Microscale heterogeneity controls macroscopic soil heterotrophic respiration by regulating resource availability and environmental stress

Increasing evidence indicates that microscale heterogeneity is critical to interpret and predict macroscopic soil processes and functions. However, the difficulty of measuring soil characteristics, such as soil organic carbon (SOC) and microbial biomass, at the micron level greatly hinders our understanding of how microscale soil heterogeneity quantitatively regulates macroscopic soil behaviors. Here, we investigated the effect of microscale distributions of soil water saturation ( S ), SOC content ( C SOC ), and microbial biomass ( C MB ) on soil heterotrophic respiration (SHR) using a microscale process-based model. The microscale distributions of S , C SOC , and C MB were mathematically determined by assuming they varied linearly with local soil porosity, which was derived from X-ray computed tomography, and the effect of microscale heterogeneity on SHR rates was examined under different water saturations and carbon availabilities. The results show that microscale soil heterogeneity stimulated macroscopic SHR only when it alleviated resource limits or environmental stress. For instance, the heterogeneous S improved the SHR rate by reducing microbial water stress or enhancing dissolved organic carbon (DOC) diffusion, whereas the heterogeneous C SOC enhanced CO 2 flux by increasing DOC availability. In addition, the heterogeneous C MB promoted CO 2 flux by increasing microbial accessibility of substrates. The interactions among water, SOC, and microbes at the micron scale may stimulate or restrict CO 2 emission, depending on soil water saturation and DOC availability. Given the nature of soil heterogeneity at the micron scale and the challenge of measuring microscale soil characteristics, this study provided invaluable insights into how microscale heterogeneity regulates macroscale soil behaviors.