Enzymatic mechanism of organic nitrogen conversion and ammonia formation during vegetable waste composting using two amendments

Elucidating the mechanism of nitrogen conversion during composting is crucial for controlling nutrient loss and improving the quality of compost. To explore the enzymatic mechanism of organic conversion during composting, composting experiments using vegetable waste and chicken manure mixed with wheat straw and corn stalk as two separate treatments: WS and CS, respectively, were conducted in 63 L aerated static pile reactors for 33 d. The changes in the nitrogen fractions and related-enzymes activities were analyzed during different periods. The total nitrogen content increased by 34.3% during WS and decreased by 6.22% during CS after 33d of composting. The ammounium nitrogen content decreased by 79.6% and 51.4% during WS and CS. The nitrate, nitrite, organic, acid-insoluble organic nitrogen contents increased by approximately 52.6–123.9%, 590.9–5875%, 59.1–213.8%, and 764.4–7834.1%, respectively. The amount of total hydrolysable organic nitrogen increased by 18.8% during WS and decreased by 26.7% in CS. Structural equation modeling revealed that the contributions of different types of nitrogen to the formation of NH₄⁺ during WS composting decreased as follows: amine nitrogen (AN) > amino acid nitrogen (AAN) > amino sugar nitrogen (ASN) > hydrolysable unknown nitrogen (HUN), while the corresponding nitrogen contributions during CS decreased as follows: AAN > AN > HUN > ASN. The AN and AAN were most easily converted into NH₄⁺ during WS and CS, respectively, while ASN was synthesized from NH₄⁺ during vegetable waste composting. Using redundancy analysis it was revealed that nitrate reductase (50.1%), nitrite reductase (23.2%) and urease (7.1%) played leading roles in nitrogen transformation. Furthermore, total organic carbon (59.6%) was the main factor that affected enzymes activities.