Interactive effects of salinity and straw on the soil aggregate stability and organic carbon sequestration in saline soils in the Hetao area, China

Soil organic carbon (SOC) and aggregate stability are the critical factors in evaluate soil quality and carbon sinks in agricultural soils. Especially in complex saline soils containing larges amount of sodium and calcium ions. To quantitatively estimate the importance of salinity factors, SOC fractions, and soil aggregate fractions on SOC contents and the mean weight diameter (MWD), as well as to reveal the transformation mechanism of SOC under the interactions between salinity and straw, a laboratory experiment was conducted on three soils with different salinity, with and without straw addition. The results showed that straw and salinity interactions significantly increased the proportion of large macro-aggregate fractions and the MWD, decreased micro-aggregate fractions, but promoted the conversion of SOC fractions to mineral-associated organic carbon (MAOC) of micro-aggregates in moderately saline soils. The responses of SOC contents and MWD to electrical conductivity (EC1:5) were nonlinear. The exchangeable sodium to calcium ions ratio (E-Na/E-Ca) (%IncMSE = 11.4, p < 0.01), and MAOC contents (%IncMSE = 17.0, p < 0.01) provided the best explanations of SOC contents, while the MWD was more explained by calcium carbonate (CaCO3) contents (%IncMSE = 9.6, p < 0.01), and MAOC/SOC (%IncMSE = 10.5, p < 0.01). The proportion of micro-aggregate fractions was significant drivers of both SOC contents and MWD. The increased MAOC contents was mainly caused by improving the binding sensitivity of aliphatic compounds to salt-related compounds, and the contents of C-C/C-H and C=O functional groups. The findings offer new perspectives into the mechanism of SOC sequestration and sustainable development of moderately saline soils under straw returning in the Hetao area.