Soil organic carbon fractions and their associated bacterial and fungal abundance in alpine ecosystems.

Soil organic matter (SOM) has a key role in the carbon (C) cycle and consists of particulate organic matter (POM) and mineral-associated organic matter (MAOM), which differ in stability and turnover. This study investigates C dynamics and microbial abundance at two alpine sites (1526 and 2500 m a.s.l). Soil samples were fractionated, and the C:N ratio, pH, and microbial abundances were analysed. While the MAOM/POM ratio remained stable across sites, the higher-elevation soil, dominated by N-poor alpine graminoids, showed an increased C:N ratio, consistent with reduced decomposition and transfer of litter into mineral-associated pools under colder, more acidic conditions. Bacteria predominated in MAOM, supporting their role in SOM stabilisation, whereas fungal abundance was highest in MAOM only at 2500 m. Fungal abundance remained stable across sites, indicating greater tolerance to low temperatures and pH compared to bacteria, which declined at higher altitudes. This suggests fungi play a key role in decomposition in colder environments. Correlations between fungi and bacteria were context-dependent: negative in MAOM and positive in POM, but only at 2500 m. These findings highlight how the composition and stability of SOM and microbial abundance differ between fractions and at different elevations, underscoring the value of integrating microbial data with SOM fractionation to better understand alpine soil C dynamics.