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.