Materials and methods
In August 2017, we evaluated 94 sites of saline land in Yellow River Delta (37º33′-38º09′N, 118º35′-119º13′E), China, with 0.50–19.24‰ soil salinity (Fig. S1), and pH 7.04-8.08 (Fig. S2). Vegetation types primarily included crops (maize, cotton etc.), Phragmites ,Tamarix chinensis , Suaeda salsa , and mudflat.
Five soil cores were randomly collected at each site, pooled, and homogenized into a representative soil sample. Soil pH(1:2.5 w/v) was determined using a pH meter; total soluble salinity concentration measured by gravimetric method with water ecxtraction (Chi et al., 2019). Carbonate was determined by Acid-base titration, representing SIC (Fu et al., 2020); SOC was analyzed on a Vario MACRO Cube (Elementar, Langenselbold, Germany) after removing SIC with hydrochloric acid. Amino sugars and microbial residues were analyzed using previously reported methods (Zhang and Amelung, 1996; Engelking et al., 2007).
Generalized linear regression models were developed to assess associations among SIC, SOC, and microbial residues (R software). Structural equation modeling (SEM) was performed to trace causality among SIC, microbial residues, and SOC via root-mean-square error of approximation (AMOS software).