Physical protection of soil carbon in macroaggregates does not reduce the temperature dependence of soil CO sub(2) emissions

In a warmer world, soil CO sub(2) emissions are likely to increase. There is still much discussion about which soil organic C (SOC) pools are more sensitive to increasing temperatures. While the temperature sensitivity of C stabilized by biochemical recalcitrance or by sorption to mineral surfaces has been characterized, few studies have been carried out on the temperature sensitivity-expressed as Q sub(10)-of C physically protected inside soil macroaggregates (0.2-2mm). It has been suggested that increasing the availability of labile SOC by exposing C through macroaggregate crushing would decrease Q sub(10), i.e., the temperature dependence of soil CO sub(2) emissions. To test this hypothesis, the temperature dependence of CO sub(2) emissions from C physically protected in macroaggregates was measured through 21-d laboratory incubations of crushed and uncrushed soils, at 18 degree C and 28 degree C. 199 topsoil samples, acidic or calcareous, with SOC ranging from 2 to 121gkg super(-1) soil were investigated. The CO sub(2) emissions were slightly more sensible to temperature than to C deprotection: about 0.3mgCg super(-1)soil (=13 mgC g super(-1) SOC) and 0.2 mgC g super(-1) (=12 mgC g super(-1) SOC) were additionally mineralized, in average, by increasing the temperature or by disrupting the soil structure, respectively. The mean Q sub(10) index ratio of CO sub(2) emitted at 28 degree C and 18 degree C was similar for crushed and uncrushed soil samples and equaled 1.6. This was partly explained because Q sub(10) of macro-aggregate-protected C was 1. The results did not support the initial hypothesis of lower temperature dependence of soil CO sub(2) emissions after macroaggregate disruption, although a slight decrease of Q sub(10) was noticeable after crushing for soils with high amounts of macroaggregate-protected C. Field research is now needed to confirm that soil tillage might have no effect on the temperature sensitivity of SOC stocks.