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WETLAND SOILS

Major Biogeochemical Processes in Soils-A Microcosm Incubation from Reducing to Oxidizing Conditions

^a Wetland Biogeochemistry Institute, School of the Coast and Environment, Louisiana State Univ., Baton Rouge, LA 70803
^b Helmholtz Centre for Environmental Research–UFZ, Dep. of Soil Chemistry, Theodor-Lieser-Str. 4, 06120 Halle/Saale, Germany

^* Corresponding author (kyu1{at}lsu.edu).

Six soils used for rice (Oryza sativa L.) production were incubated using an automatic microcosm system. Production of trace gases (CO₂, CH₄, and N₂O) and transformation of N, S, and metals (Fe and Mn) were studied in soil suspensions incubated from reducing to oxidizing conditions. Results show that soil pH variation was inversely correlated to soil redox potential (E_H) change (P < 0.01). Soil CO₂ production exponentially increased with soil E_H increase. In contrast, soil CH₄ production and DOC showed an exponential decrease with soil E_H increase. Without the presence of soil oxidants, methanogenesis occurred across the entire E_H range, with probable H₂–supported methanogenesis at higher soil E_H conditions constituting up to 200f total CH₄ production. The CH₄ compensation point, where CH₄ concentration became constant due to equilibrium between CH₄ production and consumption, exponentially decreased with soil E_H increase. At pH 7, the critical E_H above which soils consumed atmospheric CH₄ varied among the soils, but was generally >400 mV. Significant N₂O production was observed between 200 and 500 mV. Nitrification could also contribute to N₂O production when E_H is >500 mV, a possible critical E_H for the initiation of nitrification. The critical E_H for substantial immobilization of Fe and Mn was estimated to be around 50 and 250 mV, respectively. The intermediate E_H range (approximately –150 to 180 mV) provided optimum conditions for minimizing cumulative global warming potential resulting from CO₂, CH₄, and N₂O production in soils. Our results have implications in interpreting the overall benefits of soil C sequestration efforts.

Abbreviations: DOC, dissolved organic carbon • E_H, redox potential • GC, gas chromatograph • OM, organic matter

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