Sulfur forms in bulk soils and alkaline soil extracts of tropical mountain ecosystems in northern Thailand

Abstract

Sulfur, besides phosphorus, is crucial for the nutrition of plants on tropical soils. Its availability is closely related to the turnover of soil organic matter. To get a better insight into transformation of soil S forms during the decomposition of organic matter, we studied inorganic and organic S pools in bulk samples and alkaline extracts of soils under different land uses representative of the tropical highlands of northern Thailand. Samples were taken from a cabbage cultivation, a Pinus reforestation, a secondary forest, and a primary forest. Total S ranged from 483 549 mg/kg in the subsoil to 1909 376 mg/kg in the organic layers, which is relatively high for tropical soils. The major S component in soil was organic S, comprising 75–99% of total S. Organic S was significantly correlated with total S, organic C, and total N, indicating that there is a close relationship between C, N, and S cycling in soil. C-bonded S was the predominant form in the topsoils (35–99% of total S) but its presence decreased with soil depth. The maximum concentrations of ester SO4-S were found in the A horizons (128 49 mg/kg), whereas the concentrations of inorganic SO4-S were small in all horizons. Compared with the forest site, the cabbage cultivation site was strongly depleted in S. C-bonded S was more depleted than ester SO4-S. A comparison of the S forms in NaOH extracts with S forms in bulk soil and C forms as indicated by 13C-NMR spectroscopy showed (i) that the extracts were very representative of soil organic S fractions and (ii) that ester SO4-S was mainly associated with O-substituted aliphatic C. In contrast, C-bonded S seemed to be connected to more-or-less all C binding types. transformation of soil organic matter, sulfate.

Anthropogenic and climate influences on biogeochemical dynamics and molecular‐level speciation of soil sulfur

“…Total soil S in the surface layers of the native subhumid tropical forest ecosystems of Kenya ranged from 2916 mg S/kg soil (Kakamega) to 3706 mg S/kg soil (Nandi) and was considerably higher than the values from grasslandderived soils of temperate and subtropical ecosystems. However, these results are in line with the values reported for a variety of forest-derived soils from tropical ecosystems (Stanko-Golden and Fitzgerald 1991, Solomon et al 2001, Mo¨ller et al 2002. The higher concentration of S in the subhumid tropical forest ecosystems of Kenya compared with the native temperate and subtropical grassland ecosystems could be broadly attributed to differences in climate, vegetation, and soil type.…”

“…These results concur positively with the XANES analysis of organic S in humic fractions extracted from peat (Morra et al 1997) and forest-derived soils (Xia et al 1998, Solomon et al 2003, as well as with XANES spectra taken directly from peat (Martı´nez et al 2002) and organic layers of forest soils (Prietzel et al 2003). Our results agree also with those of Bettany et al (1980) and Mo¨ller et al (2002), who reported higher proportions of C-bonded S (70-84% of the total organic S) in the humic fractions of variety of temperate and tropical soils using conventional wet-chemical techniques. The dominance of organic S directly bonded to C in the surface layers of grassland-derived temperate and subtropical soils and forest-derived tropical soils could be attributed to the fact that this organic S fraction originates directly from plant exudates, root and leaf litterfall, animal residues, as well as microbial metabolites; which make up the bulk of soil organic matter input to the surface layers of these undisturbed ecosystems.…”

“…3) indicate that S directly linked to C is the predominant form of organic S in the studied humic fractions from surface soils of undisturbed temperate prairie grassland ecosystems of North America (77%), subtropical Highveld grassland ecosystems of South Africa (60%) and subhumid tropical forest ecosystems of Kenya (70%). Our results agree also with those of Bettany et al (1980) and Mo¨ller et al (2002), who reported higher proportions of C-bonded S (70-84% of the total organic S) in the humic fractions of variety of temperate and tropical soils using conventional wet-chemical techniques. These results concur positively with the XANES analysis of organic S in humic fractions extracted from peat (Morra et al 1997) and forest-derived soils (Xia et al 1998, Solomon et al 2003, as well as with XANES spectra taken directly from peat (Martı´nez et al 2002) and organic layers of forest soils (Prietzel et al 2003).…”

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

“…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.…”