The Effect of Carbon Mineralization on Denitrification Kinetics in Mineral and Organic Soils

Rates of denitrification and organic carbon (C) mineralization were measured simultaneously in soil suspensions maintained at 30°C under anoxic conditions. Nine mineral and seven organic soils were used in the study. Disappearance of NO3~ and production of CO2 were measured at various times during the 12-day incubation. Labeled NO, was used to differentiate denitrification from immobilization and reduction to NH4-N. The rate of organic C mineralization followed first-order kinetics in all soils with the mineralization rate coefficient (kc) values varying from 0.075 to 0.405 day'. The denitrification rates in anaerobic soils were shown to be proportional to the concentration of the two substrates: NO, and available C. The denitrification rate coefficient (&„) value was essentially constant for the mineral soils [0.00147 ± 25% day" (fig C/mi)"], while kn values for the organic soils were somewhat more variable [0.00155 ± 65% day" (jig C/ml)"']. Significant correlations were observed between NO," consumption and CO, production. The molar ratio of NO, consumption to CO, production ranged from 0.6 to 1.8. Significant relationships were also observed between water-soluble C (WSC) and total organic C (TOC), maximum available C (C^,,), and WSC and Cmls, respectively. Watersoluble C represented 0.4 to 0.9% of TOC, while Cm represented about 0.6 to 1.4% of TOC. Results also showed that denitrification rates were influenced by the rate at which available C is mineralized and made available to the organisms. Additional Index Words: nitrate reduction, water-soluble carbon, first-order kinetics, anaerobic soil, flooded soil, Michaelis-Menton kinetics. Reddy, K. R., P. S. C. Rao, and R. E. Jessup. 1982. The effect of carbon mineralization on denitrification kinetics in mineral and organic soils. Soil Sci. Soc. Am. J. 46:62-68. T ROLE of organic carbon (C) as an electron donor in the denitrification process has been widely recognized. Several researchers (Wijler and Delwiche, 1954; Bremner and Shaw, 1958; Nommik, 1956; Bowman and Focht, 1974; Reddy et al., 1978) have shown increased denitrification rates following the addition of energy source (organic C) to a soil. Significant relationship was reported between NOf losses via denitrification and "available" C, as evaluated either by glucose-equivalent C (Stanford et al., 1975a), by watersoluble C (Burford and Bremner, 1975; Reddy et al., 1980), or by mineralizable C (Burford and Bremner, 1975). A considerable portion of available C would be used in normal oxidative respiration by denitrifiers and 1 Florida Agric. Exp. Stn. Journal Series no. 2,902. Received 15 Jan. 1981. Approved 31 Aug. 1981. 2 Assistant Professor, Agric. Res. and Ed. Cen., Univ. of Florida, Sanford, Fl 32771, and Assistant Professor and Scientific Programmer, 'respectively, Soil Sci. Dep., Univ. of Florida, Gainesville, FL 32611. other microorganisms (using O2 as an electron acceptor) until the system becomes anoxic. When O2 is depleted from the system, the denitrifiers use NO3~ as an electron acceptor during their oxidative respiration. Once the easily decomposible C is used by the denitrifiers, the rate of NO3~ loss would depend on the rate of soil organic C conversion to mineralizable organic C and to soluble organic C (Focht and Verstraete, 1977), while C loss in the same system may not depend on the availability of NO3~, because some organisms use Fe, Mn, and SO4 as their electron acceptors during C oxidation. Denitrification rates, expressed either as zero-order (Wiljer and Delwiche, 1954; Nommik, 1956; Patrick, 1960; Reddy et al., 1978) or first-order (Stanford et al., 1975a; Reddy et al., 1980) rate coefficients, and which do not account for the soluble soil organic C availability will be specific to the soil and a given set of environmental conditions, and not of general value in modeling the denitrification process. Also, most researchers (Stanford et al., 1975a; Bowman and Focht, 1974; Burford and Bremner, 1975; Kohl et al., 1976) measured denitrification rates under static conditions where NO3~ was present in the overlying water and the underlying soil. Since denitrification occurs only in the anaerobic soil layer, diffusion of NO3~ from the floodwater to the anaerobic soil layer also controls the rate of denitrification (Reddy et al., 1978). No data are available to date on the kinetics of available C mineralization coupled with the kinetics of denitrification measured under diffusion nonlimiting conditions. The objectives of the present investigation were (i) to determine simultaneously the rates of denitrification and C mineralization in several mineral and organic soils; (ii) to develop a relationship between the kinetics of denitrification and organic C mineralization; and (iii) to obtain a denitrification rate coefficient as a function only of available C (i.e., independent of soil type). REACTION RATE EQUATIONS In this study, the rate of available C mineralization during denitrification was assumed to follow first-order kinetics: