The carbon economy of drought: comparing respiration responses of roots, mycorrhizal fungi, and free-living microbes to an extreme dry-rewet cycle

Aims: The effects of drying and wetting on soil carbon processes are regulated by the responses of plants, plant-associated microbes, and free-living microbes. Whether these groups respond similarly to drying and wetting is not clear, however, making it difficult to predict the net effect of drought on soil carbon processes. Methods: We imposed a drought-rewetting event on mesocosms planted with maple (Acer saccharum Marshall; arbuscular mycorrhizal fungi host) or oak (Quercus alba L.; ectomycorrhizal fungi host) saplings. In each mesocosm, we used mesh to create chambers separating roots, mycorrhizal fungi hyphae, and free-living microbes. We compared respiration and carbon degrading enzyme activity across chambers throughout the dry-rewetting cycle. Results: Respiration from all chambers declined during drought, and chambers differed in their drought-sensitivity. In maple mesocosms, respiration from the Hyphae+Microbes chamber was the most drought-resistant; in oak mesocosms respiration from the Microbes chamber was the most drought-sensitive. Respiration did not recover after rewatering, indicating a persistent drought legacy. In contrast, enzyme activity returned to control functioning after 2 weeks of well-watered conditions. Conclusions: Our results suggest that belowground biota differ in their sensitivity to and recovery from drought, which affects the carbon processes differently. An improved ability to partition carbon fluxes into biotic sources can help to constrain predicted carbon fluxes under future climate scenarios.