Mixed carbon source strategy sustains long-term nutrient removal in pyrite-based bioretention systems: Insights into...

Bioretention systems (BSs) are widely used for urban stormwater management, yet sustaining nutrient removal over long-term operation remains challenging. A 483-day experiment evaluated a 1:1 woodchip-corncob mixed-carbon formulation in pyrite-based BSs by comparing a mixed carbon source-pyrite system (WCP) with woodchip-pyrite, corncob-pyrite, and pyrite-only systems. Under the tested conditions, the WCP system showed higher observed nutrient removal and a smaller long-term performance decline than the comparison systems, maintaining total dissolved nitrogen and total dissolved phosphorus removal efficiencies of 81.4 ± 3.7% and 73.3 ± 5.8%, respectively, during the long-term stage. Pore-water and fluorescence analyses indicated differences in dissolved organic matter release intensity and composition associated with the mixed-carbon configuration, consistent with more balanced organic electron-donor availability during alternating rainfall and drought periods. 16S rRNA gene sequencing revealed differences in microbial community composition and functional-group distribution, while absolute quantitative metagenomics revealed differences in nitrogen- and sulfur-related functional gene profiles among the composite samples. At the long-term endpoint, the observed electron transport system activity in the WCP system was 21.3%-239.0% higher than the values observed in the comparison systems. The metagenomic profiles and sulfur-related functional patterns were consistent with nitrous oxide-reduction potential and potential coordination between nitrogen and sulfur transformations. Overall, the tested mixed-carbon configuration sustained long-term nutrient removal and was associated with more balanced organic electron-donor availability and potential coordination between nitrogen and sulfur transformations, highlighting its potential for long-term stormwater bioretention.