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[Display omitted] • S0&BM boosted NO 3 –-N removal by 15.4% and slashed N 2 O release by 59.0% vs. S0. • Boron and magnesium of S0&BM optimized the microenvironment and ecological niche. • Oxygen vacancies enabled a closed-loop sulfur cycle, converting chem-S0 to bio-S0. • S0&BM boosted gene expression for full denitrification and sulfur cycling. • S0&BM utilized for boron mud recycling supported nitrogen sink formation in CWs. Constructed wetlands (CWs) utilizing elemental sulfur (S0) as an electron donor are effective for nitrate (NO 3 –-N) removal from low C/N wastewater. However, the limited bioavailability of chemical S0 (chem-S0) due to low water solubility and inadequate mass transfer impedes complete denitrification. In this study, boron mud waste and S0 (S0&BM) were processed and combined to convert chem-S0 to biological S0 (bio-S0) in CWs. The employment of S0&BM improved NO 3 –-N and total nitrogen (TN) removal rates by 15.36 % and 10.55 % compared to S0 alone. The 15N isotope tracing experiment unveiled the pathways of N 2 O production and demonstrated that S0&BM facilitated full denitrification, resulting in a higher percentage of nitrogen being degraded in a non-polluting form (54.70 %). Releasing trace amounts of boron and magnesium in S0&BM mitigated the excessive acidification caused by the sulfur autotrophic process and promoted biofilm formation, thereby pronouncing vertical oxygen fluctuations. Meanwhile, oxygen vacancies generated in the S0&BM promoted the closed-loop sulfur cycle, starting from the oxidation of chem-S0 to sulfate (SO 4 2-), the important process of SO 4 2--reducing bacteria using partial organic compounds as electron donors to reduce SO 4 2- to sulfide (S2-) was realized, and finally the S2- was oxidized to bio-S0 with better properties. S0&BM enriched functional microbial communities in the biofilm related to nitrogen and sulfur transformation. Functional gene analysis showed an increase in the narG , nirS , norB , and nosZ genes supporting denitrification. The dsrA and sqr genes responsible for the sulfur cycle exhibited the highest abundance. This realization allowed for the utilization of waste materials for treatment, enhancing the energy efficiency of carbon reduction and pollution reduction in CWs. [ABSTRACT FROM AUTHOR]

[Display omitted] • S0&BM boosted NO 3 –-N removal by 15.4% and slashed N 2 O release by 59.0% vs. S0. • Boron and magnesium of S0&BM optimized the microenvironment and ecological niche. • Oxygen vacancies enabled a closed-loop sulfur cycle, converting chem-S0 to bio-S0. • S0&BM boosted gene expression for full denitrification and sulfur cycling. • S0&BM utilized for boron mud recycling supported nitrogen sink formation in CWs. Constructed wetlands (CWs) utilizing elemental sulfur (S0) as an electron donor are effective for nitrate (NO 3 –-N) removal from low C/N wastewater. However, the limited bioavailability of chemical S0 (chem-S0) due to low water solubility and inadequate mass transfer impedes complete denitrification. In this study, boron mud waste and S0 (S0&BM) were processed and combined to convert chem-S0 to biological S0 (bio-S0) in CWs. The employment of S0&BM improved NO 3 –-N and total nitrogen (TN) removal rates by 15.36 % and 10.55 % compared to S0 alone. The 15N isotope tracing experiment unveiled the pathways of N 2 O production and demonstrated that S0&BM facilitated full denitrification, resulting in a higher percentage of nitrogen being degraded in a non-polluting form (54.70 %). Releasing trace amounts of boron and magnesium in S0&BM mitigated the excessive acidification caused by the sulfur autotrophic process and promoted biofilm formation, thereby pronouncing vertical oxygen fluctuations. Meanwhile, oxygen vacancies generated in the S0&BM promoted the closed-loop sulfur cycle, starting from the oxidation of chem-S0 to sulfate (SO 4 2-), the important process of SO 4 2--reducing bacteria using partial organic compounds as electron donors to reduce SO 4 2- to sulfide (S2-) was realized, and finally the S2- was oxidized to bio-S0 with better properties. S0&BM enriched functional microbial communities in the biofilm related to nitrogen and sulfur transformation. Functional gene analysis showed an increase in the narG , nirS , norB , and nosZ genes supporting denitrification. The dsrA and sqr genes responsible for the sulfur cycle exhibited the highest abundance. This realization allowed for the utilization of waste materials for treatment, enhancing the energy efficiency of carbon reduction and pollution reduction in CWs. [ABSTRACT FROM AUTHOR]