## 4 Discussion
### NH4+-N, NO3-N, TN, and TOC  
The accumulation of NH4+-N is mainly due to the mineralization and ammonification. The NH4+-N content of C1 treatment was significantly higher than that of I1 and I2 treatment on the second day, and the difference reached the maximum between all treatments. It showed that the higher temperature and pH value of continuous aerated treatment was beneficial to the conversion of organic nitrogen into NH4+-N by ammonification [32,33]. As the higher nitrogen content in the early composting materials, microorganisms accelerated the decomposition of available nitrogen through ammonification which leads to a rapid accumulation in NH4+-N content [34]. Subsequently, the NH4+-N content gradually decreased due to the intense ammonia volatilization under high pH and temperature, nitrification, and biological immobilization [10,35]. At the end of composoting, the NH4+-N content of intermittent aerated treatment decreased more than that of continuous aerated treatment, which may be due to the better growth of nitrifying bacteria at about 30 \({}^{\circ}\)C in the I1 and I2 treatments [36], thereby enhancing the nitrification and promoting the transformation of NH4+-N to NO3-N.  
The NO3-N concentration of all treatments was maintained within 0.2 g/kg DM during the thermophilic phase. High temperature and pH value limited the activity of nitrite bacteria and nitrobacteria [33,37], so the NO3-N concentration remained at a low level. [PERSON] et al. [38] reported that nitrification occurs when the temperature is lower than 40 \({}^{\circ}\)C and the aeration is favorable. After the 12 th day, the ammonification was weakened and the nitrification was intensified, which promoted the conversion of NH4+-N to NO3-N [27], so the concentration of NO3-N increased rapidly. Compared with continuous aerated treatment, the NO3-N concentration of intermittent aerated treatments was lower. The optimum temperature of nitrifying bacteria was about 30 \({}^{\circ}\)C [36]. Therefore, intermittent aerated treatments enhanced nitrification and promoted the conversion of NH4+-N to NO3-N.  
TOC showed a downward trend throughout the composting process, which was possibly due to the production of CO2 by bacterial and fungal metabolism and the rapid decomposition of degradable carbon in Compost material [10]. At the end of composting, Higher TOC content occurred with the C1 treatment due to the higher CO2 emissions. [PERSON] et al. [33] reported CO2 was the main source of carbon loss (over 70% of the total carbon losses), and the rest of the carbon losses were caused by the emission of CH4 and other VOCs.  
TN content in all treatments decreased at the beginning of composting, due to the decomposition of organic nitrogen and massive NH3 volatilization under high temperature and high pH value [2,39]. TN peak content of C1 treatment (13.74 g/kg DM) was the lowest among the three treatments, which was consistent with its NH3 emissions (Figure 4a). [PERSON] et al. [40] reported that the decomposition of massive nitrogenous organic matter by microorganisms requires a large amount of oxygen, so the oxygen consumption rate was higher than supply rate in the reactors. Therefore, anaerobic microorganisms further decompose decomposition of nitrogenous organic matter to N2O and N2, resulting in a decrease in TN content. With the start of the concentration effect, the reduction rate of composting materials quality was faster than that of nitrogen [22], so the final TN content was higher than the initial TN content [13,41]. Among the three treatments, the TN content of C1 treatment was the lowest on day 20 because of its highest TN loss. This was consistent with the results in Figure 5b.