Effect of Sodium Pyrosulfite on the Quality of Sweet Sorghum Silage in Saline–Alkali Soil: Microbial Diversity and Metagenomic Analysis

This study examined the impact of sodium pyrosulfite (Na₂S₂O₅) as a silage additive on the fermentation, quality, microbial diversity, and metabolic function of sweet sorghum silage in saline–alkali soil. Sweet sorghum grown in local saline–alkali soil was harvested, defoliated, and sprayed with a Na₂S₂O₅ solution (contained 400–2000 ppm SO₂), vacuum-sealed, and fermented at 25 °C. Samples were stored for 0–64 days and analyzed for chemical composition and microbial diversity. After 8 days, the pH of all silages dropped below 3.8. The 16th day silage chemical analysis revealed Na₂S₂O₅ inhibiting ethanol and acetic acid production; S12 showed the best results: 5.497% total sugar, 2.357% lactic acid, 0.825% acetic acid, and 0.669% ethanol and achieved the highest silage quality scores (DB50/T 669-2016: 82, “Excellent”; DGL: 19, “Very Good”). Microbial analysis showed that Na₂S₂O₅ inhibited spoilage microbes, reduced sugar consumption by nonlactic acid bacteria (such as genus Rehenlla1), and promoted the fermentation of lactic acid bacteria, and the original pathogenic genus did not hinder lactic acid fermentation. Predominant genera like Rehenlla1 and Lactobacillus contributed the most to the key metabolic pathways of silage in the best treatment period.