Local temperature optimum of N2O production rates in tropical rain forest soils of Australia

Laboratory experiments are well-accepted approaches to study the temperature dependency of soil N-turnover processes under defined boundary conditions. However, many of these experiments have several drawbacks: the soil temperature regimes investigated are far wider than the span of soil temperatures observed in the field, the increments of the sampled soil temperatures are wide and may hide interesting details such as non-linear reaction of N-turnover processes, and due to the successive sampling over several days nutrient supply and soil moisture conditions change over time. The first 2 drawbacks are especially important when investigating soil samples from tropical rain forests as these sites are characterised by only small temperature amplitudes. Here we show an approach that allows the study of N-turnover processes at realistic soil temperature regimes observed in the field and small temperature increments within a short period of time. N2O production rates are measured for soil samples from 3 tropical rain forest sites in Queensland, Australia. Kinetics of N2O production follow exponential increases with increasing soil temperatures but are additionally characterised by local temperature optima at 2 of the 3 sites. The temperature response of N2O production rates at these 2 sites can be best described by the use of a combined exponential and optimum temperature function, which improves the coefficient of determination from 1% and 13% to 95% and 99%, respectively. The results further indicate that the microbial processes responsible for N2O production are well adapted to the mean annual temperature conditions of these sites. Successful application of the combined exponential and optimum function to other published results of N-turnover studies support this assumption.