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).