Discrete functional pools of soil organic matter in a UK grassland soil are differentially affected by temperature and priming

We show that both temperature and priming act differently on distinct C pools in a temperate grassland soil. We used SOM which was ¹⁴C-labelled in four different ways: by labelling soil with ¹⁴C-glucose, by adding leaf litter from plants pre-labelled with ¹⁴CO ₂, and by labelling in situ with ¹⁴CO ₂ applied to the ryegrass canopy either 6 or 18 months earlier. Samples of each type of ¹⁴C labelled soil were incubated at either 4, 10, 15, or 20°C and the exponential loss of ¹⁴CO ₂ used to characterise treatment effects. ¹⁴C allocation to microbial fractions was greater, and so overall mineralization by microbes was greater, as temperature rose, but turnover of the microbial labile pool was temperature-insensitive, and the turnover of microbial structural material was reduced as temperature rose. The ability of the microbial population to degrade just one fraction of plant litter was increased greatly by temperature. A pool of SOM with a half-life of about 70d was degraded faster at higher temperatures. Less tractable but abundant pools of SOM were not accessed more readily at higher temperatures by the microbial population. Priming with glucose or amino-acids only speeded the mineralization of recent SOM (probably from the living microbial biomass), and was not altered by temperature. These results have implications for the impacts of climate change on soil C cycling.