Role of organic sulphur in supplying sulphate for plant growth

An open system of incubation was developed to measure sulphur mineralisation in a wide range of New Zealand soils. During 10 weeks of incubation at 30°C, the amount of sulphur mineralised varied from 2.9 to 26.8 µg sulphur g⁻¹ soil (3.4 to 32.2 kg ha⁻¹). In an attempt to explain the variation in sulphur mineralisation between soils correlations were examined between the amounts of mineralised-SO₄²⁻and some Individual soil chemical properties. C-bonded forms of sulphur (C-S) in soils showed the best single factor correlation with the amounts of mineralised- SO₄²⁻and this relationship was able to account for up to 63% of the variation in mineralised- SO₄²⁻between the soils. These findings were further supported by a series of experiments in which C-bonded sulphur was identified as a major contributor to mineralised- SO₄²⁻. The best multiple correlation was obtained from the combination of C-bonded sulphur and the C:N ratio of the soil and this relationship accounted for 71 % of the variation In sulphur mineralisation between the soils. The dichotomous model of soil sulphur cycling as proposed by McGill and Cole (1981) was tested by altering the amounts of readily available C, N and S in the soil. Addition of C to soils resulted a significant decrease in the mineralisation of C-bonded forms of sulphur. This would suggest that mineralisation of C-bonded forms of sulphur is controlled by the availability of metabolisable C in soils. In addition, this study showed that soil micro-organisms could mineralise C-bonded sulphur to satisfy their sulphur or possibly nitrogen requirements. It was also found that In the presence of low levels of SO₄²⁻ increased microbial activity (where C was added) did not necessarily result in the mineralisation of HI-reducible forms of sulphur. In most cases, microbes apparently selectively mineralised more C-bonded than HI-reducible forms of sulphur. Sulphur-35 was used as a tracer both in a carrier-free form and with sulphate as a carrier to examine the cycling of sulphur in a closed incubation system. Soil treatment prior to the addition of sulphur-35 I.e. preconditioning, air-drying or adding glucose showed marked differences In the rate of sulphur-35 incorporation into the soil and also affected the nature of the sulphur-35 Incorporation into organic fractions. Recovery of sulphur-35 in the microbial biomass showed that a considerable amount of sulphur Is incorporated through the biomass. In some cases the amount of sulphur cycled through the biomass reached 90% of the total Incorporation. Addition of sulphate as carrier decreased the amount of sulphur-35 Incorporation. Reincubation of the labelled soil, where sulphur-35 was incorporated into organic fractions during the original Incubation showed that the longer the sulphur remained In organic fractions the less it was mineralised. A technique was developed to remove the HI-reducible sulphur from the soil organic sulphur fraction. The method effectively removed more than 98% of the HI-reducible sulphur from the soil. Such a separation enabled the study of sulphur mineralisation characteristics from C-bonded sulphur. In a field experiment, the effects of seasonal variation on sulphate and microbial biomass-S levels in soils were assessed. Attempts were made to explain these variations through changes in rainfall and temperature. The amounts of sulphur held in microbial biomass tissues was higher in the autumn and summer seasons compared to winter season