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Sulfur forms in organic substrates affecting S mineralization in soil

Churka Blum, Susana, Lehmann, Johannes, Solomon, Dawit, Caires, Eduardo Fávero, Alleoni, Luís Reynaldo Ferracciú
Geoderma 2013 v.200-201 pp. 156-164
Avena strigosa, Oxisols, Pisum sativum, Raphanus raphanistrum, Triticum aestivum, X-radiation, Zea mays, absorption, biochar, carbon, carbon dioxide, corn stover, energy, enzymatic hydrolysis, fractionation, hydrolysis, leaching, microbial activity, microorganisms, mineralization, peas, soil enzymes, spectroscopy, sulfur, wheat
The effects of sulfur (S) forms beyond total S contents for S release from litter to soil are not well understood. In this study, the effect of plant residues (black oat — Avena strigosa Schreb, pea — Pisum sativum L., rape — Raphanus raphanistrum L., wheat — Triticum aestivum L., corn stalk — Zea mays L. and corn stalk biochar applied on an equivalent sulfur basis) with greatly varying S contents, C/S ratios and organic S forms on S mineralization and immobilization in an Oxisol was monitored for 90days using a laboratory incubation experiment. Soil microbial activity (CO2 evolution) and N mineralization were also evaluated. At 3 and 90days of the incubation experiment, the samples were analyzed to assess the main transformations in the soil S pools. Plant residues and biochar had a considerable effect on S mineralization. The highest leaching of sulfate occurred after the application of biochar (11.05mgkg−1 at the first leaching, corresponding to 29.1% of the total S added), and the main mechanisms involved in this process were the abiotic release of mineral sulfur and the hydrolysis of ester-S mediated by soil enzymes, since no relationship with CO2 evolution was observed. Our results suggest that the forms of S in the starting materials seem to drive S mineralization. Increases in mineralized S at earlier stages of the incubation after the incorporation of plant residues and biochar to the soil were correlated with the most oxidized S species (+6) in the organic amendments as revealed by X-ray Absorption Near-Edge Structure (XANES) spectroscopy (r=0.92, p<0.01 at 15days and r=0.80, p<0.05 at 30days; n=6). These findings seem to confirm the hypothesis that S forms rather than S concentration in the tissue plays a major role in S mineralization. In addition, during the first three days of incubation an increase of soil contents of ester-S was associated with a decrease in C-bonded S. Our results, obtained by wet-chemical S fractionation, indicated that in highly oxidized S containing residues, the process of S mineralization was mostly governed by the enzymatic hydrolysis of the ester-S pool rather than the need for carbon to provide energy to the microorganisms. With the application of C-bonded S rich residues, the dominant mechanism was biological mineralization, thus liberating S as a secondary product.