Biochar-driven regulation of anammox systems under varying nitrogen loads: performance, microbial community and...

This study examined the effects of wheat straw biochar on the anaerobic ammonium oxidation (anammox) process during stepwise decrease in influent substrate concentrations. Biochar exerted a dual role depending on nitrogen load. During phase I (300 mg·L^-1 NH₄⁺-N and NO₂^--N), the biochar group exhibited 1-4% higher relative abundances of selected anammox related genes, including hzs and hdh, than the control group. At the end of phases III and IV, the electron transport system activity in the biochar-amended reactor was 10% and 46% above the corresponding control values, respectively, whereas improvement in specific anammox activity (SAA) and total nitrogen removal efficiency (TNRE) was observed after stabilization in phase IV. At 50 mg·L^-1 NH₄⁺-N and 50 mg·L^-1 NO₂^--N, the SAA reached 12.5 mg N·(g volatile suspended solids (VSS)·d) ^-1, 7% higher than that of the control group, while the TNRE was approximately 6% higher. Metagenomic analysis revealed phase-dependent differences in functional-gene relative abundance. The biochar group showed higher relative abundances of denitrification genes (nirS, norB, and nosZ) and genes related to dissimilatory nitrate reduction to ammonium (DNRA), including nrfA, together with lower relative abundances of nitrification genes (amoA, amoB, and amoC). These differences were consistent with reduced substrate competition and potential coupling between anammox and denitrification. These findings provided a mechanistic basis for applying wheat straw biochar to anammox systems operated under changing nitrogen loading conditions.