Research articleSulfur compounds, potential turnover of sulfate and thiosulfate, and numbers of sulfate-reducing bacteria in planted and unplanted paddy soil
Sulfate reduction potentials (SRP), thiosulfate consumption potentials (TCP), numbers of sulfate-reducing bacteria (SRB) and the vertical profiles of sulfate, thiosulfate, acid volatile sulfides (AVS) and chromium reducible sulfides (CRS) were measured within 10-cm-deep 13-week-old planted and unplanted paddy soil microcosms. The soil pore water of unplanted microcosms showed sulfate concentrations 300 μM and > 150 μM, respectively, indicating that oxidation of reduced sulfur was stimulated in this zone (the root zone of the rice plants). On the other hand, concentrations of AVS were also much higher in the upper layers of planted versus unplanted microcosms indicating that reduction of oxidized sulfur compounds was also stimulated in this zone. The highest AVS and CRS concentrations were 1.6 and 1.3 μmol cm−3 soil, respectively. Indeed, planted soils showed a two- to five-fold higher SRP and TCP (< 2.8 and < 1.9 μmol cm−3 d−1, respectively) compared to unplanted microcosms (< 0.56 μmol cm−3 d−1). Concentrations of acetate and lactate were also higher, especially in the uppermost soil layers. However, SRP and TCP were only stimulated by the addition of hydrogen. SRB were enumerated by the MPN technique using hydrogen, acetate, propionate, lactate, butyrate, succinate and benzoate as electron donors. Vertical profiles indicated that the SRB were relatively homogenously distributed in the paddy soil microcosms. The SRB populations growing on H2, propionate and succinate appeared to be higher in planted than in unplanted paddy soil microcosms although at a relatively low statistic significance (α < 0.1). Enrichment cultures showed a relatively high diversity of sulfate-reducing bacteria with respect to utilization of at least eight different substrates out of 21 substrates tested. The genera observed included Desulfovibrio, Desulfotomaculum, Desulfobulbus, and Desulfobotulus. Our results indicate a very dynamic cycling of both reduced and oxidized sulfur species in the rhizosphere of planted paddy soil.