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

There is ample evidence about the impact of land-use and climatic variables such as temperature and precipitation on the biogeochemical cycling of organic carbon (C), nitrogen (N), phosphorus (P) in terrestrial ecosystems. However, although sulfur (S) is one of the essential nutrients for plant growth, the impact of these factors on soil organic S (SOS) forms has received little attention. The objective of this investigation was to assess the influence of long-term anthropogenic management and climate on the amount, form and distribution of SOS in bulk soils and in the clay and silt sized separates of soils from the Great Plains of North America using degradative wet-chemical reduction technique. For this purpose, samples were collected from surface soils (0 to 10 cm) of native grassland and cultivated sites along temperature and precipitation transects from Saskatoon, Canada to Texas, USA and total SOS, C-bonded S and ester-SO4 S contents were determined. The concentration of SOS in bulk soils ranged from 194 to 853 mg kg 1 soil and from 135 to 441 mg kg 1 soil in the native grassland and cultivated soils, representing on the average 96% of the total soil S. Carbon-bonded S was the major organic S form comprising about two-thirds of the total SOS in clay and about three-fourth of the total SOS in silt sized separates. Only a small portion of the total SOS was present in ester linkages. Significantly larger concentration (P b0.05) of SOS was found in the clay sized separates compared to the silt. Breaking of native grasslands and converting them into long-term cropping significantly reduced (P b0.05) organic S in the bulk soils and in the size separates. Relatively larger proportion of SOS was lost from ester-SO4 S (39%) than C-bonded S (25%), suggesting that it may represents the more labile form of organic S compared to C-bonded S. However, the result should be interpreted with caution since classification of organic S on the bases of bond classes may not have enough sensitivity to distinguish between labile and refractory organic S forms. The concentration of SOS (r = 0.64* and 0.79**), C-bonded S (r = 0.69* and 0.83**) and ester-SO4 S (r = 0.57* and 0.76**) significantly decreased (P b0.05) with an increase in mean annual temperature (MAT) in bulk soils of the native grassland and cultivated sites, respectively, indicating that temperature is the most important site attribute governing turnover of SOS pool in the Great Plains of North America. Compared to MAT, the impact of precipitation on SOS forms was not pronounced in the native grassland sites. However, the