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

Abstract

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 CO 2 were measured at various times during the 12‐day incubation. Labeled NO ‐ 3 was used to differentiate denitrification from immobilization and reduction to NH 4 ‐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 CO 2 production. The molar ratio of NO ‐ 3 consumption to CO 2 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.