Soil S availability in upland pastures of NE Scotland: relationship of extractable soil S and soil respiration to soil and site characteristics

The mean extractable sulphur (S) concentration in 315 upland topsoil sample s collected in 1988/89 from beneath pasture in NE Scotland was 13 mu g S g( -1) (range 2-77 mu g S g(-1) More than two thirds of the samples had S conc entrations less than that acceptable for productive soils. Continued decrea ses in atmospheric S inputs may have increased this proportion subsequently The analysis of herbage S also indicated that two-thirds of the samples we re below 0.2% S. A 'respirometric index: namely CO2 produced during cellulo se decomposition without added S as a percentage of that produced with adde d S, was significantly less than 100% in a quarter of the soils. Results of three different extraction procedures suggested that sulphate in the soils was present mainly as free plus adsorbed rather than precipitated forms. S oil extraction identified a significant non-sulphate S fraction, presumably organic S. The variability in extractable S stemmed from a combination of geographical, depositional and local site and soil factors. Extractable S w as significantly correlated with soil organic matter content and inversely with soil pH and together these factors explained 37% of the variability Wh ile significant differences in mean concentrations between geographical are a, soil association and drainage status were evident, no trends could be ob served between the major soil subgroups or with altitude.