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 &lt; 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="italic">SOC</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="true">¯</mml:mo></mml:mrow></mml:mover><mml:mi>c</mml:mi></mml:msub></mml:math> = 18.47 g kg −1 ) to 0.46 g kg −1 below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="italic">SOC</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="true">¯</mml:mo></mml:mrow></mml:mover><mml:mi>c</mml:mi></mml:msub></mml:math> (~−430%), while for noncroplands, such decline is sharper, from 0.96 above <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="italic">SOC</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="true">¯</mml:mo></mml:mrow></mml:mover><mml:mrow><mml:mi mathvariant="italic">nc</mml:mi></mml:mrow></mml:msub></mml:math> = 35.96 g kg −1 to 4.99 below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="italic">SOC</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="true">¯</mml:mo></mml:mrow></mml:mover><mml:mrow><mml:mi mathvariant="italic">nc</mml:mi></mml:mrow></mml:msub></mml:math> (~−620%). Although salinity’s significance in explaining SOC variability is minor (&lt;6%), we estimate a one SD increase in salinity of topsoil samples (0 to 7 cm) correlates with respective <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="italic">SOC</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="true">¯</mml:mo></mml:mrow></mml:mover></mml:math> declines of ~4.4% and ~9.26%, relative to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="italic">SOC</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="true">¯</mml:mo></mml:mrow></mml:mover><mml:mi>c</mml:mi></mml:msub></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="italic">SOC</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="true">¯</mml:mo></mml:mrow></mml:mover><mml:mrow><mml:mi mathvariant="italic">nc</mml:mi></mml:mrow></mml:msub></mml:math> . The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="italic">SOC</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="true">¯</mml:mo></mml:mrow></mml:mover></mml:math> decline in croplands is greatest in vegetation/cropland mosaics while lands covered with evergreen needle-leaved trees are estimated with the highest <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="italic">SOC</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="true">¯</mml:mo></mml:mrow></mml:mover></mml:math> 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.