Biochar Addition Increases the Rates of Dissimilatory Iron Reduction and Methanogenesis in Ferrihydrite Enrichments

Abstract

Biochar contains quinones and aromatic structures that facilitate extracellular electron transfer between microbial cells and insoluble minerals. In this study, granulated biochar (1.2-2 mm) and powdered biochar (<0.15 mm) were amended to two ferrihydrite (<i>in situ</i> ferrihydrite and <i>ex situ</i> ferrihydrite) enrichments to investigate the effect of biochar with different particle sizes on dissimilatory iron(III)-reducing bacteria (DIRB) and methanogens. Biochar addition significantly stimulated the reduction of both <i>in situ</i> ferrihydrite and <i>ex situ</i> ferrihydrite and the production of methane. Powdered biochar amendments increased iron reduction compared to granulated biochar amendment in both the <i>in situ</i> ferrihydrite and <i>ex situ</i> ferrihydrite enrichments. However, no significant difference was observed in methane production between the powdered biochar and granulated biochar amendments in the two ferrihydrite enrichments. Analysis of 16S <i>rRNA</i> gene sequences showed that both DIRB and methanogens were enriched after biochar amendments in the <i>in situ</i> ferrihydrite and <i>ex situ</i> ferrihydrite enrichments. Taxa belonging to the Geobacteraceae and methanogenic genus affiliated to <i>Methanosarcina</i> were detected with significantly higher relative abundances in powdered biochar amendments than those in granulated biochar amendments in both the ferrihydrite enrichments. X-ray diffraction analysis indicated green rust [Fe₂(CO₃) (OH)] and vivianite [Fe₃(PO₄)₂ 8(H₂O)] formed in the <i>ex situ</i> ferrihydrite and <i>in situ</i> ferrihydrite enrichments without biochar addition, respectively. After granulated biochar amendment, the mineral phase changed from the green rust to vivianite in the <i>ex situ</i> ferrihydrite enrichment, while crystalline vivianite and iron oxide (γ-Fe₂O₃) were detected simultaneously in the <i>in situ</i> ferrihydrite enrichment. No crystalline iron compound was found in the powdered biochar amendments in both ferrihydrite enrichments. Overall, our study illustrated that the addition of biochar affected iron-reducing and methane-generating microbial communities to some extent.