Impact of suburban cropland intensification and afforestation on microbial biodiversity and C sequestration in paddy soils

Rapid urbanization drives changes in suburban cropland and afforestation, but the impact on soil microbes and carbon (C) dynamics remains unclear. Using genetic sequencing (16S rRNA sequence and functional prediction), we assessed the effects of cropland intensification and afforestation on the soil microbial communities and C sequestration potential in a suburban agricultural area. Traditional rotation (rice–wheat) homogenized microbial diversity along soil depth (0–100 cm). Cropland intensification (rice–wheat to rice–vegetables) and afforestation (rice–wheat to forest) decreased fungal diversity below 20 cm, while bacterial diversity and dominant taxa were less affected, due to the advantages of bacterial diversity and dominant taxa distribution. Cropland intensification increased profile C accumulation, mainly due to excessive fertilization (obviously decreased aerobic_chemoheterotrophy below 20 cm in bacterial function), but not C stability (significantly increased permanganate oxidizable C (POXC), and microbial-dependent C sources tend to from crop litter to soil sources). Afforestation offset C accumulation through no fertilization and inundation, but increased saprophytic function for C sequestration. In addition, pH and C had opposing effects on dominant microbial taxa, mitigating extreme differentiation. Subsoil was more sensitive to changes than topsoil. Altogether, soil C stability decoupled from C accumulation in the suburban agricultural area. Cropland intensification increased C accumulation yet decreased bacterial C sequestration potential and C stability; cropland afforestation facilitated fungi-derived C accumulation and its C sequestration. Deep soil C sequestration was more vulnerable to suburban agriculture practices, emphasizing the need to consider subsoil C-cycling in agricultural practices amid urbanization.