4.1 Effect of N and P additions on SOC decomposition
The SOC decomposition was determined by microbial basic respiration and PEs due to exogenous substrates input to soil, of which the latter is a vital factor regulating SOC decomposition and content. Our data showed nutrient additions affected the magnitude and direction of PEs (Figure 2a). N and P addition promoted native SOC decomposition with the input of glucose, resulting in positive PE, while depressed native SOC decomposition with the input of vanillin, resulting in negative PE (Figure 2a). This was in accordance with our predictions and other studies that easily degradable substrate produce positive PE and complex C produce negative PE (Aye et al., 2018; Di Lonardo et al., 2017; Nottingham et al., 2009; Wang et al., 2015). The positive PE was mainly due to easily degradable C substrate was easier to be utilized by microbes than recalcitrant C substrate (Di Lonardo et al., 2017; Fontaine et al., 2011; Wang et al., 2015). The added substrates provided energy and promoted microbial growth, as shown in the higher MBC with glucose input than with vanillin input under corresponding N and P additions (Figure S1). In response to nutrient limitation under N and P addition, microbial community shifted their composition and structure and mineralized more native SOC to meet their biomass stoichiometric ratio requirements (Fontaine et al., 2011; Lin et al., 2019; Riggs & Hobbie, 2016; Zhu et al., 2018), which resulted in higher PEs of glucose than of vanillin and positive PEs with glucose amendment. The negative PE might be resulted from “preferential substrate utilization”, that microorganisms preferred utilize amended substrate vanillin rather than recalcitrant native SOC (Kuzyakov & Bol, 2006; Werth & Kuzyakov, 2010). The differential PEs among N and P addition might be resulted from nutrient additions-induced changes in N and P availability and soil stoichiometric characteristics which modified the N or P limitation status for microorganisms. The higher soil N:P under N addition but lower N:P under P and NP addition, as well as the lower microbial N:P compared with Control indicated that microbes were more of P limited under N addition but more of N limited under P and NP addition (Figure 1) (Chen et al., 2016; Craine et al., 2007; Zhao et al., 2017). This resulted in higher PE under N and P additions than Control, and the highest PE under P addition.
In our study, the differences in PEs among N and P additions were likely responsible for the differential SOC decomposition. In general, the higher PEs, the higher SOC decomposition was, as indicated by SOC decomposition pattern: P> NP >N (Figure 3a). However, SOC decomposition with vanillin under N addition did not follow this pattern, with N < NP (Figure 3a). This might be due to N addition-induced decrease in MBC and pH (Table S1) and modifications in microbial community structure (Lin et al., 2019; Leff et al., 2015; Riggs & Hobbie, 2016), which constrained microbial respiration (Li et al., 2018b; Riggs & Hobbie, 2016) and thus led to lower SOC decomposition.
The significant relationships between PE with glucose amendment and ecological stoichiometric ratios of soil and microorganisms suggested that PE was mainly generated by “nutrient mining”. However, the PE with vanillin amendment might be generated by the balanced two main mechanisms “co-metabolism” and “nutrient mining” (Fontaine et al., 2004; Chen et al., 2014; Razanamalala et al., 2018). Our results suggested the increased productivity and modified plant composition predominance due to N and P additions affected the magnitude and direction of PEs, and thus SOC decomposition in alpine meadows. The underlying mechanisms on PEs: “co-metabolism” theory and “nutrient mining” theory may occur simultaneously, depending on soil nutrient availability and stoichiometric properties, and the composition of C substrate (Craine et al., 2007; Wang et al., 2015; Razanamalala et al., 2018).