All microbial activity, including denitrification, is influenced by changes in temperature. Activity is very slow below 5 °C, increases linearly to a maximum around 25-30 °C and thereafter decreases to a minimum at around 65 ° C, where growth stops due to enzyme dena-turation. There are exceptions and some bacteria have evolved to grow in extremely hot (>100 °C) or cold (-12 ° C) conditions. Temperature optima in a particular soil appear to be the same for a range of microbial processes, including nitrification and denitrification, but temperature optima differ for different soils. The temperature optima do not appear to be linked to climate.

Strong seasonal and diurnal variations in N2O and NO fluxes are observed in all marine and terrestrial systems, but only when carbon and nitrogen supply and anaerobi-city is not limiting. Under nonlimiting conditions, the temperature coefficient of denitrification, Qj0, is mostly reported around 2-4. Frequently, the temperature response of denitrification and other microbial reactions is expressed as an activation energy, which can be calculated from the plot of the natural logarithm of the flux against the inverse absolute temperature. Usually, activation energies range from 50 to 100 kJ mol_ , and again are similar for a range of microbial processes.

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