SULFUR-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 pr ofiles of sulfate, thiosulfate, acid volatile sulfides (AVS) and chrom ium reducible sulfides (CRS) were measured within 10-cm-deep 13-week-o ld planted and unplanted paddy soil microcosms. The soil pole water of unplanted microcosms showed sulfate concentrations <110 mu M and no d electable thiosulfate. The upper layers of planted microcosms, in cont rast, showed concentrations of sulfate and thiosulfate that reached > 300 mu M and > 150 mu M. respectively, indicating that oxidation of re duced sulfur was stimulated in this zone (the loot zone of the I icr p lants). 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 th is zone. The highest AVS and CRS concentrations were 1.6 and 1.3 mu mo l cm(-3) soil, respectively. Indeed, planted soils showed a two- to fi ve-fold higher SRP and TCP (< 2.8 and < 1.9 mu mol cm(-3) d(-1), respe ctively) compared to unplanted microcosms (< 0.56 mu mol cm(-3) d(-1)) . Concentrations of acetate and lactate were also higher, especially i n 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 H-2, propionate and succinate appeare d to be higher in planted than in unplanted paddy soil microcosms alth ough at a relatively low statistic significance (alpha < 0.1). Enrichm ent cultures showed a relatively high diversity of sulfate-reducing ba cteria with respect to utilization of at least eight different substra tes out of 21 substrates tested. The genera observed included Desulfou ibrio, Desulfotomaculum, Desulfobulbus, and Desulfobotulus. Our result s indicate a very dynamic cycling of both reduced and oxidized sulfur species in the rhizosphere of planted paddy soil.