4.2 Effect of N and P additions on substrate decomposition
N and P addition increased the decomposition of added substrate with
differential magnitude among N and P addition (Figure 2b). This might be
might be due to two main reasons. Firstly, nutrient addition-induced
changes in soil N availability (Table S1) and microbial stoichiometric
ratios were associated with substrate mineralization (Table 2). Studies
have shown that the higher N availability, the higher substrate
(glucose) mineralization was (Craine et al., 2007; Fontaine et al.,
2011; Paterson & Sim 2013). Thus, the increased N availability, with
the highest value under N addition, led to the increased substrate
decomposition in response to N and P addition and the highest glucose
mineralization under N addition (Figure 2b). Secondly, the nutrient
addition-induced changes in microbial composition and structure
regulated substrate decomposition because of preferential substrate
utilization (Blagodatskaya et al., 2007; Fontaine et al., 2011; Di
Lonardo et al., 2017). Fungi have an advantage in breaking down complex
polymers such as lignin and cellulose while bacteria prefer soluble
organic C such as glucose (Wang et al., 2015; Fontaine et al., 2011).
Therefore fungi are thought to be more important than bacteria in
mediating long-term soil C sequestration (Fontaine et al., 2011;
Blagodatskaya et al., 2014). The soil pH (above 6.7) in our study site
(Table S1) and the higher microbial C:N under N addition than P and NP
addition (Figure 1b) indicated bacterial dominance in microbial
community and shifts in microbial composition and structure due to
nutrient additions. Our recent results from the same study site also
showed N and P additions increased bacterial and fungal abundance of
those who decompose complex C and those bacteria, such as
Actinobacteria, who utilize available C to grow fast (Li et al., 2020
submitted). Many other studies also reported nutrient addition-induced
modification in microbial composition and structure (Blagodatskaya et
al., 2007; Leff et al., 2015; Lin et al., 2019; Zeng et al., 2016) and N
addition increased the relative abundance of fast growing bacteria (Leff
et al., 2015; Zeng et al., 2016). Nutrient addition-induced increase in
the abundance of bacteria and fungi would accelerate the decomposition
of both easily degradable C and recalcitrant organic C, as shown by
other studies (Diamond et al., 2019; Riggs & Hobbie, 2016). The
increased substrate mineralization (Figure 2b) and decreased CUE (Figure
4) showed that long-term N and P additions resulted in greater losses of
easily degraded C and recalcitrant organic C by microbial respiration
and led to lower substrate C accumulation. The higher vanillin
mineralization than glucose mineralization (Figure 2b) and higher CEU of
glucose-C than vanillin-C (Figure 4) implied that N and P
addition-induced changes in plant dominance and litter chemical
structure would accelerate decomposition of both plant-derived
substrates and soil-derived C and reduce SOC sequestration in alpine
meadows.