Depth-resolved carbon-pool composition and climate-scenario sensitivity across contrasting peatland sites in the Altai Mountains

Peatland carbon stability reflects the vertical distribution of organic-carbon fractions and their response to changing hydrothermal conditions. We quantified soil organic carbon (SOC), particulate organic carbon (POC), mineral-associated organic carbon (MAOC), dissolved organic carbon (DOC), measured carbon stability index (CSI), soil properties, and extracellular enzyme activities across five depth intervals within the upper 0–80 cm profiles of three contrasting Altai peatland sites. At D1, SOC and POC followed a non-monotonic profile, with maxima of 304.42 and 211.17 g kg⁻¹ at 0–10 cm and a secondary maximum at 20–40 cm. At D2, SOC remained within 104.14–117.36 g kg⁻¹ and POC within 11.42–19.09 g kg⁻¹, while MAOC constituted 75.47–94.40% of SOC. At D3, SOC decreased from 147.61 to 148.09 g kg⁻¹ in the upper 20 cm to 93.12 g kg⁻¹ at 60–80 cm; measured CSI reached 23.68 at 20–40 cm. Principal components 1 and 2 explained 41.9% and 14.3% of standardized variance. PC1 contrasted pH, electrical conductivity, bulk density, and moisture with LAP, NAG, and POC, whereas PC2 contrasted total phosphorus with N/P, total nitrogen, and SOC. Under SSP5–8.5, simulated SOC changed by − 23.29%, + 5.11%, and − 2.11% from 2025 to 2100 at D1, D2, and D3, respectively; corresponding SSP2–4.5 changes were − 13.45%, + 16.79%, and + 5.43%. These results provide depth-resolved carbon-pool profiles and quantitative site-specific scenario trajectories for Altai peatlands.