Topography matters: Contrasting hilltop and valley habitats shape soil organic carbon pools in Danxia landforms

Topography is a key regulator of carbon cycling in terrestrial ecosystems, yet how local-scale, landform-driven habitat heterogeneity controls soil organic carbon (SOC) pools remains poorly understood, particularly in unique geomorphological systems such as the Danxia landform. In this study, we collected soil samples from two typical mountains (Hailuofeng and Yangyuanshan) of Danxiashan Mountain in Guangdong Province, China. The effects of habitat difference among landform positions on the spatial variation in SOC pools were investigated based on the information of SOC contents, SOC stocks, SOC fractions (particulate OC (POC) and mineral-associated OC (MAOC)), C:N ratios, and the relationship between environmental factors and SOC. We found that SOC contents and stocks of the bulk soil and soil fractions were significantly higher at hilltops than at valleys, a topographic inversion opposite to the pattern in erosional toposequences. This hilltop SOC enrichment was associated with greater plant-derived C inputs (higher tree layer biomass and litter mass), finer soil texture, and higher soil moisture, together with substrate-driven increases in microbial biomass and C- and N-acquiring enzyme activities. Although the absolute POC and MAOC pools were larger at hilltops, hilltop soils had a higher POC proportion and higher C:N ratios. This pattern may reflect a stoichiometric constraint, in which N-poor, high-C:N substrates are inferred to reduce microbial carbon-use efficiency and limit the relative formation of MAOC. Consequently, the larger hilltop SOC pool is relatively more vulnerable to decomposition than the valley pool. The habitat differences between hilltops and valleys induced by Danxia landforms led to spatial variation in SOC stocks and altered SOC pool properties. Recognizing these landform-induced hilltop and valley effects will therefore improve the understanding and assessment of carbon cycling in terrestrial ecosystems.