## 3 Results and Discussions
### Assessment of Litter Characteristics
The environmental temperatures in the two rearing conditions were consistent, with a lab temperature of 25 \({}^{\circ}\)C. As shown in Table 2, the pH values of litter samples in the nonlitter group and waste-tea-litter group were 7.8 \(\pm\) 0.1 and 6.4 \(\pm\) 0.2, respectively. The pH value of waste tea was 5.8 \(\pm\) 0.1, appearing mildly acidic. As the tea was mildly acidic, H+ increased in the broiler litter in the waste-tea-litter group, making the balance relationship in Formula (4) shift to the NH\({}_{4}\)+ form. In addition, the microorganisms had lower activity in a low-pH environment, and the decomposition rates of urea and protein were slowed down [23]. Generally, litter additives can control NH\({}_{3}\) generation and emission, and mainly include adsorbent, microorganism, inhibitor, and acidifier types. Among them, the acidifier has been recognized as one of the most effective methods to prevent nitrogen from flowing away to the atmosphere, and has been popular in poultry-rearing processes. Theoretically, as the bacteria for transforming uric acid or urea into NH\({}_{3}\) grows well in a high-pH environment, reducing the pH value of litter in litter can reduce the amount of bacteria and the NH\({}_{3}\) generation. The pH value of litter has been proved as significant in the growth and survival of pathogens. Moreover, studies proved that litter acidification also could inhibit the urease activity [37]. Urease is a muramidase mainly existing in manure bacteria, and its activity is affected by pH value. Studies showed that urease inhibitor in livestock litter can inhibit the original microorganism bacteria activity, thus inhibiting the NH\({}_{3}\) generation [38]. [PERSON] and [PERSON] [39] reported that tea extract had an equivalent inhibiting effect on urease as the urease inhibitor did. The microbial urease is a nickel-bearing enzyme, and needs six Ni\({}_{2}\)+ atoms to come into play [40]. Studies have showed that adding nickel can affect the generation of urease [41]. It has been reported that tea is capable of absorbing heavy metals [42], since some chemical substances in tea can react with the Ni\({}_{2}\)+ atom [43; 44]. This nickel-absorption process might inhibit the activity of bacteria that produce uricase and urease, and reduce the generation of urease. Therefore, this are probably why the mildly acidic tea with tea polyphenols can inhibit the growth of urease bacteria and urease actinomycetes, as well as urease activity [45], and weaken the reaction processes given in Formula (1) and Formula (2), which further reduce the decomposition of urea and uric acid. C/N is another important factor, as the NH\({}_{3}\) in litter is mainly generated by bacteria's decomposition of organic nitrogen. This catabolic pathway can provide the energy required for bacterial growth, which can occur in both aerobic and anaerobic conditions, but is higher in the aerobic conditions. Adding straw and other litters can increase the number of degradable carbon substances, which is favorable for aerobic conditions, and induces anabolism [8]. Studies [46] showed that the C/N ratio in pig manure slurry in an anaerobic environment was between 4 and 10 with almost no anabolism; the C/N ratio in livestock litter varied from 1:1 to 27:1. As the litter C/N increased, released CO\({}_{2}\)-C and NH\({}_{3}\)-N quantities tended to decrease [47]. Therefore, the C element can be increased and the N element can be decreased in litter by controlling the C/N in the litter to fix NH\({}_{4}\)\({}^{+}\) in an organic form, thus reducing NH\({}_{3}\) generation and emission. Studies showed that by adding litter with high carbon organics in litter, the ammonium nitrogen can be transformed into organic nitrogen during the additive decomposition. If the added litter has a high C/N ratio, microorganisms can use inorganic nitrogen (ammonium and nitrate) in the litter, which can be fixed to a new microbial biomass under microbial action [48].