Drivers of soil organic carbon from temperate to alpine forests: a model-based analysis of the Swiss forest soil inventory

Predicting soil organic carbon (SOC) stocks in forest ecosystems is crucial for assessing forest C balance, but the relative importance of key drivers, including soil mineral properties, litter inputs, and climate, remains uncertain. Here, we linked SOC stocks measured to 100 cm depth at 556 old-growth Swiss forest sites (350 to 2000 m a.s.l.) to soil properties, net primary production (NPP), climate (mean annual precipitation, MAP: from 700 to 2100 mm; mean annual temperature, MAT from 0 to 12°C), and forest type. We compared measured SOC stocks with stocks simulated by Yasso20 model, commonly used for reporting SOC stock changes in greenhouse gas inventories. Since Yasso20 accounts only for litter inputs and climatic conditions, deviations between modelled and measured stocks can reveal the significance of organo-mineral interactions that we hypothesize to be crucial for SOC stocks.

Total SOC stocks exhibited distinct regional patterns, with highest values in the Southern Alps, characterized by soils rich in Fe and Al oxides. On average, SOC stocks simulated by Yasso20 aligned with measured SOC stocks (13.7 vs 13.2 kg C m^-2). In soils with pH ≤ 5, SOC stocks and model deviations were driven by exchangeable Fe, while in soil with pH > 5, exchangeable Ca was the dominant controlling factor. Beyond soil mineral properties, MAP emerged as an important driver of SOC stocks, while NPP remained unrelated to SOC stocks.

Our study demonstrates that soil mineral properties play a dominant role for SOC stocks across Swiss forest soils. Incorporating mineral-driven SOC stabilization into models can thus improve our ability to predict SOC stocks and its long-term dynamics.