The vulnerability of plant xylem to embolism can be described as the water potential at which xylem conductivity is lost by 50

Summary It is well‐known that the mycorrhizal type of plants correlates with different modes of nutrient cycling and availability. However, the differences in drought tolerance between arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) plants remains poorly characterized. We synthesized a global dataset of four hydraulic traits associated with drought tolerance of 1457 woody species (1139 AM and 318 EcM species) at 308 field sites. We compared these traits between AM and EcM species, with evolutionary history (i.e. angiosperms vs gymnosperms), water availability (i.e. aridity index) and biomes considered as additional factors. Overall, we found that evolutionary history and biogeography influenced differences in hydraulic traits between mycorrhizal types. Specifically, we found that (1) AM angiosperms are less drought‐tolerant than EcM angiosperms in wet regions or biomes, but AM gymnosperms are more drought‐tolerant than EcM gymnosperms in dry regions or biomes, and (2) in both angiosperms and gymnosperms, variation in hydraulic traits as well as their sensitivity to water availability were higher in AM species than in EcM species. Our results suggest that global shifts in water availability (especially drought) may alter the biogeographic distribution and abundance of AM and EcM plants, with consequences for ecosystem element cycling and ultimately, the land carbon sink.

Fungi and bacteria are the two dominant groups of soil microbial communities worldwide. By controlling the turnover of soil organic matter, these organisms directly regulate the cycling of carbon between the soil and the atmosphere. Fundamental differences in the physiology and life history of bacteria and fungi suggest that variation in the biogeography of relative abundance of soil fungi versus bacteria could drive striking differences in carbon decomposition and soil organic matter formation between different biomes. However, a lack of global and predictive information on the distribution of these organisms in terrestrial ecosystems has prevented the inclusion of relative abundance of soil fungi versus bacteria and the associated processes in global biogeochemical models. Here, we used a global-scale dataset of > 3000 distinct observations of abundance of soil fungi versus bacteria in the surface topsoil (up to 15 cm) to generate the first quantitative and high-spatialresolution (1 km(2)) explicit map of soil fungal proportion, defined as fungi/fungi + bacteria, across terrestrial ecosystems. We reveal striking latitudinal trends where fungal dominance increases in cold and high-latitude environments with large soil carbon stocks. There was a strong nonlinear response of fungal dominance to the environmental gradient, i.e., mean annual temperature (MAT) and net primary productivity (NPP). Fungi dominated in regions with low MAT and NPP and bacteria dominated in regions with high MAT and NPP, thus representing slow vs. fast soil energy channels, respectively, a concept with a long history in soil ecology. These high-resolution models provide the first steps towards representing the major soil microbial groups and their functional differences in global biogeochemical models to improve predictions of soil organic matter turnover under current and future climate scenarios. Raw datasets and global maps generated in this study are available at https://doi.org/10.6084/m9.figshare.19556419 (Yu, 2022).

The biogeography of relative abundance of soil fungi

The hydraulic vulnerability segmentation hypothesis proposes that plant branches are more resistant to cavitation than roots, namely, the difference in vulnerability to cavitation between branches and roots is positive ( P > 0). However, it is not clear how this phenomenon can vary along environmental aridity gradients. We compiled the above hypothesis with 105 woody species from four biomes with increasing aridity, by compiling functional traits related to hydraulic properties and anatomical structures of branches and roots. We investigated the relationships between P and several environmental factors that are associated with aridity. We found a positive P across species, which supported the hydraulic vulnerability segmentation hypothesis, and P increased significantly with environmental aridity. Branch xylem hydraulic conductivity changed from “more efficient” (e.g., wider conduit, higher hydraulic conductivity) to “safer” (e.g., narrower conduit, more negative P) in response to increased aridity, while root xylem hydraulic conductivity remained unchanged across aridity gradients. Our results demonstrated that hydraulic vulnerability segmentation is more pronounced for species from arid regions. Changes in branch traits may be responsible for hydraulic vulnerability segmentation between branches and roots.