The response of metabolically active Clostridium community to initial pH shift is closely correlated with microbial Fe(III) reduction in flooded paddy soils

Abstract

Soil pH could be greatly affected by agriculture practices in paddy soils, in which microbial Fe(III) reduction is a prevalent and important biogeochemical process. There is increasing evidence that microbial fermentative processes can play an important role in Fe(III) reduction. In this study, the response of metabolically active Clostridium community to initial pH adjustment was investigated to elucidate the mechanism of the effects of soil pH on microbial Fe(III) reduction through fermentative microbes. Two paddy soils with different original pH were incubated anaerobically for 40 days. The initial pH of alkaline paddy soil (Baodi district; pH 7.89) was adjusted to acidic levels (approximately pH 5.2), and the initial pH of acidic paddy soil (Nanchang county; pH 5.17) was adjusted to alkaline levels (approximately pH 7.9). The abundance and the structure of metabolically active Clostridium community were measured on days 0, 1, 5, 10, 20, and 40 during anaerobic incubation. Decreasing the initial pH of alkaline paddy soil decreased the Clostridium abundance by 3.77 × 104 to 1.30 × 107 copies g−1 soil, while increasing the initial pH of acidic paddy soil decreased the Clostridium abundance by 1.25 × 108 to 1.80 × 1010 copies g−1 soil within the first 20 days. Decreased initial pH stimulated the growth of H2-producing Clostridium species, while increased initial pH was conducive to the growth of Fe(III)-reducing Clostridium species. The dynamics of the abundance and structure of Clostridium community were closely correlated with processes of hydrogen production and Fe(III) reduction. Decreased pH could inhibit Fe(III) reduction by reducing the abundance and changing the structure of Clostridium community. However, the effects of increased pH on Fe(III) reduction were not obvious as a result of stimulation of the growth of Fe(III)-reducing Clostridium species and H2 utilization ability. Decreasing the initial pH of alkaline paddy soil and increasing the initial pH of acidic paddy soil led to significant decreases in the abundances and great differences in the structures of metabolically active Clostridium community. This study further demonstrated that fermentative microbes could play an important role in Fe(III) reduction via direct and indirect effects, and their optimum pH for growth was closely related to the inherent soil pH.