Beyond climate: Modelling the distinct pH sensitivities of radiocarbon-based turnover across forest soil carbon fractions

Soil pH affects organic carbon (OC) turnover through its linkage with microbial processes and soil properties. However, the pH dependency of the turnover of individual OC fractions has so far not been quantified and incorporated into biogeochemical models. We analyzed the relationship between soil pH and Δ14C-derived turnover rates of the surface organic layer (F and H horizons combined), as well as particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) fractions in the 0-20 cm mineral soil, across a wide pH gradient (2.8-7.7) in Swiss forests.We found that acidic pH (pHCaCl2 < 3.5) reduced the Δ14C-based turnover rates of all OC fractions, but their C stocks were insensitive except for an increase in the FH horizon. Moreover, the relationships of turnover rates and pH represent a logistic function that can be characterized by two parameters: logistic steepness (s) and inflection pH (c). These pH sensitivity parameters varied consistently across the OC fractions: for inflection pH, FH has the lowest inflection pH (most acid-tolerant), followed by POC and MAOC. For steepness, FH has the steepest response, again followed by POC and MAOC.Lastly, the estimated logistic equations were used to build new rate modifiers to simulate the effect of pH on OC turnover in the CENTURY model. We evaluated the new formulation against the original pH rate modifier in CENTURY with a different order of inflection pH. We showed that the new model improved most of the simulated C stocks and turnover times of the fractions, especially in acidic soils. Collectively, we conclude that our findings present an important advance towards the development of soil organic matter turnover models beyond the effect of climate, and may help account for some of the unexplained soil organic carbon variations in past model simulations.