Effects of new acidification methods on nitrogen conversion during agricultural waste composting

Aerobic composting is a common method for treating agricultural waste. However, a large amount of nitrogen is lost during composting, which is an important problem in agricultural waste composting. Material acidification is an effective method to reduce nitrogen loss during composting, while conventional acidification using inorganic acids has disadvantages such as high cost and secondary pollution. Optimizing the acidification process is of great significance for reducing nutrient loss and environmental pollution during composting. In this study, an acid conditioner was prepared by anaerobic fermentation of lactic acid bacteria with food residues (apple pomace and soybean dregs) as substrates, which were rich in lactic acid (70 mmol·L⁻¹) and lactic acid bacteria (10⁶ cfu·mL⁻¹). Two acidizing methods were designed using the acid conditioner: 1) A certain amount of acid conditioner (30%, w/w) was added directly to acidify the material (MA); 2) a small amount of acid conditioner (3%, w/w) was added with no forced ventilation for the first 3 days of composting to enable the lactic acid bacteria under the acidic conditioners to produce lactic acid achieving self-acidification of compost materials (LA). Meanwhile, we set up two experimental treatments consisting of adding sulfuric acid (SA) and no acidification (CK). Changes in physicochemical properties (temperature, pH, electrical conductivity, germination index of Oenanthe javanica treated with different composts, contents of organic matter and total nitrogen) and treansformation of nitrogen forms (emissions of NH₃ and N₂O; contents of organic nitrogen, NH₄⁺-N, NO₂⁻-N, and NO₃⁻-N) during the composting of agricultural waste were analyzed. The results showed that the compost products treated by the three acidification methods all reached the maturity standard (germination index > 80%), and the MA treatment was the best (germination index = 117.8%). The duration of the thermophilic phases (> 50 ℃) of CK, MA, SA, and LA were 10, 10, 9, and 7 days, respectively, all of which reached the harmless standard (> 50 ℃ for at least 7 days). The total nitrogen losses of MA, SA, and LA decreased by 14.0%, 25.6%, and 22.2%, and NH₃ volatilization decreased by 26.0%, 36.5%, and 54.9%, compared with CK, respectively. The acidification treatments increased the NH₄⁺-N content, promoted nitrification, and indirectly enhanced denitrification. MA and LA treatments reduced N₂O emissions by 23.1% and 69.4%, respectively, whereas SA treatment inhibited N₂O reduction and increased N₂O emissions by 18.3%. The ReCiPe evaluation method was used to evaluate the total environmental burden of different acidification treatments. The total environmental burden of MA, SA, and LA decreased by 34.5%, 11.0%, and 55.9%, respectively, compared with that of CK, indicating that acidification is an effective way to reduce the environmental burden of composting. By comparing the economic benefits of the three acidifying methods, it was found that the costs of MA and LA treatments were 18.4 Yuan and 0.87 Yuan, respectively, for reducing the emission of 1 kg active nitrogen, which was far lower than that of SA treatment at 91.3 Yuan. These results indicate that MA and LA acidification methods are economically feasible. In conclusion, MA and LA treatments can be feasible methods to reduce nitrogen loss during composting. This study provides a new theoretical basis for composting acidification and nitrogen conservation technology as well as a new scheme for the collaborative treatment of multi-source waste.