The temperature response of CO2 production from bulk soils and soil fractions is related to soil organic matter quality

The projected increase in global mean temperature could accelerate the turnover of soil organic matter (SOM). Enhanced soil CO2 emissions could feedback on the climate system, depending on the balance between the sensitivity to temperature of net carbon fixation by vegetation and SOM decomposition. Most of the SOM is stabilised by several physico-chemical mechanisms within the soil architecture, but the response of this quantitatively important fraction to increasing temperature is largely unknown. The aim of this study was to relate the temperature sensitivity of decomposition of physical and chemical soil fractions ( size fractions, hydrolysis residues), and of bulk soil, to their quality and turnover time. Soil samples were taken from arable and grassland soils from the Swiss Central Plateau, and CO2 production was measured under strictly controlled conditions at 5, 15, 25, and 35 degrees C by using sequential incubation. Physico-chemical properties of the samples were characterised by measuring elemental composition, surface area, 14 C age, and by using DRIFT spectroscopy. CO2 production rates per unit (g) organic carbon (OC) strongly varied between samples, in relation to the difference in the biochemical quality of the substrates. The temperature response of all samples was exponential up to 25 degrees C, with the largest variability at lower temperatures. Q(10) values were negatively related to CO2 production over the whole temperature range, indicating higher temperature sensitivity of SOM of lower quality. In particular, hydrolysis residues, representing a more stabilised SOM pool containing older C, produced less CO2 g(-1) OC than non-hydrolysed fractions or bulk samples at lower temperatures, but similar rates at >= 25 degrees C, leading to higher Q(10) values than in other samples. Based on these results and provided that they apply also to other soils it is suggested that because of the higher sensitivity of passive SOM the overall response of SOM to increasing temperatures might be higher than previously expected from SOM models. Finally, surface area measurements revealed that micro-aggregation rather than organo-mineral association mainly contributes to the longer turnover time of SOM isolated by acid hydrolysis.