Warming may decelerate mineralization rate of the extant soil organic carbon in the rhizosphere

Abstract:

The temperature sensitivity of soil organic matter (SOM) mineralization is a very important yet controversial question concerning global C cycle in the context of predicted changing climates. One reason may have been the soil-only incubations without substrate supply or artificial substrate supply ignoring the potential of rhizosphere processes to modulate the effect of temperature. Another reason may be the assumption that soil enzymes are activated till certain optimum temperature and then on they are denatured. On the contrary, in enzyme assay studies it has been shown that thermal activation and inactivation of enzymes occur simultaneously and thermal inactivation is more temperature sensitive than activation. Here we present a simple model which takes into account the labile substrate supply and thermal inactivation and activation of extra-cellular enzymes. It predicts that the temperature sensitivity of SOM mineralization decreases with increasing temperature. We validated it by incubating permanently ¹³C labelled Lolium perenne plants under different temperatures. The labelling permitted us to separate soil derived and plant derived carbon. Though apparently temperature increased rhizosphere priming effect (soil derived C efflux in planted soils minus that in control soils) but closer analysis revealed that it was due to enhanced rhizodeposition (root exudates) under higher temperatures and that temperature has no direct relationship of RPE. These data fitted well into our model thus confirming that under future climates when temperature would increase, lessened the temperature sensitivity of SOM mineralization under plant ecosystems would make the soils C sink rather than source.