Soil drying with experimental warming depends on ecosystem type and warming method: First results of the Soil Warming to Depth Data Integration Effort (SWEDDIE)

Abstract. Field-warming experiments offer insight into the response of ecosystems to rising temperatures, but cross-site comparison is needed to determine both the general tendencies of warming responses and the context dependencies of deviations from those norms. These responses are not limited to the direct effects of temperature but also their indirect effects on soil moisture, a critical factor controlling ecosystem productivity and carbon fluxes. Here we introduce SWEDDIE: the first database to characterize the whole soil profile warming response across 26 distinct soil warming experiments, encompassing forest, grassland, cropland, tundra, and wetland ecosystems. SWEDDIE is needed because prior databases and syntheses of warming effects on ecosystems were dominated by aboveground warming studies, many of which warmed soil modestly or negligibly during much of the growing season and reported only growing season averages.

We demonstrate the potential of the SWEDDIE database by quantifying soil temperature and moisture changes for each experiment as a function of depth, warming methodology, ambient climate conditions, and ecosystem, as well as the relationship between soil moisture and imposed warming. Warming attenuated with depth at sites with aboveground warming only but increased with depth at sites with belowground warming only, as hypothesized. Warming led to soil drying at most sites, and drying was positively correlated with the magnitude of warming. However, the relationship between soil warming and soil drying varied by ecosystem: forest soils dried the most, while tundra soils became wetter with warming. Ambient climatic conditions also significantly influenced the relationship between experimental warming and drying, with more drying per degree of warming observed in soils with higher ambient moisture.

The inconsistency of soil moisture changes with warming across ecosystems and warming methodologies demonstrates the importance of quantifying shifts in temperature and moisture in both space and time in order to overcome site-specific bias in ecosystem warming responses. The high temporal resolution and depth-resolved observations of the fundamental ecosystem properties of soil temperature and moisture in SWEDDIE v1.0.0 serve as a foundation for future experimental soil warming synthesis efforts and demonstrate the power of this actively growing community resource.