Influence of in situ biochar capping on microbial dynamics and ammonia nitrogen release in sediment

To study the influence of in situ biochar (BC) capping technique on the release of ammonia nitrogen (NH₄⁺-N) from sediments, a field mesocosm experiment was conducted in Baiyangdian Lake (BYDL), a critical water body often referred to as the "kidney of North China" where sediment pollution poses a significant threat to water quality. This study also assessed the impact of BC on sediment microorganisms. The results showed that the NH₄⁺-N concentration in the overlying water of the BC-treated mesocosms was the lowest among four treatments, decreasing to 0.051 mg L^-1 by the 60th day. More importantly, the BC treatment showed the least increase in NH₄⁺-N concentrations in sediments compared to other treatments. For sediments capped with a 4 cm layer of BC, the potential release flux of NH₄⁺-N was reduced from 1.84 mg m^-2 d^-1 to -0.76 mg m^-2 d^-1. This reduction is likely due to the negatively charged surfaces of biochar, which enhance NH₄⁺-N adsorption through electrostatic interactions. Additionally, BC modified the physical and chemical properties of the surface sediment, improving pH and increasing both organic content and the carbon/nitrogen (C/N) ratio. These changes influenced the microbial community structure within the sediments, enhancing NH₄⁺-N removal. After 60 days, a significant alteration in the microbial community was observed in the BC-treated surface sediments. The addition of BC significantly increased the abundance of Proteobacteria and Firmicutes of the phyla in the sediments. Furthermore, BC enhanced the expression of functional genes including amoA, amoB, nirK, nirS, hzsB, nrfA and ureC, which are likely the primary microbial mechanisms promoting NH₄⁺-N conversion in sediments for final removal.