Effects of historical land use changes on soil carbon, nitrogen, and microbial communities in an alpine sandy region of northwestern China

Land use change plays a crucial role in the dynamics of soil carbon and nitrogen, thereby influencing soil fertility. However, the effects of historical land use changes on deep soil carbon and nitrogen dynamics, as well as microbial community composition, in alpine sandy regions remain poorly understood. Therefore, this study aimed to investigate how different historical land-use types regulate soil carbon and nitrogen and shape microbial community structure along a deep soil profile in an alpine sandy ecosystem. The study site located at elevation of 2800 m, experiences an arid climate, with an annual mean temperature of 3.9°C, an average annual precipitation of 246.3 mm, and an annual potential evaporation of 1,716.7 mm, thereby classifying the area as an alpine arid region with predominantly sandy soils. This study investigated a 23-year-old Caragana microphylla shrub forest in the Gonghe Basin, northwestern China. Three land-use types were established: post-agricultural reforestation on sandy land (PR), where former cropland was converted to forest 23 years ago, direct afforestation on sandy land (PF), established directly on sandy land without prior agricultural use, and bare sandy land as a control (CK), which remained uncultivated and unafforested. Soil carbon, nitrogen, and microbial community structure were examined across the 0–500 cm soil profile among the three land-use types. Results indicated that historical land-use changes significantly influenced the storage of soil organic carbon (SOC), inorganic carbon (SIC), and total nitrogen (STN). Average concentrations of SOC, SIC, and STN across the 0–500 cm soil profile were highest in PR (2.40, 10.37, and 0.28 g·kg⁻¹, respectively), followed by PF (1.46, 9.53, and 0.17 g·kg⁻¹), and lowest in CK (0.89, 8.31, and 0.11 g·kg⁻¹). SOC and STN storage within each 100 cm depth increment were also greater in PR than in PF and CK. Soil water content emerged as a critical environmental factor regulating deep soil carbon and nitrogen cycling. Microbial diversity was highest in the 0–40 cm layer under PR, whereas PF exhibited greater diversity in deeper soil layers (100–500 cm). Bacterial communities were more sensitive to historical land-use changes than fungal communities. In CK, microbial communities were primarily influenced by soil physical factors, including pH, soil water content, and electrical conductivity, whereas in PF, SOC and STN were the dominant controlling factors. In PR, SIC content, soil bulk density, and soil water content played major regulatory roles. Overall, the post-agricultural reforestation model in alpine sandy regions demonstrates greater effectiveness than direct afforestation on sandy land in enhancing SOC, SIC, and STN storage across the 0–500 cm soil profile and in promoting surface soil microbial diversity. In contrast, direct afforestation on sandy land plays a distinct ecological role in maintaining microbial diversity in deeper soil layers. These findings highlight that, in sandy land restoration, consideration of the long-term legacy effects of historical land-use conversion is essential for promoting the sustainable development of desertification control strategies.