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A constructed wetland microcosm was employed to investigate the sulfur cycle-mediated electron transfer between carbon and nitrate.Sulfate accepted electrons from organics at the average rate of 0.84 mol/(m3.d) through sulfate reduction,which accounted for 20.0% of the electron input rate.The remainder of the electrons derived from organics were accepted by dissolved oxygen (2.6%),nitrate (26.8%),and iron(Ⅲ) (39.9%).The sulfide produced from sulfate reduction was transformed into acidvolatile sulfide,pyrite,and elemental sulfur,which were deposited in the substratum,storing electrons in the microcosm at the average rate of 0.52 mol/(m · d).In the presence of nitrate,the acid-volatile and elemental sulfur were oxidized to sulfate,donating electrons at the average rate of 0.14 mol/(m3· d) and driving autotrophic denitrification at the average rate of 0.30 g N/(m · d).The overall electron transfer efficiency of the sulfur cycle for autotrophic denitrification was 15.3%.A mass balance assessment indicated that approximately 50% of the input sulfur was discharged from the microcosm,and the remainder was removed through deposition (49%) and plant uptake (1%).Dominant sulfatereducing (i.e.,Desulfovirga,Desulforhopalus,Desulfatitalea,and Desulfatirhabdium) and sulfuroxidizing bacteria (i.e.,Thiohalobacter,Thiobacillus,Sulfuritalea,and Sulfurisoma),which jointly fulfilled a sustainable sulfur cycle,were identified.These results improved understanding of electron transfers among carbon,nitrogen,and sulfur cycles in constructed wetlands,and are of engineering significance.