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The Effect of Carbon Mineralization on Denitrification Kinetics in Mineral and Organic Soils¹

ABSTRACT

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 NO^-₃ and production of CO₂ were measured at various times during the 12-day incubation. Labeled NO^-₃ 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 (k_c) values varying from 0.075 to 0.405 day^-1. The denitrification rates in anaerobic soils were shown to be proportional to the concentration of the two substrates: NO^-₃ and available C. The denitrification rate coefficient (k_n) value was essentially constant for the mineral soils [0.00147 ± 25% day^-1 (µg C/ml)^-1], while k_n values for the organic soils were somewhat more variable [0.00155 ± 65% day^-1 (µg C/ml)^-1].

Significant correlations were observed between NO^-₃ consumption and CO₂ production. The molar ratio of NO^-₃ consumption to CO₂ 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_max), and WSC and C_max, respectively. Water-soluble C represented 0.4 to 0.9% of TOC, while C_max 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.

Key Words: nitrate reduction • water-soluble carbon • first-order kinetics • anaerobic soil • flooded soil • Michaelis-Menton kinetics

¹ Florida Agric. Exp. Stn. Journal Series no. 2,902.

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.

Received for publication January 15, 1981. Accepted for publication August 31, 1981.

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