Reply to Comments on “Factors Influencing Oxygen Consumption Rates in Flooded Soils”

Thirty-seven surface soil samples varying in physico-chemical properties were incubated under reduced (anaerobic) conditions to study the factors influencing oxygen consumption rates. Oxygen consumption rates were measured after introduction of a known amount of oxygen into a reduced (anaerobic) soil system and following the disappearance of oxygen as a function of time. The time dependence of oxygen consumption in all flooded soils could be described as a two-phase first order reaction process. Rapid oxygen consumption during Phase I was followed by relatively slower consumption during Phase II. The first-order rate coefficient (kt, hour-) for Phase I was approximately the same for all 37 soils evaluated, whereas each soil was characterized by a different rate coefficient (kn, hour') for Phase II. The average value for *, was 0.15 hour' (coefficient of variation = 20%) while the values of fen ranged from 0.0027 to 0.054 hour". Step-wise regression analysis of the data showed that NaOAc-extractable Fe of a reduced soil was the single best predictor of ftn and also the fractional consumption associated with Phase I. Significant improvement in these regressions was obtained when total NH,-N content of the reduced soil, in addition to reducible Fe* was considered. Additional Index Words: oxygen uptake, reducible Fe, reducible Mn, first-order kinetics, waterloggged soils, anaerobic soils. Reddy, K. R., P. S. C. Rao, and W. H. Patrick, Jr. 1980. Factors influencing oxygen consumption rates in flooded soils. Soil Sci. Soc. Am. J. 44:741-744. F SOILS AND SEDIMENTS are characterized by the absence of oxygen (Mortimer, 1941; Krinstensen and Enoch, 1964; Yunkervich et al., 1966; Armstrong and Boatman, 1967). Turner and Patrick (1968) could detect no oxygen in four soil suspensions within 36 hours of withdrawal of oxygen supply. Upon depletion of oxygen, the requirements of facultative anaerobic and true anaerobic organisms for electron acceptors result in the reduction of several oxidized compounds in the soil. Nitrate, nitrite, the higher oxides of Mn, hydrated ferric oxide and sulfate will be reduced if an energy source is available to the microorganisms. Flooded soils are characterized by the accumulation of Fe, Mn, NH4, and S~ (Ponnamperuma, 1972; Patrick and Mikkelsen, 1971). Oxygen diffusing into a flooded soil may be consumed as a result of (i) microbial respiration where it is used as an electron acceptor, (ii) chemical oxidation of reduced Fe and Mn, (iii) biological oxidation of NH4 and carbon, and (iv) oxidation of sulfides. 1 Joint Contribution from the Center for Wetland Resources, Louisiana State Univ. and the Inst. of Food and Agric. Sciences (IFAS), Univ. of Florida. This research was done while the senior author was located at the Louisiana State Univ. Received 13 Nov. 1979. Approved 3 April 1980. "Assistant Professor, Agric. Res. and Educ. Center, IFAS, Univ. of Florida, P.O. Box 909, Sanford, FL 32771. "Assistant Professor, Soil Sci. Dep., IFAS, Univ. of Florida, Gainesville, FL 32611. * Boyd Professor, Laboratory of Flooded Soils and Sediments, Center for Wetland Resources, Louisiana State Univ., Baton Rouge, LA 70803. These processes result in the development of a surface oxidized (aerobic) layer (Pearsall and Mortimer, 1939; Mortimer, 1941, 1942; Alberda, 1953). Patrick and Delaune (1972) characterized the oxidized and reduced soil layers based on the vertical distribution of Fe, Mn, NH4 and S~. The rates of oxygen consumption and the thickness of the oxidized layer are influenced by the concentration of these oxygen consuming species in a flooded soil. The objectives of this study were to determine (i) the rates of oxygen consumption by several soils incubated under reduced (anaerobic) conditions, and (ii) the relationship between the rates of oxygen consumption and the oxygen-consuming parameters. MATERIALS AND METHODS Thirty-seven surface soil samples obtained from various locations in southern Louisiana were used in this study. Selected properties of these soils are shown in Table 1. Twenty-five grams of soil (oven-dry basis), with an equal weight of water were added to a narrow mouth bottle (about 250-ml capacity) fitted with a serum cap, and each of the bottles was sealed with plastic rubber. There were a total of four replications for each soil. All bottles were purged with argon to create oxygen-free conditions and the samples were incubated for a period of 15 days at 30°C in the dark to insure reducing (anaerobic) conditions in all soils. At the end of the incubation period, two replications were used to determine the extractable Fe, Mn, and NH,-N and the remaining two replications were used to determine the oxygen consumption by these reduced soil systems. The incubation bottles used for measuring the oxygen consumption rates were purged with air containing 21% oxygen at a high flow rate for a period of 3 min, to insure displacepent of the argon from the system. This procedure resulted in an oxygen content of 1,667 Mg/g of s°UThe bottles were then sealed and further incubated at 30°C in the dark under continuous shaking. The oxygen content of the air in the bottle was measured at 0, 1, 2, 4, 6, 8, 12, 24, 48, and 72 hours, by using a modification of the procedure described by Patrick (1977). This method was modified to suit the laboratory conditions. The oxygen analyzer system consists of a specially constructed stainless steel cell, which was machined to thread onto the tip of the oxygen electrode and was designed so that the internal volume of the cell was < 0.5 cm. An O-ring seal in the cell permitted contact of the air sample with the oxygen membrane surface. A schematic diagram of the system used is shown in Fig. 1. To measure oxygen content, the air inlet of the analyzer cell was connected to a syringe needle by means of small tubing (approx. 0.2 cm i.d., and 2 cm long), and the air outlet was connected to a 10-ml capacity plastic syringe (Fig. 1), that was used to draw air into the analyzer cell. Before being used, the gas sampling cell was calibrated by drawing an air sample into the cell and setting the meter to 21% oxygen, and by drawing pure argon into the cell to check the zero oxygen point of the meter. At the end of each incubation period, approximately 5 ml of air was drawn from the bottle through the cell by alternately pulling and pushing the syringe piston several times and the oxygen content determined. Between each measurement, the meter was checked for zero oxygen content by allowing argon to flow through the cell. The decrease in oxygen content of the incubation bottle was followed until it reached a minimum value. Analytical Procedure.—After the preincubation period, duplicate samples were extracted with a soil to extracting solution (IN NaOAc) ratio of 1:5, under oxygen-free conditions (Gambrell et al., 1977) and filtered through a 0.45-^m millipore filter into a flask containing three drops of cone HC1. The extracted solutions were analyzed for Fe and Mn using an atomic adsorption spectrophotometer and NH,-N using a specific