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

•Soil organic and inorganic C content were higher under organic ameliorants.•Humic acid plus manure alleviated microbial N and P limitation by reducing soil electric conductance.•Humic acid plus manure promoted the positive regulation of soil organic C on inorganic C.•Electric conductance and microbial resource limitation were the main factors affecting soil C content.•Humic acid plus manure is more recommended in saline soil.