Graduate Thesis Or Dissertation

Influence of cropping and sulfur fertilization on sulfur transformations in soil

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  • A 2-year field study was conducted on the effects of two sulfur fertilizers (calcium sulfate or elemental S, 0 or 50 kg S ha⁻¹ yr⁻¹) on the distribution of sulfur fractions (inorganic sulfate, C-bonded, ester sulfate, and residual S), arylsulfatase enzyme activity, and microbial biomass carbon (C) and sulfur (S) over time in an Aquultic Argixeroll in cropped (winter rape, Brassica napus) and uncropped treatments. Despite high rainfall, winter SO₄⁻² levels in the 0 to 15 cm depth ranged from 7 to 13 mg SO₄⁻²-S kg⁻¹ soil in calcium sulfate treated plots, compared to control plots which ranged from 2 to 7 mg SO₄⁻²-S kg⁻¹ soil. However, in the months from March to May, SO₄⁻² levels (0 to 15 cm depth) decreased to < 4 and < 6 mg SO₄⁻²-S kg⁻¹ soil (years 1 and 2, respectively) and were not significantly different among treatments. From March to May of 1988 biomass S increased from 2 to nearly 6 mg S kg⁻¹ soil, which provided evidence that S immobilization is occurring in the spring. Overall, there was a significant (P< 0.01) effect of the sulfur fertilizer on SO₄⁻² levels in the surface soil, but the effect of cropping was not significant. Results of this study indicate that residual S varied seasonally, and is not a completely inert S fraction. Cropping increased biological activity, as cropped plots had significantly greater arylsulfatase activities and microbial biomass C than uncropped plots. Conversely, distributions of organic S fractions were unaffected by the presence of plants during the two-year period. Subsoil SO₄⁻² appeared to be both a source and a sink for S in surface soil. After 2 years, extractable SO₄⁻² in the 60-90 cm depth was significantly affected by cropping and S fertilization, with uncropped calcium sulfate treated plots having the greatest accumulation of SO₄⁻² which increased from 17 to 27 mg SO₄⁻²-S kg⁻¹ soil over the course of the study. During the unusually dry, warm period in the fall of 1987, total S levels in the surface soil increased, even in uncropped control plots, suggesting the possibility of upward movement of SO₄⁻² from the subsoil. At the end of the study (July, 1988), the SO₄⁻² rich subsoil zone below the 60 cm depth showed significant differences in extractable SO₄⁻² due to cropping and S fertilization. Results from this study raise a concern about using extractable SO₄⁻² as an S soil test. First, SO₄⁻² levels varied widely over time, even in the control which ranged from 2 to 14 mg SO₄⁻²-S kg⁻¹ soil. Secondly, S fertilizer applications increased SO₄⁻² levels in the calcium sulfate treated plots, but elemental S treatments remained similar to the controls. Because seed yield responses to elemental S were observed each year, it is apparent that extractable SO₄⁻² does not predict plant availability of S from elemental S applications. Critical levels have been established at 2-3 mg SO₄⁻²-S kg⁻¹ soil, thus depending on what time of year a soil sample is taken different conclusions would be reached as to whether this soil would be responsive to S fertilization.
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