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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

View larger version (33K): [in this window] [in a new window]   Fig. 1. Components of the soil microcosm system used in an incubation study with six soils: (1) thermometer; (2) redox potential (EH) electrode; (3) pH electrode; (4) dispersion tube for N2; (5) dispersion tube for O2; (6) stirrer; (7) sampling tube; (8) microcosm vessel; (9) temperature control by a thermostat and water circulation; (10) data logger for EH, pH, and temperature; (11) automatic redox regulation by N2 and O2 valves; (12) control computer for data logger, pump, and valve system (gas sampling), and gas chromatograph (start signal); (13) gas chromatograph (GC) with flame ionization detector/electron capture detector for trace gas measurements (CO2, CH4, and N2O); and (14) computer for GC control and GC data storage.

View larger version (17K): [in this window] [in a new window]   Fig. 2. Carbon dioxide production rates under different redox potential (EH) conditions in a microcosm incubation study with six soils. Only results when soil dissolved organic carbon (DOC) measurements were conducted are included.

View larger version (12K): [in this window] [in a new window]   Fig. 3. Soil CH4 production rates and dissolved organic carbon (DOC) concentrations in soil suspensions under different redox potential (EH) conditions in a microcosm incubation study with six soils: (i) DOC measurement; (ii) original CH4 flux rate; and (iii) DOC adjusted CH4 flux rate. Only results when soil DOC measurements were conducted are included. Adjusted CH4 flux rates were calculated by: DOC adjusted CH4 flux rate = measured CH4 flux rate x DOC content.

View larger version (18K): [in this window] [in a new window]   Fig. 4. Determination of CH4 compensation point under different redox potential (EH) conditions, and exponential regression analysis of CH4 compensation point and soil EH in a microcosm incubation study with six soils: (i) CH4 concentration increases to a steady point; (ii) CH4 concentration decreases to a steady point; and (iii) relationship between CH4 compensation points and soil EH conditions from analysis of all six soils. Examples in determining CH4 compensation point are given from analysis of the Indonesian soil.

View larger version (17K): [in this window] [in a new window]   Fig. 5. Soil N2O production rates and NH4+ and NO3– concentrations in soil suspensions under different redox potential (EH) conditions in a microcosm incubation study with six soils. Additional NO3– was amended when EH > 0 mV. Soil EH occasionally fluctuated beyond the desired level, especially during the period before NO3– was added, where measurements were also conducted.

View larger version (16K): [in this window] [in a new window]   Fig. 6. Soluble (i) Fe, (ii) Mn, and (iii) S concentrations in soil suspensions under different redox potential (EH) conditions in a microcosm incubation study with six soils. Data represent the cumulative results from all six soils. The vertical lines represent the approximate EH conditions at which (i) Fe and (ii) Mn become immobilized when EH further increases. To use the same scale for the y axis, values of each analyte are standardized by multiplying by a standardized factor. For Fe concentration (soil x standardized factor): Arkansas x 10, California x 1.67, Louisiana x 10, Texas x 1, China x 1.5, Indonesia x 3. For Mn concentration (soil x standardized factor): Arkansas x 1, California x 2, Louisiana x 4, Texas x 4.8, China x 2.4, Indonesia x 1. For S concentration (soil x standardized factor): Arkansas x 3.56, California x 1.33, Louisiana x 3.56, Texas x 1.78, China x 1, Indonesia x 1.