Distribution of Sulfur Fractions in Soil as Influenced by Management of Organic Residues
There is increasing concern about the long-term effects of crop management on soil properties that may be affecting soil productivity. Consequently, a study of the long-term (55 yr) effect of organic residue and inorganic N applications on the distribution of S fractions (inorganic sulfate, C-bonded, ester sulfate, residual, and sulfolipid S) in a Typic Haploxeroll under a wheat-fallow (Triticum aestivum L.) rotation was undertaken. The biennial treatments were: (i) straw (control), (ii) straw plus fertilizer (+N) (34 kg N ha−1 from 1931–1966 and 90 kg N ha−1 from 1967–1986), (iii) straw plus 2.24 Mg ha−1 of pea vine residue (PV), and (iv) straw plus 22.4 Mg ha−1 of strawy manure (M). There was a significant treatment effect (P < 0.01) with the M treatment having 7 to 41% greater total S, and 9 to 22% greater ester sulfate hydriodic reducible S), and nearly two times more C-bonded (Raney-Ni reducible S) and sulfolipid S than the remaining treatments. The lowest level for total S occurred in the +N treatment (114 vs. 161 mg S kg−1 soil in the M treatment). A significantly lower level of residual S (nonreducible) for the +N treatment suggests that the addition of inorganic N causes a depletion of S from the residual pool. On a biennial basis, treatments receiving only straw (control and +N) had net inputs (amount of S added with residue minus S removed in grain) of −0.51 and −1.72 kg S ha−1, respectively, and PV and M treatments had net S inputs of 1.92 and 31.05 kg S ha−1, respectively. Calculation of the coefficient of simple determination (r2) among S fractions and other soil properties showed that whereas organic C and total N had a high r2 value (0.81), total S had a low r2 (0.29) with organic C and was more closely related to total N (r2 = 0.55). The r2 values among the organic S fractions were quite low (<0.23), except for sulfolipid S and C-bonded (r2 = 0.45).
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