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.