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).