Effects of magnetized water combined with nitrogen-preserving agents on nitrogen loss and microbial communities during composting

Aerobic composting is one of the most typical biological processes to decompose organic wastes into organic fertilizers or soil amendments. However, the existing composting has commonly suffered from severe nitrogen loss and long composting periods. Therefore, the additives are required to reduce the composting period and nitrogen loss. Taking the mixture of cow dung and straw as the research material, this study aims to explore the nitrogen loss during rapid composting. The experiment utilized magnetized water (T1), magnetized water combined with biochar (T2), magnetized water combined with ferrous sulfate (T3), and magnetized water combined with Bacillus megaterium (T4), with a control group designated as CK, for a 25-day aerobic composting. The results showed that the high-temperature periods for CK, T1, T2, T3, and T4 were 3-5, 2-8, 1-8, 4-8, and 2-8 d, respectively. All T1, T2, T3, and T4 were extended the high-temperature period of composting, compared with the CK. The temperature in the T1 treatment coincided with the ambient temperature from day 14 to day 25 of composting, without the secondary fermentation, thereby shortening the composting period. T1, T2, and T4 increased the highest temperature during composting. The most significant effect of T3 was achieved at the maximum temperature of 64.5℃. The total nitrogen content of each treatment group at the end of composting increased by 14.04%, 18.89%, 37.56%, 36.27%, and 28.47%, respectively, compared with the initial total nitrogen content of composting. The total nitrogen content increased the most in the T3 treatment, compared with the CK. The total ammonia emissions in each treatment group were 8.12, 4.37, 6.60, 7.32, and 3.95 g/d, respectively, during composting. The T4 treatment emitted the least ammonia, while the CK was the most. The T3 treatment reduced the pH of the compost during the whole process, whereas the low pH reduced the NH3 emissions. The T4 treatment promoted the nitrate nitrogen content and GI value, with the GI value reaching 110.03% at the end of composting. As such, nitrogen functional genes were then determined to explore the effects of different microbial communities on nitrogen and nitrogen transformation during microbiological composting. The T2 treatment promoted the absolute abundance of the nitrogen cycling gene gdhA, and then suppressed the relative abundance of Actinobacteria, for the high nitrogen cycling. The T3 treatment promoted the absolute abundance of the nitrogen-fixing gene nifH and the nitrogen-cycling gene gdhA, indicating the relative abundance of Proteobacteria. Additionally, structural equations showed that T2 treatment shared a significant negative effect on ammonia emissions, while ammonia emissions had a significant positive effect on total nitrogen loss. T3 treatment had a significant negative effect on pH, and then pH had a significant positive effect on ammonia emissions, whereas, the ammonia emissions had a significant negative effect on total nitrogen loss. Therefore, the biochar was used to adsorb ammonia for less ammonia emissions, thereby reducing the nitrogen loss. Ferrous sulfate was used to lower the pH of the compost, thus reducing ammonia emissions and nitrogen loss. Magnetized water combined with biochar showed the best nitrogen retention and ideal degree of maturation in the rapid composting, in terms of composting temperature, total nitrogen content, absolute abundance of nitrogen functional genes, relative abundance of microorganisms, and structural equation.