5.9 µg S/g soil for pots with and without plants, respectively. Rates of soil 32S mineralization with time as fitted by regression models were different in the three systems, showing the differential impact of leaching, crop removal and plant effects on soil S mineralization. Similar results were obtained using radioactive 35S tracer. Overall, the results suggested that contrary to commonly postulated ideas, soil S mineralization determined by periodic leaching of soil in the open incubation system does not simulate crop removal of S or provide a means of predicting plant S availability.