Parent material geochemistry – and not plant biomass – as the key factor shaping soil organic carbon stocks in European alpine grasslands

Abstract. Soils represent the largest terrestrial carbon (C) reservoir on Earth. Within terrestrial ecosystems, soil geochemistry can be a strong driver of plant-soil-carbon dynamics, especially in young, less weathered soils. Here, we investigate the impact of potential plant biomass input, soil fertility parameters, and soil organic carbon (SOC) stabilization mechanisms on the distribution of SOC in European alpine grasslands across gradients of geochemically distinct parent materials. We demonstrate that SOC stock accrual and persistence in geochemically young soils, with fraction modern (F14C) values ranging from 0.77–1.06, is heavily dependent on soil mineralogy as a result of parent material weathering, but is not strongly linked to plant biomass. We show potential differences in the importance of geochemical variables and SOC stabilization mechanisms, with the microaggregate soil fraction contributing ≥ 50 % to bulk SOC in the majority of cases. We further show that concentrations of Fe, Al and Mn pedogenic oxides coincide with SOC stock magnitude across an alpine soil geochemical gradient, where SOC stocks range between 8.1–23.2 kg C m−2. Our results highlight that soil fertility and soil mineralogical characteristics, which govern plant C inputs and control C stabilization respectively, play equally crucial roles in predicting SOC contents in alpine soils at an early development stage.