4.2. The dynamics of litter nutrient concentration during litter decomposition
Lignin, a recalcitrant component of litter, is difficult to be decomposed by various types of hydrolytic enzymes in soil due to its complex aromatic structure, and its degradation is an important process in the decomposition of plant litter. As a whole, in each treatment the lignin remaining rate was decreased faster in the early and middle stages than in late stage, which indicated that both N and P could significantly affect lignin degradation in the early and middle litter decomposition process (Fig.2), which was consistent with previous reports (Song et al., 2017; He et al., 2019). This may be because the main influencing factor of early lignin degradation is the initial nutrient content of litter, and the input of N and P increased the integrated level of initial effective nutrients and reduced the critical point of decomposition of lignin. (Song et al., 2017; He et al., 2019). The study showed that in the late stage P addition could accelerate the lignin decomposition, while the high concentration of N and NP addition would accumulated the lignin, which indicated that high concentration of N input inhibits lignin degradation and the alleviating effect of P input on the limitation of energy supply for soil microbial activity diminishes with increasing concentration of N input, and the high concentration of NP treatment reduces the synergistic effect of N and P for litter decomposition (Jiang et al., 2019). High concentrations of N input altered microbial habitat and community composition, inhibited lignin-related decomposition enzyme activities, and reduced microbial decomposition capacity (Rinkes et al., 2016; Gao et al., 2017). Plant cellulose is an important carbon source for soil microbial activity and significantly influenced by soil enzyme activity as a relatively abundant biopolymer in the ecosystem (Thomson et al., 2013). During the decomposition process, the cellulose remaining rate of litter showed a decreasing trend and the decrease of cellulose under NP treatment was higher than N and P treatment alone. This was due to the fact that N and P inputs reduced C/N and C/P ratios, increased microbial activity and cellulase activity, resulting in a decrease of cellulose content (Jiang et al., 2014; Talbot et al., 2012). The correlation and relative influence analysis showed that the cellulose content had a significant positive correlation with the litter decomposition rate, and its contribution to litter decomposition was the largest (Fig.5-7). This may be because cellulose is the most abundant polysaccharide compound in litter, and the exogenous nutrients addition change the community structure of decomposers, enabling them to decompose more cellulose to obtain energy materials (Štursová et al., 2012; Talbot and Treseder, 2012). At the same time, cellulose mainly require decomposition by cellulase and invertase (Xiao et al., 2018), thus cellulase and invertase activities were also significantly positively correlated with litter decomposition rate.
Litter nutrients are key factors affecting the composition of bacterial communities during decomposition, and C, N, and P stoichiometry significantly affect the structure and function of microorganisms, thus altering the fixation and mineralization processes of litter N and P (Barantal et al., 2014; Xu et al., 2020). The results showed that N, P and NP addition significantly promoted the degradation of carbon (C) in litter (Fig.3), because the nutrient input compensated for the environmental nutrient deficiency of microbial growth in the study area and accelerate the consuming of carbon sources by microorganisms (Ren et al., 2018). In the late stage of decomposition, the NP treatment had a significantly lower C remaining rate than the N and P additions alone, and had the highest contribution to the litter C release, which indicated that N and P additions had interactive effects on litter C release (Wang et al., 2017).However, the synergistic effect of high N and P addition might lead to microbial carbon utilization above the critical level, resulting in an increase of litter C remaining rate in the late stage of decomposition. In the study, N and P addition were able to accelerate the litter P and N release , while causing the accumulation of N and P, respectively, and the changes were aggravated with the increase of concentration. This is because, on the one hand, N and P addition reduced litter C/N and C/P, increased N and P hydrolase activity, which accelerated the decomposition of litter N and P (Jian et al., 2016); on the other hand, long-term N and P addition alone increased nutrient limitation in the ecosystem and produced nutrient enrichment with the increase of N and P input (Jie et al., 2014; Chen et al., 2012), while NP treatment could alleviate this nutrient limitation to some extent. During the decomposition process, the gradual decrease of litter N/P reflected that the study area was mainly N-limited and the litter N release rate of Salix cupularis after removing N, P limitation was higher than P.