Soif &of. &o&em. Vol. 24, No. 4, pp. 331-341,1992 Printed in Great Britain. All rights reserved OO38-0717/92 %5.00 + 0.00 Copyright Q t992 Pergamon Press plc SULPHUR ~INERALISATIUN AND TRANSF~R~AT~UNS IN SOILS AS INFLUENCED BY ADDITIONS OF CARBON, NITROGEN AND SULPHUR A. &iANNI,’ R. G. MCLAREN~ and R. S. SWPT~ Department of Soil Science* Lincoln University, Canterbury, New Zealand zyxwvutsrqponmlkjihgf (Accepted 5 November 1991) Summary-The mineralisation af soil organic sulphur was examined for two soils using an open incubation system. Combinations of glucose-carbon, nitrate-nitrogen and sulphate-S were added to the incubated soils at regular intervals to examine the eiTects of these nutrients on S mineralisation. Soil microbial activity was monitored by CQ evolution from the incubated samples. At the end of the iii-week incubation, the incubated soils were anatysed to determine the sources of S mineralised during the incubation. In botk soils, the addition of C, N and S had a considerable effect on S mineralisation. Additions of C or S decreased S mineralisation whereas N additions slightly enhanced mineralisation. The mineralised S appeared to be derived almost exclusively from C-bonded forms of soil organic S. In both soils, there were substantial increases in hydriodic acid (WI)-reducible farms of S during the incubation. Additions of C, N and S influenced the amounts of S mineralised or transformed from or to C-bonded and HI-reducible forms of soil organic S. The results from this study are discussed in relation to the dichotomous modef of S cycling in soii as proposed by McGill and Cole (Geoderma, 26, X7-286, 1981). In some respects, the S mineralisation data appeared to support the model. However, in relation to the source of mineralised S, there were large differences between what t,he model would predict for certain treatments and what was actually observed. This was particularly true with regard to the transformations of HI-reducible forms of organic S. The transformation of soil organic sulphur to inor- ganic sulphate-S, i.e. S mineralisation, and the reverse process, the incorporation of sulphate-S into soil organic compounds, play important roles in the cycfing of S within the soil. The incorporation of sufphate-S into the soil organic pool, including the microbia1 biomass, is commonly referrred to as S immobilisation, e.g. Freney (1986), and this definition of S immobilisation is adhered to in the present paper. The definition therefore excludes other poss- ible processes such as plant uptake of S and sorption or precipitation reactions which may also sometimes be referred to as “S immobilisatian”. Both S mineral- isation and immobilisation are microbiologically mediated and thus depend on the type and size of soil microbial population and the physiological state of the organisms. The activity of the microbial popu- lation will be affected by the prevaiiing soif physical and chemical conditions including the avaiIabilit~ of essential nutrients, particularly carbon, nitrogen and S. The forms of organic S present in the soil will also play an important role in determining the ease of sulphur mineralisation. Both of the main groups of organic S compounds, i-e, hydriodic acid (HI)- *Present address: MAFTech, Ruakura Agricultural Research Centre, Private Bag, Hamilton, New Zealand. TPresent address: Department of Soil Science, University of Reading, London Road, Reading, England. SAuthor for correspondence. reducible and carbon-bonded as defined by Freney (1986), have been shown to be mineral&d to sulphate-S when added to soils (Houghton and Rose, 1976; Fitzgerald er al., 1984). Field and laboratory studies have also shown that mineralised S can originate from both HI-reducible and C-bonded forms of soil organic S (e.g. Freney et al., 1975; McLaren and Swift, 1977). However, there is little information on either the relative ease of mineralis- ation of the two forms of S or on factors controlling their mineralisation. McGill and Cole (1981) have presented a concep- tual model to explain the comparative cycling of C, N, S and P through soil organic matter. Based on the stoichiometric relationship between C, N, S and organic P in soils, McGill and Cole (1981) have suggested that two types of process are involved in the mineralisation of organic S; biological and b~#~~e~~~ul minera~sation‘ Biological ~neralis~tion is considered to be driven by the microbial consump- tion of organic C to provide energy. Thus sulphur which is directly bonded to C in soil organic com- pounds (C-bonded S), and is released as sulphate (SO:-) during the oxidation of C to carbon dioxide, is considered essentiafly a by-product oftbe microbial search for energy. Biochemical mineralisation of S is defined by McGill and Cole (198 1) as the release of inorganic S from organic forms through enzymatic activity external ta the cell membrane, and is controlled by the supply of S rather than the need for energy. In this 331