Effect of Temperature on Rate, Affinity, and 15N Fractionation of NO3– During Biological Denitrification in Soils
Soil isotopes are commonly used in environmental, agricultural, and biogeochemical studies to track sources and fate of labeled compounds, and also because they facilitate quantification of the intensity of a process relative to others. In a recent study, researchers worked to (1) elucidate the linear and nonlinear contributions of temperature to the reaction rate of isotopically labeled reactants, (2) highlight whether effects arise in other parameters, and (3) provide a comprehensive sensitivity analysis of kinetic isotopic effects over the concentration-temperature space using mathematical modeling of the effects in (1) and (2). To accomplish this, nine independent experiments of nitrate (NO3–) denitrification were analyzed using the Arrhenius law and the Eyring’s transition-state theory to highlight how temperature affects reaction rate constants, affinities, and kinetic isotopic effects. For temperatures between 20 and 35 °C, the Arrhenius law and the transition-state theory described equally well observed temperature increases in 14NO3– and 15NO3– denitrification rates. These increases were partly caused by an increase in frequency factor and a slight decrease in activation energy (enthalpy and entropy). Parametric analysis also showed that the affinity of 14NO3– and 15NO3– toward a microbial enzyme increased exponentially with temperature and a strong correlation with the rate constants was found. Experimental time and temperature-averaged fractionation factor αP/S showed only a slight increase with increasing temperature (i.e., lower isotopic effects); however, a comprehensive sensitivity analysis in the concentration temperature domain using average thermodynamic quantities estimated here showed a more complex response; αP/S was relatively constant for initial bulk concentrations [NO3–]0 ≤ 0.01 mol kg-1, while substantial nonlinearities developed for [NO3–]0 ≥ 0.01 mol kg-1 and appeared to be strongly correlated with microbial biomass, whose concentration and activity varied primarily as a function of temperature and available substrate. Values of αP/S ranging between 0.9 and 0.98 for the tested temperatures suggested that interpretations of environmental isotopic signatures should include a sensitivity analysis to the temperature as this affects directly the rate constants and affinities in biochemical reactions and may hide process- and source-related isotopic effects.
Maggi, F., and W. J. Riley. 2015. “The Effect of Temperature on the Rate, Affinity, and 15N Fractionation of NO3– During Biological Denitrification in Soils,” Biogeochemistry 124(1), 235–53. DOI 10.1007/s10533-015-0095-2.