4.Discussion
4.1. Effects of nitrogen and phosphorus addition on litter decomposition rate
The input of N into the ecosystem changes processes such as microbial community structure and extracellular enzyme activity, which can affect the decomposition of litter matter (Hernandez et al., 2019; Wu et al., 2013). The mass remaining of litter in the early stage of decomposition was significantly lower than CK at low N concentration (Fig.1), indicating that the moderate N addition in the early stage was beneficial to litter decomposition, while too much was inhibitory, which is consistent with previous reports (Chen et al., 2013; Song et al 2015). Similarly, the low concentration N treatment had higher kvalues and was able to accelerate the decomposition of litter (Table 2). This is because moderate N input reduces the litter C/N ratio and adjusts the suitable conditions for litter decomposition, while high N input significantly reduces the soil pH, which is not conducive to microbial decomposition activities and tends to break the N/P balance and enhance ecosystem phosphorus limitation (Chen et al., 2013; Song et al 2015; Heuck et al., 2018). During the decomposition process, the high P concentration treatment had a larger k value and lower mass remaining of litter, indicating that P addition significantly stimulated litter decomposition, and the higher the concentration the stronger the stimulating effect. This could be explained by the fact that, on the one hand, P addition relieved the P nutrient limitation of ecosystem in the early stage of litter decomposition, and on the other hand, it increased the soil nutrient effectiveness, thus improving microbial activity as well as phosphorus use efficiency (Lin et al., 2019). The mass remaining rate of litter in all concentrations of NP treatment was lower than N and P treatment during the litter decomposition, indicating that NP addition had the strongest effect on litter decomposition. The results also indicated that N and P addition had an interactive effect on litter decomposition, and P addition may alleviate the inhibitory effect of high N on litter decomposition (Hao et al., 2013). In the study, increasing NP concentration slowed down the decomposition rate of litter, which may be related to the chemical regulation of phosphorus content in litter (Zheng et al., 2017). Overall, the decomposition rate of litter was higher in the early stage than in the middle and late stages, which could be because the study area is in highland alpine region, and rapid reproduction and growth of soil microorganisms would consume a large amount of nutrients and accelerate the litter decomposition in the early stage (July-October).