, Volume 37, Issue 3, pp 253-278

Localization of sulfate reduction in planted and unplanted rice field soil

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Abstract

Rates of in situ sulfate reduction (SRR) in planted and unplanted rice fieldsoil were measured by the 35SO2− 4-radiotracermethod using soil microcosms. The concentration of 35SO2− 4 decreased exponentially with time.However, time course experiments indicated that incubation times of10–30 min were appropriate for measurements of SRRusing a single time point in routine assays. Unplanted microcosmsshowed high SRR of 177 nmol cm-3 d-1 inthe uppermost centimeter where average sulfate concentrations were<33 µM. Fine scaled measurements (1 mmresolution) localized highest SRR (<100 nmol cm-3d-1) at the oxic/anoxic interface at 2–5 mmdepth. In planted rice field soil, SRR of <310 nmolcm-3 d-1 were observed at 0–2cm depth. Sulfate reduction rates were determined at a millimeter-scalewith distance to a two dimensional root compartment. The SRR was highestat 0–1.5 mm distance to the root layer with rates up to500 nmol cm-3 d-1, indicating a highstimulation potential of the rice roots. SRR seemed to be mainlydependent on the in situ sulfate porewater concentrations. At thesoil surface of unplanted microcosms sulfate concentration decreasedfrom <150 µM to <10 µM within the first 8 mm of depth. In planted microcosmssulfate concentration varied from 87–99 µMsulfate at the 0–3 mm distance to the root layer to48–62 µM sulfate at a root distance>4 mm from the roots.

The depth distribution of inorganic sulfur compounds was determinedfor planted and unplanted rice field soil. Sulfate, acid volatilesulfide (AVS) and chromium reducible sulfide (CRS) were up to 20 foldhigher in planted than in unplanted microcosms. CRS was the majorinsoluble sulfur fraction with concentrations >1.7µmol cm-3. Organic sulfur accounted for25–46% of the total sulfurpresent (269 µg/g dw) in an unplanted microcosm.The biogeochemical role of sulfate reduction forshort-term accumulation of inorganic sulfur compounds(FeS, FeS_2 and S°) in rice soil wasdetermined in a time course experiment with incubationperiods of 5, 10, 20, 30 and 60 min. The relativedistribution of CRS and AVS formation showedlittle depth dependence, whereas the formation of35S° seemed to be the highest in themore oxidized upper soil layers and near the root surface.AV35S was the first major product of sulfatereduction after 20–30 min, whereas CR35Swas formed, as AV35S and 35S°decreased, at longer incubation periods of >30 min.