Effective abatement of ammonium and nitrate release from sediments by biochar coverage

Inorganic forms of N from sediments and runoff water, among others, remain some of the key sources of pollution of water bodies. However, the release of NH₄⁺-N from sediment to water can be effectively reduced by biochar coverage due to high adsorption capacity, unlike NO₃^-N, where biochar has a low affinity. The feasibility of biochar coverage to abate NO₃^--N release needs to be evaluated. This study collected four sediments from Lake Taihu (China). Three types of biochar pyrolyzed from ordinary wastes, coconut shell (coBC), algal and excess sludge, were prepared to cover them and were incubated for 90 days. Results showed that the terminal total nitrogen (TN) and NO₃^--N concentrations decreased from 5.35 to 2.31-3.04 mg/L, 3.05 to 0.34-1.11 mg/L, respectively. CoBC coverage showed the best performance for reducing NO₃^--N release flux from 26.99 ± 0.19 to 9.30 ± 0.02 mg/m²·d (63.6 %). Potential denitrifiers, such as Flavobacterium and Exiguobacterium, were enriched in the biochar-coverage layer, and the absolute abundance of N-related functional genes (narG, nirS, nosZ and anammox) was increased by 1.76-4.21 times (p < 0.05). Jar tests by ¹⁵N isotope labeling further indicated that biochar addition increased the denitrification and anammox rates by 53.5-83.4 %. Experiments combining exogenous organic‑carbon addition and ¹⁵N labeling demonstrated that biochar's key role was regulating organic matter's bioavailability. Analysis with partial least square path modeling (PLS-PM) implied biochar with higher adsorption enhanced the denitrification and anammox processes in sediments via modifying the niche with suitable DOC, TN, and pH. This study suggested that biochar coverage could effectively abate NO₃^--N release from sediments by affecting the denitrification and anammox processes.