Immediate and adaptational temperature effects on nitric: oxide productionand nitrous oxide release from nitrification and denitrification in two soils

Nitrification and denitrification are, like all biological processes, influ enced by temperature. We investigated temperature effects on N trace gas tu rnover by nitrification and denitrification in two soils under two experime ntal conditions. In the first approach ("temperature shift experiment") soi l samples were preincubated at 25 degrees C and then exposed to gradually i ncreasing temperatures (starting at 4 degrees C and finishing at 40-45 degr ees C). Under these conditions the immediate effect of temperature change w as assessed. In the second approach ("discrete temperature experiment") the soil samples were preincubated at different temperatures (4-35 degrees C) for 5 days and then tested at the same temperatures. The different experime ntal conditions affected the results of the study. In the temperature shift experiment the NO release increased steadily with increasing temperature i n both soils. In the discrete temperature experiment, however, the producti on rates of NO and N2O showed a minimum at intermediate temperatures (13-25 degrees C). In one of the soils (soil B9), the percent contribution of nit rification to NO production in the discrete temperature experiment reached a maximum (>95% contribution) at 25 degrees C. In the temperature shift exp eriment nitrification was always the dominant process for NO release and sh owed no systematic temperature dependency. In the second soil (soil B14), t he percent contribution of nitrification to NO release decreased from 50 to 10% as the temperature was increased from 4 degrees C to 45 degrees C, but no differences were evident in the discrete temperature experiment. The N2 O production rates were measured in the discrete-temperature experiment onl y. The contribution of nitrification to N2O production in soil B9 was consi derably higher at 25-35 degrees C (60-80% contribution) than at 4-13 degree s C (15-20% contribution). In soil B14 the contribution of nitrification to N2O production was lowest at 4 degrees C. The effects of temperature on N trace gas turnover differed between the two soils and incubation conditions . The experimental set-up allowed us to distinguish between immediate effec ts of short-term changes in temperature on the process rates, and longer-te rm effects by which preincubation at a particular temperature presumably re sulted in the adaptation of the soil microorganisms to this temperature. Bo th types of effects were important in regulating the release of NO and N2O from soil.