Accumulation of soil sulphur fractions in grazed pastures receiving long‐term superphosphate applications

Topsoil samples (0–75mm) collected at the beginning ofeach growing season over a 35‐year period from grazed pastures receiving annual superphosphate applications at rates of 0, 188, and 376 kg/ha per year were analysed for changes in soil organic nitrogen (N), total carbon (C),organic sulphur (So), hydriodic acid‐reducible S (HI‐reducible S), carbon‐bonded S (C‐bonded S), and readily‐soluble plus adsorbed S. In addition, soils from subsoil depths (75–300 mm) were also sampled at the beginning of each growing season for 3 years to investigate the effects of superphosphate applications on So, readily‐soluble plus adsorbed S, microbial S, and arylsulphatase activity. Results obtained showed that over 95% of soil S was in So and the remainder was in readily‐soluble plus adsorbed S. Applications of superphosphate to increase S fertiliser inputs from 0 to either 21 or 42 kg S/ha per year, substantially enhanced the accumulation of So, HI‐reducible S, and C‐bonded S fractions. However, there was no further increase in these So fractions by increasing annual S inputs from 21 t0 42 kgS/ha. Approximately 50% of this additional S was considered to be leached from the top 300 mm of the soil, which has a low capacity to adsorb S. The accumulation of So Received 21 August 1989; accepted 17 October 1989 reached an apparent steady state after25‐27 years of superphosphate applications, whereas N and C attained a steady state at 3–4 and 9–12 years earlier respectively, thus suggesting that these elements were stabilised independently in soil organic matter. Both C‐bonded S and HI‐reducible S were the major So fractions constituting 50–60% and 40–50%, respectively. The proportion of So as C‐bonded S decreased with depth, whereas the reverse occurred with HI‐reducible S. The arylsulphatase activity reflected the previous topdressing history and decreased with depth. In contrast, microbial S was not affected by superphosphate applications and showed inconsistent trends with depth. It accounted for less than 3% of total soil S in all treatments.