Soil salinization and sodication represent a pervasive form of soil degradation worldwide, characterized by the deterioration of soil physical structure and a reduction in saturated hydraulic conductivity (Ks). This has an adverse effect on crop water uptake, which in turn results in a reduction in food production. Nevertheless, there is a lack of research on the limiting mechanisms of Ks by individual limiting factors in soils with different salinity and sodicity. The Songnen Plain in north-eastern China, the site of this research, is one of the regions with a concentration of saline sodic soils globally. In this study, structural equation modelling (SEM) was employed to evaluate the influence of soil properties on soil Ks, with the objective of identifying the principal limiting factors and barrier mechanisms of soil Ks. The results showed that Ks exhibited significant spatial autocorrelation and its spatial variation depended on soil properties. Soil Ks was significantly lower in the central plains of the study area, with a notable increase observed in both the eastern and western regions. The presence of substantial quantities of exchangeable sodium on the surface of soil colloids is the root cause of the low soil Ks, which, in conjunction with total soil alkalinity, exerts a considerable direct influence on soil pH (55%). Soil sand content and bulk density are important physical factors affecting soil Ks (22% and 31%, respectively), which influences water transport in the soil by affecting soil pore structure. The three predictors of saturated hydraulic conductivity for soils with different salinity and sodicity were pH, bulk density, and water-stable aggregate content greater than 0.25 mm (WSA >0.25). A predictive model was developed using the logarithmic transformation of soil Ks and a regression equation based on the support vector mechanism model. The regression equation was K s = e 26 . 5 + 3 . 72 WSA > 0 . 25 - 4 . 15 BD - 2 . 71 pH . The findings indicate that lowering soil pH and bulk density and promoting the formation of aggregate structures are key to increasing Ks in saline sodic soils.

Leymus chinensis (Trin.) Tzvel. (Poaceae) is the dominant plant species in the Songnen grassland of China. Nitrogen addition is an important measure to prevent degradation of grassland due to soil salinization. To clarify the effects of long-term nitrogen addition on the quality of L. chinensis and soil properties, a field experiment was continuous conducted for 10 years in moderately saline-sodic grassland from 2009 to 2018, and the annual nitrogen addition rate was 0，30，60，90，120，150，180 and 210 kg N ha-1, respectively. The results showed that with nitrogen addition rates increasing, the contents of protein and fat in shoot increased 20.5% to 80.9% and 6.9% to 69.3% compared with those of the control without nitrogen addition, respectively. When nitrogen addition rates reached 90 kg ha-1 to 210 kg ha-1, The ratio of K+/Na+ and Ca2+/Na+ in L. chinensis shoot increased 1.26 to 3.37 times and 1.56 to 2.98 times compared with those of the control, respectively. Nitrogen addition decreased significantly soil pH, electrical conductivity and exchange sodium percentage, and increased significantly the contents of soil organic matter, total nitrogen, available nitrogen, the activities of sucrase and urease and ecosystem multifunctionality(P＜0.05). The redundancy analysis showed that there was a significant positive correlation between the improvement of L. chinensis quality and soil desalination and soil fertility increasing. Therefore, the suitable nitrogen addition rate of 90-120 kg N ha-1 was an effective and necessary measure to maintain the healthy growth of L. chinensis and inhibit land degradation in moderately saline-sodic grassland of Northern China.