Humic acid plus manure increases the soil carbon pool by inhibiting salinity and alleviating the microbial resource limitation in saline soils

Fertilization strategies have been widely studied as an effective strategy to improve the structure and fertility of saline soils. However, the effect of the combination of different organic ameliorants on the soil carbon (C) pool is still not clear. We conducted a two-year field experiment to investigate the influences of organic ameliorants on soil organic C (SOC) and inorganic C (SIC) stocks, and enzymatic stoichiometry for both top and subsoil (i.e., 0–20 cm and 20–40 cm) in the semi-arid saline region of Western China. Compared to chemical fertilizer (CK) soils, the content and stock of total C and SOC at 0–40 cm soil layer under organic ameliorant (OM), humus acid plus OM (HA + OM), and biofertilizer plus OM (B + OM) were 9–40% higher, whilst the content and stock of SIC at 0–40 cm soil layer under HA + OM were 18% higher after two years. In addition, soil microorganisms were generally limited by phosphorus (P), and HA + OM reduced microbial P and nitrogen (N) limitations after two years application. Structural equation modeling showed that reduced microbial N and P limitation under HA + OM was mainly directly driven by the decrease in electrical conductivity (EC) content. Redundancy analysis and correlation analysis further confirmed that soil EC and microbial resource limitation were the main factors affecting soil C content. Furthermore, HA + OM promoted the positive regulation of SOC on SIC, which facilitated the formation of SIC content. The higher accumulation of SOC and SIC content under HA + OM were both beneficial to the enhancement of the total C stock in saline soil. Overall, our results provided an effective regime to improve the soil saline and microbial environments, and increase the potential for rapid enhancement of the C stock of saline soil in Northwest China.