Temperature and moisture are minor drivers of regional-scale soil organic carbon dynamics

Storing large amounts of organic carbon, soils are a key but uncertain component of the global carbon cycle, and accordingly, of Earth System Models (ESMs). Soil organic carbon (SOC) dynamics are regulated by a complex interplay of drivers. Climate, generally represented by temperature and moisture, is regarded as one of the fundamental controls. Here, we use 54 forest sites in Switzerland, systematically selected to span near-independent gradients in temperature and moisture, to disentangle the effects of climate, soil properties, and landform on SOC dynamics. We estimated two SOC turnover times, based on bulk soil ¹⁴C measurements (τ_14C) and on a 6-month laboratory soil incubation (τ_i). In addition, upon incubation, we measured the ¹⁴C signature of the CO₂ evolved and quantified the cumulated production of dissolved organic carbon (DOC). Our results demonstrate that τ_i and τ_14C capture the dynamics of contrasting fractions of the SOC continuum. The ¹⁴C-based τ_14C primarily reflects the dynamics of an older, stabilised pool, whereas the incubation-based τ_i mainly captures fresh readily available SOC. Mean site temperature did not raise as a critical driver of SOC dynamics, and site moisture was only significant for τ_i. However, soil pH emerged as a key control of both turnover times. The production of DOC was independent of τ_i and not driven by climate, but primarily by the content of clay and, secondarily by the slope of the site. At the regional scale, soil physicochemical properties and landform appear to override the effect of climate on SOC dynamics.