Ca(H₂PO₄)₂ and MgSO₄ activated nitrogen-related bacteria and genes in thermophilic stage of compost

This study was conducted to investigate the effects of Ca(H₂PO₄)₂ and MgSO₄ on the bacterial community and nitrogen metabolism genes in the aerobic composting of pig manure. The experimental treatments were set up as control (C), 1% Ca(H₂PO₄)₂ + 2% MgSO₄ (CaPM1), and 1.5% Ca(H₂PO₄)₂ + 3% MgSO₄ (CaPM2), which were used at the end of composting for potting trials. The results showed that Ca(H₂PO₄)₂ and MgSO₄ played an excellent role in retaining nitrogen and increasing the alkali-hydrolyzed nitrogen (AN), available phosphorus (AP), and available potassium (AK) contents of the composts. Adding Ca(H₂PO₄)₂ and MgSO₄ changed the microbial community structure of the compost. The microorganisms associated with nitrogen retention were activated. The complexity of the microbial network was enhanced. Genetic prediction analysis showed that the addition of Ca(H₂PO₄)₂ and MgSO₄ reduced the accumulation of nitroso-nitrogen and the process of denitrification. At the same time, despite the reduction of genes related to nitrogen fixation, the conversion of ammonia to nitrogenous organic compounds was promoted and the stability of nitrogen was increased. Mantel test analysis showed that Ca(H₂PO₄)₂ and MgSO₄ can affect nitrogen transformation-related bacteria and thus indirectly affect nitrogen metabolism genes by influencing the temperature, pH, and organic matter (OM) of the compost and also directly affected nitrogen metabolism genes through PO₄^3- and Mg²⁺. The pot experiment showed that composting with 1.5% Ca(H₂PO₄)₂ + 3% MgSO₄ produced the compost product that improved the growth yield and nutrient content of cilantro and increased the fertility of the soil. In conclusion, Ca(H₂PO₄)₂ and MgSO₄ reduces the loss of nitrogen from compost, activates nitrogen-related bacteria and genes in the thermophilic phase of composting, and improves the fertilizer efficiency of compost products. KEY POINTS: • Ca(H₂PO₄)₂ and MgSO₄ reduced the nitrogen loss and improved the compost effect • Activated nitrogen-related bacteria and altered nitrogen metabolism genes • Improved the yield and quality of cilantro and fertility of soil.