4.3. The dynamics of soil enzyme activities during litter
decomposition
Invertase and cellulase are produced by microorganism and play important
roles in the decomposition of litter, and their activities are limited
by the number and structural function of the microbial community, the
chemical nature of the litter substrate, and the elemental stoichiometry
and nutrient effectiveness (Wang et al., 2010).The results showed that
in each nutrient addition invertase and cellulase activities were
significantly increased with significant effect in the early stage of
decomposition, which corresponded to the rapid degradation of
polysaccharide compounds(e.g. cellulose) in the early stage (Fig.4).
Cellulose is an easily decomposable substrate at the initial stage and N
and P addition effectively stimulated the increase in carbon hydrolytic
enzyme activities and the exudation of substrates for enzymes (Wang et
al., 2010; Song et al., 2018). The decrease of INV activity with the
increase of N addition concentration might be related to the inhibition
of litter decomposition and reduction of the amount of substrate for INV
action under high N concentration treatment (Ge 2017). In the late stage
of decomposition, the CBH activity of NP treatment showed a decreasing
trend. This may be due to the fact that in the early stage, the
cellulose-like polysaccharide content of litter increased with the input
of NP and the limitation of litter N and P was removed, while litter
carbon gradually became the main influencing factor in the later stage,
and thus cellulase activity was inhibited (Chen et al., 2014). POX
activity is an important indicator of lignin degradation. In the study,
POX activity decreased with the increase of N addition, which
corresponded to the change of lignin. N input promoted the
polymerization of lignin-like aromatic compounds with phenols into
refractory substances and limited the activity of POX (Wang et al.,
2019; Song et al., 2018). Meanwhile, POX activity was significantly
higher in middle concentrations of P and NP addition than in low and
high concentrations. This suggests that the moderate amount P input
might disrupt this bio polymerization and alleviate P limitation (Kunito
et al., 2009; Jiang et al., 2014). This was also verified by the results
of the correlation analysis in this study, in which the rate of litter
decomposition was negatively correlated with lignin/P and positively
correlated with litter P content.
Soil UG and AP are important drivers of the carbon, nitrogen and
phosphorus cycle in the ecosystem, and significantly affected by changes
of litter decomposition conditions such as exogenous N, P addition
(Zhang et al., 2017). Compared to CK, UG activity showed a significant
increase in all treatments, while the effect was inhibited in high N
addition (Fig.4), which was inconsistent with the previous study (Guan
et al., 2019). This is because the substrate exudation rate was
decreased by the slowing down of the degradation process of litter under
high N treatment, resulting in lower UG activity (Zeng et al., 2019).
However, there was no significant difference between different
concentrations of P, NP additions, which might be due to the fact that
the UG on topsoil was more sensitive to N addition than P addition
(Jiang et al., 2014). In the study, AP activity was increased with
increasing of N and P concentrations, which is similar to the previous
study (Calvo-Fernández et al., 2018), because reduction of litter C/N
and C/P ratio promoted the production of microbial extracellular
phosphatases (Chen et al., 2018). However, AP activity decreased with
increasing NP concentration, which may be closely related to changes in
microbial habitat conditions caused by high concentrations of N and P
additions in the short term (Jiang et al., 2018). With the extension of
decomposition time, the soil enzyme activities of each treatment showed
a trend of decreasing and then increasing, which was probably related to
the change of climatic conditions in the study area. After the growing
season, the rapidly decreases temperature and water content in the
alpine region reduced the microbial activity, resulting in the decrease
of the type and activity of litter decomposition enzymes.