Parent material geochemistry – and not plant input – as the primary element 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 in geochemically young, developing alpine soils is dependent on soil mineralogy as a result of parent material weathering, and is not strongly linked to plant biomass input. We show potential differences in the importance of SOC stabilization mechanisms, with universally large relative contributions (≥ 50 %) of the microaggregate soil fraction to bulk SOC. 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, which governs plant C inputs, and soil mineralogical characteristics, which control C stabilization, play equally crucial roles in predicting SOC contents in alpine soils at an early development stage, corroborated by soil fraction modern (F14C) values ranging from 0.77–1.06.