Effects of elevation and slope aspect on the distribution of the soil organic carbon associated with Al and Fe mineral phases in alpine shrub–meadow soil

Mountain ecosystems store a large amount of soil organic carbon (SOC) sensitive to global climate change. The SOC associated with Al and Fe minerals is important for SOC retention because of the ubiquitous nature and highly reactive surface properties of these minerals. Topography is also known to impact the distribution and transformation of SOC by creating different microclimates. However, the effect of topography on the distribution of organo-mineral associations has seldom been reported. This study uses a selective dissolution method to quantify the soil carbon (C) fractions associated with Al and Fe minerals in alpine shrub-meadow soil. Na-pyrophosphate (PP), HCl-hydroxylamine (HH) and dithionite-HCl (DH) were used to quantify organo-metal complexes, SOC associated with short-range order (SRO) phases and crystalline phases, respectively. Results suggest that the Al and Fe mineral-associated C accounted for a small proportion of SOC (less than 30%) in each extraction. A higher concentration coupled with a lower percentage of SOC was found in the A horizon compared to the B horizon. A significant correlation was observed between Fe and C in PP and HH extractions, whereas Al was significantly correlated with C in DH extractions. Elevation and slope aspect strongly influenced soil biotic and abiotic parameters, as well as organo-mineral associations. The C fractions extracted by PP and HH were significantly higher in the NE slope aspect than the SW slope aspect. These fractions were positively correlated with soil water content and negatively correlated with soil pH. The C fractions extracted using DH decreased with increasing elevation and were positively correlated with DH extractable Al. Our results highlight the role of topography on the distribution of organo-mineral associations, which should be considered during the assessment of SOC stability in alpine soils.