Nitrate fertilization with biochar simultaneously enhanced arsenic immobilization and mitigation of greenhouse gas emissions from an organic carbon-enriched paddy soil

The organic carbon-enriched paddy soil from the weathering of black shales is the hotspot for arsenic (As) release and greenhouse gases (GHG) (e.g. CH 4 , N 2 O, and CO 2 ) emissions. Therefore, the relevant As detoxification and GHG mitigation make sense. Herein, the effects of nitrate (N), biochar (BC), and nitrate fertilization with biochar (BCN) on As transformation and GHG emissions from paddy soils from black shales were investigated. Results indicated that the composite BCN significantly decreased the concentration of available As in soil by 45.5%, and promoted the oxidative transformation of As(III) to As(V) in available As, weakening the toxicity and mobility of As. Moreover, the composite BCN decreased N 2 O emissions by 62.2% in comparison to the individual N fertilization. Compared with BC, the composite BCN inhibited CO 2 emissions by 37.9%. Furthermore, the composite BCN synergistically decreased CH 4 emissions by 75.5%, compared with individual N (32.6%) and BC (53.7%). Additionally, BCN application can significantly facilitate the oxidative transformation of Fe(II) to Fe(III) in the anaerobic paddy soils in comparison to sole BC and N, promoting formation of Fe(III) minerals and further As immobilization and CH 4 mitigation. Mechanistically, the BCN enriched some functional microorganisms (e.g., Mobilitalea , Azospirillum , Oxobacter , and Fonticella ), weakening N 2 O emissions and facilitating the oxidation and immobilization of As. The metagenomic analysis indicated that the BCN-induced changes in abundances of functional genes-related to As/N/Fe/CH 4 transformation provided molecular biological evidences for As immobilization and GHG mitigation. Overall, BCN application can simultaneously achieve the synergistic goal of GHG mitigation during As-contaminated soils remediation.