Deep arsenic mobilization in karst soils: effects of pedogenesis and grazing

Arsenic (As) contamination poses substantial ecological and human health concerns in karst regions due to high geochemical background levels and enhanced mobility associated with carbonate weathering and thin soil cover. To investigate the vertical distribution, sources, and environmental risks of soil As under different anthropogenic land uses, we analyzed soil profiles from three representative ecosystems—secondary forest, abandoned cropland, and grazing shrubland—in a typical karst area of Southwest China. The As contents were identified between secondary forest (19.00 ∼ 41.17 mg/kg), abandoned cropland (22.55 ∼ 34.47 mg/kg), and shrubland (14.19 ∼ 34.46 mg/kg), with no significant differences, indicating a predominantly geogenic origin from carbonate bedrock weathering. Long-term cultivation homogenized surface soil As through plowing and crop uptake, whereas grazing introduced additional organic-bound As via livestock excreta, promoting downward migration and accumulation in deeper soil layers. Ecological risk assessment, using contamination and enrichment indices, revealed low to moderate potential risks (contamination factor: 0.71 ∼ 2.06; enrichment factor: 0.76 ∼ 2.34). Health risk assessment indicated that non-carcinogenic risks (hazard index, HI: 0.09 ∼ 0.21) were within acceptable limits for all exposure pathways, while carcinogenic risks for children (carcinogenic risk (CR): 0.11 ∼ 0.21) approached the threshold of concern. Despite overall low risk levels, the high mobility of As and its potential co-mobilization with other toxic elements underscore the need for continued monitoring. These findings highlight the importance of sustainable land-use practices to mitigate As mobilization and safeguard soil quality in vulnerable karst ecosystems.