Soil CH₄ and CO₂ emissions mediated by water and afforestation in Horqin Sandy Land

Ecological restoration in arid and semi-arid regions plays a vital role in maintaining regional carbon balance under global change. Although water management and vegetation composition regulate soil carbon cycling, their combined effects on greenhouse gas source/sink dynamics in sandy ecosystems and the underlying microbial mechanisms remain unclear. This study examined sand dunes (Sd) and 35-year-old typical afforestation types including Caragana microphylla Lam. (Cm), Populus simonii Carr. (Ps), Pinus sylvestris var. mongolica Litv. (Pm), and mixed forests of Ps + Pm and Pm + Cm in the Horqin Sandy Land. Through a controlled incubation experiment at 30% and 60% water holding capacity (WHC), we systematically analyzed the effects of afforestation and water management on soil methane (CH₄) and carbon dioxide (CO₂) emissions. Linear regression analyses of functional gene abundances and environmental factors further revealed microbial mechanisms. All afforested soils acted as CH₄ sinks, Ps, Pm, Ps + Pm, and Pm + Cm showed the strongest CH₄ uptake, correlated with higher pmoA gene abundance and lower mcrA/pmoA. 30% WHC enhanced CH₄ oxidation by improving soil aeration, whereas CO₂ emissions increased significantly under elevated water content. Cm showed the highest CO₂ emissions, whereas mixed forests (Ps + Pm and Pm + Cm) maintained high CH₄ uptake even under high CO₂ emissions. These findings indicate that water management and afforestation legacy interactively shape carbon cycling pathways in sandy ecosystems. Mixed forests optimize microbial community function, balancing carbon cycling efficiency with greenhouse gas mitigation across water regimes, providing a scientific basis for carbon management in ecological restoration of the Horqin Sandy Land and comparable semi-arid regions.