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.