Plant productivity is a key driver of soil respiration response to climate change in a nutrient-limited soil

Despite knowledge of the interaction between climate change factors significant uncertainty exists concerning the individual and interactive effects of elevated carbon dioxide (eCO₂) and elevated temperature (eT) on the soil microbiome and function. Here we examine the individual and interactive effects of eCO₂ and eT on tree growth, soil respiration (R_soil), biomass, structural and functional composition of microbial community, nitrogen (N) mineralisation and N availability in a whole tree chamber experiment. Eucalyptus globulus plants were grown from seedling to ca. 10 m tall for 15 months in a nutrient-poor sandy soil under ambient and elevated (+ 240 ppm) atmospheric CO₂ concentrations combined with ambient or elevated temperatures (+ 3 °C) in a full factorial design. Plant growth was strongly stimulated under eCO₂, but eT had little impact on any measured plant property. In contrast, R_soil was not consistently affected by eCO₂ or eT, but correlated strongly with root and leaf biomass. The response of N-mineralisation and nutrient availability to eCO₂ and eT varied across time, and available N correlated strongly with plant height. Further, the C:N ratio of the microbial biomass and leaves were both higher under eCeT treatment. However, these functional measures were not significantly linked to either structural or functional diversity of the soil microbiome. Taken together, these results suggest that in this low-nutrient soil, belowground processes are principally driven by aboveground productivity. Our work provides novel insight into mechanisms underlying above- and belowground response to climate change, and the potential to sequester C in a low-nutrient status soil under future climatic conditions may be limited.