Enhanced iron(III) reduction following amendment of paddy soils with biochar and glucose modified biochar

Although biochar application to paddy fields has been widely studied, its effects on Fe(III) reduction have not yet been investigated. Paddy soil slurry and soil microbial inoculation incubation were conducted with unmodified biochar (UMB) or glucose-modified biochar (GMB) additions at different particle sizes. The Fe(II) concentration and pH value were determined regularly, and Fe(III) reducing capacity (FeRC) was evaluated by modeling. Fe(III) reduction potential (a) was increased by 0-1.96 mg g^-1 in response to UMBs addition, and a more remarkable increase in a was related to the decrease of particle size. The dissolved organic carbon of UMBs was responsible for the majority of the biochar reducing capacity. UMBs addition increased the contribution of free Fe and nitrate nitrogen to FeRC, while it decreased that of available phosphorus. Moreover, GMBs led to greater promotion of FeRC than the corresponding UMBs, with an increase in a of 2.9-16% in soil slurry and reduction rate of 13-35% in microbial inoculation incubation. The maximum Fe(III) reduction rate (V _max) with GMBs addition was faster or invariable than UMBs, while the time to V _max (T _Vmax) was shorter or stable. The effect of GMBs on Fe(III) reduction was less sensitive as GMB particle size increased. Compared with UMBs addition, pH declined remarkably in response to GMBs. These findings suggest that GMBs can effectively stimulate Fe(III) reduction in paddy fields, while simultaneously alleviating the pH increase usually caused by pristine biochar application.