Changes in Soil Organic Sulphur Fractions Due to the Long Term Cultivation of Soils

Abstract

The amount and chemical nature of soil organic sulphur was determined in several pairs of soils taken from long-term pasture and adjacent, continuously cultivated sites. Similar determinations were also carried out on organic matter extracts obtained from the soils. The lower levels of sulphur in the arable soils compared with pasture soils were assumed to have resulted from the mineralisation of organic matter brought about by cultivation. Losses of sulphur caused by this mineralisation were found to occur in all three organic fractions examined.A high proportion (75%) of the sulphur lost consisted of carbon-bonded sulphur with only 25% derived from HI-reducible forms. Despite this, it is suggested that, of the two forms, HI-reducible sulphur has a more transitory nature and is possibly of greater importance in the short-term mineralisation of sulphur whereas carbon-bonded sulphur passes through an HI-reducible form prior to release as inorganic sulphur. It is suggested that the sulphur present in the fulvic acid pool could, likewise, be most important for short-term mineralisation. IntroductionI N MOST soils organically-bound sulphur is the predominant form of sulphur, inorganic forms accounting for only a few per cent of the total. Organic sulphur must be mineralised t o sulphate in order t o become available for plant uptake. The process of mineralisation becomes increasingly important for maintaining supplies of sulphur t o the plant under agricultural conditions where other inputs of sulphur t o the soil-plant system, mainly fertilisers and atmospheric inputs, are decreasing.Although there is considerable information on the forms of soil organic sulphur (Williams, 1975) there have been few attempts t o investigate their rates of turnover in order t o determine their relative importance in the availability of sulphur t o plants. The investigations that have been undertaken so far have consisted of relatively short-term incubations o r pot experiments (Freney et al., 1971 ;Bettany et al., 1974; Freney et al., 1975). The main problem with such experiments would appear t o be the accurate detection of the small amounts of sulphur turnover encountered in such systems, often amounting to a few per cent o r less of the total sulphur present.The present study attempts to overcome this problem by examination of changes in the distribution of sulphur in soils brought about by long-term, natural mineralisation processes. T o achieve this a compari-

Soil organic sulfur forms and dynamics in the Great Plains of North America as influenced by long-term cultivation and climate

“…According to this table, total organic S in bulk soils of the native grassland sites ranged from 194 to 853 mg kg À 1 soil, with an average of 441 mg kg À 1 soil ( Table 2, the Arvada site is excluded), while the concentration of organic S in the cultivated soils varied from 135 to 489 mg kg À 1 soil, the average being 300 mg kg À 1 soil. These results also indicated that organic S was the predominant form of soil S in the Great Plains comprising on the average 96% of the total soil S. The concentration of total SOS reported in this study falls within the range reported for temperate soils by Tabatabai and Bremner (1972), Neptune et al (1975), McLaren and Swift (1977) and Eriksen (1997). However, it is considerably lower than the values reported for humid and sub-humid tropical soils by Stanko-Golden and Fitzgerald (1991), Lehmann et al (2001) Solomon et al (2001) and Mö ller et al (2002).…”

“…These results concur positively with the suggestions of Lowe (1964), McLaren et al (1985), Schindler and Mitchell (1987) and Zhao et al (1996) that ester-SO 4 S can be stabilized independent of the main moiety of organic matter, may represents slightly more labile organic S fraction that can serves as a readily available S pool through biochemical mineralization process. In contrast to these findings, however, other studies using incubation and field experiments (Freney et al, 1975;McLaren and Swift, 1977;Ghani et al, 1991;Solomon et al, 2001;Mö ller et al, 2002) indicated that relatively larger proportion of SOS loss following land-use changes occurs from C-bonded S than ester-SO 4 S and stated that this organic S fraction represents the major source of mineralizable S in soils supporting the suggestion by McGill and Cole (1981). This view was also supported by our previous investigation using S XANES spectroscopy where Cbonded S (SOS in highly reduced and intermediate oxidation states) forms seems to represent the more labile forms of SOS compounds compared to ester-SO 4 S (SOS in highly oxidized states) forms and the dynamics of organic S was primarily driven by SOC turnover (Solomon et al, 2003(Solomon et al, , 2005.…”

“…All extractions were made on triplicate samples. Although most S speciation studies (Neptune et al, 1975;McLaren and Swift, 1977;Lehmann et al, 2001;Mö ller et al, 2002) were conducted in the past using air dried soils, it is worth mentioning the fact that this sample preparation procedure may have some effect on the proportions of extractable organic S and sulfate S (Watkinson and Kear, 1996;Tan et al, 1994), and thus direct comparison of results should be limited to investigations using similar sample preparation strategy.…”

“…These studies included characterization (Freney et al, 1970), cycling (McLaren and Swift, 1977;Strickland et al, 1987;Eriksen, 1997) and management impacts (Tabatabai and Bremner, 1972;Bettany et al, 1980;Zucker and Zech, 1985;Solomon et al, 2001) on organic S compounds in soils. In contrast, the impact of long-term cropping and climatic variables (temperature and precipitation) on the amount, form and composition of organic S compounds in soils along the temperature and precipitation transects of the Great Plains of North America has been largely neglected.…”

Pasture production and changes in phosphorus and sulphur status in irrigated pastures receiving long-term applications of superphosphate fertiliser

“…The decline in soil inorganic P and organic S after the cessation of superphosphate fertiliser could be attributed to the desorption of inorganic P from soil colloids (Syers & Iskander 1981) and the mineralisation of soil organic S (McLachlan 1975;McLaren & Swift 1977). This mobilisation of soil P and S reserves could be the main contributing factor in sustaining pasture production and herbage uptake of P and S in both the 376 and 564 kg/ha residual treatments above the unfertilised control.…”