An overlooked mechanism underlying the attenuated temperature response of soil heterotrophic respiration

Biogeochemical reactions occurring in soil pore space underpin gaseous emissions measured at macroscopic scales but are difficult to quantify due to their complexity and heterogeneity. We develop a volumetric-average method to calculate aerobic respiration rates analytically from soil with microscopic soil structure represented explicitly. Soil water content in the model is the result of the volumetric-average of the microscopic processes, and it is nonlinearly coupled with temperature and other factors. Since many biogeochemical reactions are driven by oxygen (O-2) which must overcome various resistances before reaching reactive microsites from the atmosphere, the volumetric-average results in negative feedback between temperature and soil respiration, with the magnitude of the feedback increasing with soil water content and substrate quality. Comparisons with various experiments show the model reproduces the variation of carbon dioxide emission from soils under different water content and temperature gradients, indicating that it captures the key microscopic processes underpinning soil respiration. We show that alongside thermal microbial adaptation, substrate heterogeneity and microbial turnover and carbon use efficiency, O-2 dissolution and diffusion in water associated with soil pore space is another key explanation for the attenuated temperature response of soil respiration and should be considered in developing soil organic carbon models.

Automated summary

Biogeochemical reactions in soil pore space play a crucial role in the gaseous emissions measured on a macroscopic scale, but their quantification is challenging due to the complexity and heterogeneity of soils. In this study, a volumetric-average method is developed to calculate aerobic respiration rates using a microscopic representation of soil structure. The results show that the model accurately reproduces the variations in carbon dioxide emissions under different soil water content and temperature gradients, suggesting that it captures the key microscopic processes governing soil respiration. The study highlights the importance of incorporating oxygen dissolution and diffusion in water associated with soil pore space in soil organic carbon models, in addition to thermal microbial adaptation, substrate heterogeneity, microbial turnover, and carbon use efficiency.