Rapid depletion of soil organic carbon stock threatens agricultural sustainability in IranOA

Soil organic carbon(SOC)stocks play a critical role in maintaining soil fertility,regulating biogeochemical cycles,and mitigating climate change,yet they are increasingly threatened in semi-arid agricultural regions.This study quantified spatiotemporal changes in SOC concentration and stocks across Hamadan Province,western Iran,over a 16-a period(2010–2025),a region that has undergone rapid agricultural intensification.A total of 212 soil samples were collected from the same georeferenced locations during both sampling campaigns,targeting the 0–30 cm soil layer.SOC concentration was determined using standardized laboratory methods,and SOC stock was calculated by integrating SOC concentration,bulk density,soil depth,and coarse fragment content.Geostatistical analyses,including experimental semivariograms and Ordinary Kriging,were applied to characterize spatial variability and to generate continuous maps of SOC concentration and SOC stock for both years(2010 and 2025).Results indicated a marked decline in mean SOC concentration,from 0.99%in 2010 to 0.85%in 2025.Correspondingly,mean SOC stock decreased from 126.48 to 84.45 Mg C/hm^(2),representing a reduction of 33.23%and a total carbon loss of approximately 4.59 Tg C from agricultural topsoil across the province.Spatial analysis revealed increased micro-scale variability and extended spatial dependence over time,reflecting increasingly heterogeneous management effects superimposed on broader landscape controls.Severe SOC stock depletion(SOC stock ratio(the ratio of SOC stock in 2025 to that in 2010)<0.50)occurred as scattered patches across the province,while moderate declines(0.50–1.00)dominated most agricultural areas,and some northern and southern zones showed comparatively greater stability or localized carbon gains.This study provides the first long-term,spatially explicit assessment of SOC stock dynamics in western Iran,demonstrating rapid carbon depletion driven by intensive cultivation under semi-arid conditions.The findings highlight the urgent need for soil conservation and carbon-preserving management practices to sustain soil health,agricultural productivity,and climate mitigation potential in dryland farming systems.