Salinity decreases the soil organic carbon stock while increasing its stability: A case study in the coastal region of China

The influence of soil salinity on carbon sequestration is largely unknown, and this is stopping us from being able to accurately evaluate the carbon sequestration capacity of soils with different salinity levels. Fifty-one soil samples were thus collected from Wudi County, China, along a salinity gradient. The density fractionation and Fourier-transform mid-infrared spectroscopy were employed to determine soil organic carbon (SOC) content, constitution and characteristics. The results showed that soil carbon, SOC, total nitrogen contents and carbon/nitrogen ratio significantly decreased with increasing salinity (P < 0.05). The proportion of mineral-associated organic carbon to SOC, and the band absorbance ratios of 1630/1030 cm⁻¹ (SOC stability), and 875/1030 cm⁻¹ (abundance of aromatic groups) significantly increased with increasing salinity (P < 0.05). Structure equation modeling indicated that soil salinization directly decreased the SOC stock by reducing the organic inputs, soil fine particle content (<20 μm), and specific surface area, and enhanced the SOC stability possibly due to inactive SOC fractions accumulating during salinization. Here, we prove that a reduction in the SOC stock is accompanied by an increase in its stability under saline conditions. To accurately evaluate the SOC sequestration capacity in soils with different salinity levels, the SOC formation mechanism and its sources should receive more attention in future studies.