Shifting Carbon Fractions in Forest Soils Offset C-Based Turnover Times Along a 1700 m Elevation Gradient.

Climate change impacts the soil carbon cycle, yet there is no scientific consensus on the vulnerability of soil organic carbon (SOC) stocks to global warming. Here, we studied soil organic matter (SOM) changes across 50 Swiss forest sites covering an elevation gradient from 270 to 2020 m, with dominant tree species changing from sub-Mediterranean pubescent oak to mountain pine at treeline. We sought to assess how elevation, serving as an integrating variable for climate variation, affects the stocks, transformation state, and radiocarbon (C)-based turnover of SOM fractions in the organic layer, as well as in particulate organic matter (POM) and mineral-associated organic matter (MOM) fractions in forest mineral soils (0-20 cm). Our results show consistent enrichment in C and N across all SOM fractions with increasing elevation, indicating a ubiquitous transformation state among SOM fractions regardless of environmental conditions. However, C stocks in the organic layer and in mineral soil POM increased proportionally relative to MOM with increasing elevation. Additionally, C-based turnover times in the organic layer, the free POM, and the fine MOM fractions increased with elevation, indicating slower SOM processing and a reduced transformation of POM to MOM under harsher climatic conditions. In contrast to individual SOM fractions, total SOC stocks and C-based turnover times in the bulk mineral soil showed no elevational pattern. This indicates that with increasing elevation, the shift in composition towards POM, which has a shorter turnover time than MOM, offsets the increased turnover time within each fraction. As a result, the overall SOC turnover time remains stable across the entire elevation gradient. However, the higher proportion of C stored in the more vulnerable POM fractions in high-elevation forests indicates that their SOC stocks may be at higher risk under climate change.