Cotton-Stalk Wood Vinegar Affects Nitrogen and Phosphorus Transformation in Chicken Manure Composting

This study investigated the effects of cotton-stalk-derived wood vinegar produced at different pyrolysis temperatures on nitrogen and phosphorus transformations and the associated microbial mechanisms during the aerobic composting of chicken manure. Wood vinegar produced at 300, 350, 400, 450, and 500 °C was applied in a 38-day composting experiment. Changes in physicochemical properties, nitrogen and phosphorus fractions, bacterial community succession, and potential functional profiles were analyzed using 16S rRNA gene sequencing, Spearman correlation analysis, and PICRUSt2-based COG functional prediction. The results showed that cotton-stalk-derived wood vinegar improved the composting process and regulated nutrient transformation. Compared with T0, the thermophilic phase was prolonged by 1–4 days in T1, T3, and T5. At the end of composting, the pH and electrical conductivity of all treatments ranged from 7.45 to 7.72 and from 1.37 to 1.75 mS/cm, respectively, which were within acceptable ranges for mature compost. The effects of wood vinegar on nitrogen and phosphorus transformations varied with pyrolysis temperature. All wood-vinegar-amended treatments maintained higher final TN concentrations than T0. Among them, T4 maintained a high final TN concentration of 22.90 g/kg, which was 11.17% higher than that of T0 and close to the highest value observed among all treatments. However, inorganic nitrogen responses differed among treatments: T1 showed more efficient conversion of NH4+-N to NO3−-N, whereas T3 had the lowest NH4+-N/NO3−-N ratio, indicating relatively higher compost maturity. Therefore, the nitrogen results should be interpreted on a concentration basis rather than as evidence of reactor-scale total nitrogen retention or nitrogen loss. Wood vinegar also affected phosphorus transformation, with T4 achieving a higher available phosphorus concentration. Bacterial community analysis showed that Bacillota and Actinomycetota dominated the composting process, and T4 maintained a more stable bacterial community structure during maturation. T4 was enriched in potential functional genera, including Saccharomonospora and Thermoactinomyces. Spearman correlation analysis indicated that Weissella and Pediococcus were significantly associated with nitrogen-related parameters in T4, whereas Saccharomonospora was closely linked to phosphorus transformation. PICRUSt2-based COG prediction further suggested that T4 maintained relatively stable nitrogen- and phosphorus-related functional potentials.