## 3 Results and Discussion
### Diurnal Concentration and Emission Profiles
#### 3.2.2 Nm  
The diurnal average VR in NM ranged from 11.70 to 20.80 m\({}^{3}\)/h-hen (Figure 4b), which was significantly higher than that in CC and AV (\(p<\) 0.05). The daily VR change trend in NM was similar to CC, and the peak of VR occurred from 9:00 to 18:00 because of the high temperature. It was seen that the NH\({}_{3}\) concentration gradually increased with the manure accumulation extending (Figure 4a). When the manure was accumulated for 1-3 days, the change in NH\({}_{3}\) concentration was contrary to that of VR, and the higher concentrations of NH\({}_{3}\) in NM occurred during the night, which was similar to the situation of CC. While it changed reversely from the fourth day, NH\({}_{3}\) concentration did not decrease with the increasing VR. In this case, after the manure accumulated on the belt for 4 days, NH\({}_{3}\) volatilized from manure with the temperature increasing was more than that exhausted by fans. As shown in Figure 5, the NH\({}_{3}\) concentration on day 7 was 3 times higher than that on the second day after manure removal. [PERSON] et al. [32] reported NH\({}_{3}\) concentration at air outlet with manure accumulated for 6 days was about 21 times higher than that for 2 days storage in a laboratory with the hen age of 23 to 36 weeks. Thus, manure accumulation significantly affected NH\({}_{3}\) emission rates. It was recommended that the manure should not be stored on the belt for more than 4 days in NM, which was consistent with the findings of [PERSON] et al. [32]. Otherwise, higher VR was demanded to maintain a good inner environment. NH\({}_{3}\) concentrations in NM ranged from 0.93 to 5.65 mg/m\({}^{3}\) with an average of \(3.06\pm 1.29\) mg/m\({}^{3}\), which was substantially higher than those in CC and AV (\(p<\)0.05) mainly due to much longer manure accumulation time inside the house. Figure 4: Diurnal patterns of average NH\({}_{3}\) concentration (**a**), VR and NH\({}_{3}\) emissions (**b**) in NM under different manure accumulated time. (Day 1 means the day of manure removal; Day 2 to Day 8 means the manure accumulated for 2 days to 8 days.). Figure 5: NH\({}_{3}\) concentration in NM with the manure accumulation time extending. NH\({}_{3}\) emission rates in NM gradually increased with manure accumulation, which agreed with the report by [PERSON] [33]. It also varied with time, similar to that of CC and AV, in which relatively higher NH\({}_{3}\) emission rates occurred during the day and lower emission rates occurred at night [16]. NH\({}_{3}\) emission rates in NM ranged from 1.48 to 78.93 mg/h\(\cdot\)hen (\(27.16\pm 13.12\) mg/h\(\cdot\)hen in average), which was similar to the value of 20.83 mg/h\(\cdot\)hen reported by [PERSON] et al. [34]. The NH\({}_{3}\) emission rates in NM were much higher than previously reported value (2.68 mg/h\(\cdot\)hen in average) from a caged-hen house with a 620 cm\({}^{2}\)/hen stocking density in the laboratory, inside which the manure was accumulated for 6 days [32]. In [PERSON] et al. [32], the minimum VR of laying hen house was regulated based on the maximum allowed indoor CO\({}_{2}\) concentration level of 4000 ppm\({}_{\rm v}\). The average VR inside NM was 16.03 m\({}^{3}\)/h\(\cdot\)hen, highest among the three studied laying hen houses, which had a significant and positive impact on NH\({}_{3}\) emissions [14; 16], and it was also mentioned in CC and AV houses. Besides, differences also occurred in manure belt or manure pan cleanliness between the two studies. In this study, there was inevitably some manure residue on the belt even after manure removal, which contributed to higher NH\({}_{3}\) emissions. In [PERSON] et al. [32], all the facilities were cleaned and the existing manure pans were replaced with new ones after each trial.