Sulfur species behavior in soil organic matter 3 during decomposition

7 [1] Soil organic matter (SOM) is a primary reservoir of terrestrial sulfur (S), but its role in 8 the global S cycle remains poorly understood. We examine S speciation by X-ray 9 absorption near-edge structure (XANES) spectroscopy to describe S species behavior 10 during SOM decomposition. Sulfur species in SOM were best represented by organic 11 sulfide, sulfoxide, sulfonate, and sulfate. The highest fraction of S in litter was organic 12 sulfide, but as decomposition progressed, relative fractions of sulfonate and sulfate 13 generally increased. Over 6-month laboratory incubations, organic sulfide was most 14 reactive, suggesting that a fraction of this species was associated with a highly labile pool 15 of SOM. During humification, relative concentrations of sulfoxide consistently decreased, 16 demonstrating the importance of sulfoxide as a reactive S phase in soil. Sulfonate 17 fractional abundance increased during humification irrespective of litter type, illustrating 18 its relative stability in soils. The proportion of S species did not differ systematically by 19 litter type, but organic sulfide became less abundant in conifer SOM during 20 decomposition, while sulfate fractional abundance increased. Conversely, deciduous SOM 21 exhibited lesser or nonexistent shifts in organic sulfide and sulfate fractions during 22 decomposition, possibly suggesting that S reactivity in deciduous litter is coupled to rapid 23 C mineralization and independent of S speciation. All trends were consistent in soils 24 across study sites. We conclude that S reactivity is related to speciation in SOM, 25 particularly in conifer forests, and S species fractions in SOM change during 26 decomposition. Our data highlight the importance of intermediate valence species 27 (sulfoxide and sulfonate) in the pedochemical cycling of organic bound S.