Characterisation of microbial release of available sulphur from soil to plants
The discovery of sulphur-deficient agricultural soils has focussed concern on the dynamics of sulphur transformations in soils and on the important role of the soil microbial population in mineralising organic sulphur to forms more available to plants. This thesis reports a study carried out to investigate the dynamics of supply of plant available sulphur from soils and includes development of a method for determination of microbial biomass S, comparison of a perfusion system with a plant bioassay to assess soil S-supply, characterisation of a fertilizer S budget for a crop-soil system and assessment of the role of VA mycorrhizas in plant S uptake. The first experiment was carried out to investigate the factors involved in determination of microbial biomass S in soil in order to develop a more reliable assay. Biomass S-concentrations were determined by chloroform fumigation/direct extraction (with determination of the Ks calibration constant using 35S labelled microorganisms). The effects of period of fumigation, the need for chloroform evacuation, the type of extractant, and the time of extraction were investigated. The optimium values for biomass-S were obtained using a 5 day fumigation period without chloroform evacuation (use of 35S labelled microorganisms demonstrated that there is a vacuum sensitive non-biomass S pool), with CaC12 as an extractant over a 1 h extraction period. The second experiment consisted of a simple, open perfusion system for studying the S-supplying capacity of soils. A range of soils were perfused at frequent intervals and the leachates analysed for inorganic and total sulphur. Results were compared with plant S-offtake from the same soils. Although greater amounts of sulphur were removed in the perfusion system, data from the techniques correlated strongly, suggesting the perfusion system can be effectively used to estimate the S-supplying capacity of soils.