Negative correlation between soil salinity and soil organic carbon variability

Soil organic carbon (SOC) is vital for terrestrial ecosystems, affecting biogeochemical processes, and soil health. It is known that soil salinity impacts SOC content, yet the specific direction and magnitude of SOC variability in relation to soil salinity remain poorly understood. Analyzing 43,459 mineral soil samples (SOC < 150 g kg⁻¹) collected across different land covers since 1992, we approximate a soil salinity increase from 1 to 5 dS m⁻¹ in croplands would be associated with a decline in mineral soils SOC from 0.14 g kg⁻¹ above the mean predicted SOC (SOC_c = 18.47 g kg⁻¹) to 0.46 g kg⁻¹ below SOC_c (~−430%), while for noncroplands, such decline is sharper, from 0.96 above SOC_nc = 35.96 g kg⁻¹ to 4.99 below SOC_nc (~−620%). Although salinity’s significance in explaining SOC variability is minor (<6%), we estimate a one SD increase in salinity of topsoil samples (0 to 7 cm) correlates with respective SOC declines of ~4.4% and ~9.26%, relative to SOC_c and SOC_nc. The SOC decline in croplands is greatest in vegetation/cropland mosaics while lands covered with evergreen needle-leaved trees are estimated with the highest SOC decline in noncroplands. We identify soil nitrogen, land cover, and precipitation Seasonality Index as the most significant parameters in explaining the SOC’s variability. The findings provide insights into SOC dynamics under increased soil salinity, improving understanding of SOC stock responses to land degradation and climate warming.