Drought frequency is projected to increase under future climate change. Large-scale droughts may have a huge effect on ecosystem carbon storage but despite the importance of drought events, their effect on soil carbon dynamics are less well understood. The above-below ground linkages that affect soil dynamics in forested areas during drought and immediately after rewetting can have a substantial impact on how soil carbon is stored. Soil respiration increases with addition of organic material, which also facilitates the release of stored carbon in the soil; known as a 'priming effect'. In addition, soils that have undergone drought produce a large pulse of CO2 when rewet; a phenomenon known as the 'Birch effect'.
I conducted lab incubations and a greenhouse experiment with poplar saplings to quantify soil carbon release during and after drought. I measured soil respiration to investigate potential interactions between the Birch and the priming effects and to establish whether post-drought soil CO2 release is intensified or mitigated with the addition of different amounts of leaf litter and the presence or absence of tree roots.
In both experiments, soil respiration increased with litter inputs and decreased strongly during drought. However, I observed a larger pulse of soil CO2 efflux in response to litter inputs compared to rewetting after drought. In the incubation experiments, the low carbon content of the soil explains the overriding effect of the litter treatments, because the litter inputs represented the main source of carbon and nutrients to soil microbes. In the greenhouse experiment, I observed a substantial increase in soil carbon and microbial biomass upon rewetting after drought but no clear peak in soil CO2 efflux. The apparent lack of a Birch effect in the greenhouse experiment is intriguing, because it suggests that plants have a mitigating effect on soil microbial responses to drought and rewetting.