1. INTRODUCTION
Plant inputs as shoot or root litter is the predominant source of carbon
to soil organic carbon (SOC). However, this input of plant-derived C
substrate can strongly but differentially affect the process of SOC
sequestration and loss due to their biochemical quality (Jackson et al.,
2017; Sokol et al., 2019; Zhu et al., 2018). For example, the inputs of
the exogenous organic C substrates to soil can increase or decrease the
decomposition of soil organic matter (SOM) and such effects are known as
priming effects (PEs) (Kuzyakov, 2010). However, the composition of
plant-derived fresh organic matter (FOM) varies from simple and easily
degraded compounds such as amino acids, sugars, and peptides, to
recalcitrant complex compounds such as cellulose, hemicellulose, lignin,
and proteins (Baumann et al., 2009; Sokol et al., 2019). In general, the
easily degraded compounds like glucose, greatly enhance the
decomposition of native SOC compared with the complex compounds like
those plant litter containing relatively high lignin or high C:N
(Blagodatskaya et al., 2007; Kuzyakov & Bol, 2006; Kuzyakov et al.,
2000; Qiao et al., 2014). These findings indicated that the simpler
structure of C substrate, the fast energy turnover; the more complex
structure of C substrate, the lower PE is produced (Aye et al., 2018; Di
Lonardo et al., 2017; Nottingham et al., 2009).
The PE could be generated by two different mechanisms (Fontaine et al.,
2003; Kuzyakov et al., 2000): (1) “Co-metabolism” or “stoichiometric
decomposition” theory (Blagodatskaya et al., 2014; Kuzyakov et al.,
2000; Razanamalala et al., 2018) and (2) “nutrient mining” theory
(Craine et al., 2007; Razanamalala et al., 2018; Wang et al., 2015).
“Co-metabolism” resulting from the release of extracellular enzymes by
FOM decomposers helps to break down young SOM that has a structure
similar to FOM, such as decaying vegetal tissues with a high C:N:P
ratio. “Nutrient mining” targets an old SOM that is already
transformed and rich in nutrients, with a long residence time. However,
both mechanisms might coexist in the same soil (Chen et al., 2014;
Fontaine et al., 2004; Razanamalala et al., 2018) and the balance
between the two mechanisms was driven by the nutrient content of the
soil solution (Chen et al., 2014) or SOM pool (Razanamalala et al.,
2018).
Nutrient supply, especially N and P availability, strongly affect C
cycling and storage in grasslands ecosystem (Li et al., 2018b; Luo et
al., 2020; Ramirez et al., 2010; Riggs & Hobbie, 2016). N and P
additions increase the above-ground plant biomass (Borer et al., 2017;
Li et al., 2014; Isbell et al., 2013), increase or decrease biomass
allocation to roots (Fornara & Tilman, 2012; Li et al., 2018b). These
modifications in plant biomass would in turn affect C input to soil and
SOC dynamics, however, the effects of nutrient additions on SOC
decomposition and soil C sequestration had no census. For example, N
additions increased SOC sequestration in prairie grasslands (Adair et
al., 2009; Fornara & Tilman, 2012; Fornara et al., 2013) and sandy
grasslands (Reid et al., 2012). Soudzilovskaia et al. (2007) reported NP
addition increased litter production of graminoids with low
decomposibility, thus increased SOC content in alpine tundra. Mack et
al. (2004) showed that NP fertilization decreased SOC pools in arctic
tundra. N and P additions accelerated SOM decomposition and reduced SOC
content in the topsoil of Tibetan alpine meadows (Li et al., 2018a; Luo
et al., 2019; Luo et al., 2020). The litter quality and various biomass
allocations to aboveground plant and roots in different grassland
ecosystems may resulted in varies in soil C sequestration and SOC
storage due to nutrient additions (Soudzilovskaia et al., 2007; Fornara
& Tilman, 2012; Li et al., 2018b). Besides these reasons, the
inconsistent effects might be due to the differential response of
microbial composition and their activity (Leff et al., 2015; Riggs &
Hobbie, 2016; Zhang et al., 2018) and thereby SOM decomposition (Chen et
al., 2014; Leff et al., 2015; Li et al., 2018a; Riggs et al., 2015) to N
and P additions.
N and P additions increase or decrease microbial biomass (Li et al.,
2018a; Liu & Greaver 2010; Ramirez et al., 2012), alter microbial C use
efficiency (CUE) (Riggs & Hobbie, 2016; Luo et al., 2020) and microbial
mineralization and soil C dynamics (Li et al., 2018a; Ramirez et al.,
2012; Riggs et al., 2015). Microbes can adjust CUE to maintain their
biomass stoichiometry according to stoichiometric ratio of substrates
they feed on and soil nutrient content (Craine et al., 2007; Chen et
al., 2014; Chen et al., 2016; Cleveland & Liptzin, 2007; Razanamalala
et al., 2018) and thereby regulate the fate of plant-derived-C and C
sequestration in soils (Geyer et al., 2016; Zhu et al., 2018). Microbial
activity is driven by microbial demand for resources, with an average
optimal C:N:P ratio of 60:7:1 for terrestrial ecosystems at the global
level (Cleveland & Liptzin, 2007) and 48:5:1 for Tibetan alpine meadows
(Chen et al., 2016; Zhao et al., 2017). Nutrient limitation can shift
the microbial community composition from r-strategy microbes to
K-strategy microbes which can decompose stable SOM to get access to
available N or P and stimulate SOM mineralization, leading to a positive
PE (Blagodatskaya & Kuzyakov, 2008; Chen et al., 2014; Zhu et al.,
2018). Therefore, FOM decomposition and SOC mineralization due to the N
and P additions can be explained using “stoichiometric decomposition”
theory. Higher CUE intends an increased potential for C sequestration in
soils while lower CUE implies relatively greater loss of C via microbial
respiration (Riggs & Hobbie, 2016; Sokol et al., 2019).
The Qinghai-Tibetan Plateau is the highest and largest plateau on the
earth. Alpine meadow is one of the dominant and most widely distributed
grasslands type on the plateau, with much higher soil C content in the
surface layer relative to other grasslands (Reid et al., 2012; Wen et
al., 2013; Yang et al., 2008). Thus alpine meadows take an important
role in maintaining soil functioning as C pool and sustaining ecological
safety at regional or even global level (Yang et al., 2008; Wu et al.,
2017). However, alpine meadows are particularly sensitive to climate
change and anthropogenic activities (Fayiah et al., 2020; Liu et al.,
2018). Anthropogenic drivers such as atmospheric deposition and nutrient
addition increase the availability of nitrogen (N) and phosphorus (P) in
grasslands, including alpine meadows (Galloway et al. 2008; Liu et al.,
2013). Previous studies from alpine meadows showed that input of
nitrogen (N) and phosphorus (P) modified litter quality through
increasing above-ground plant productivity and altering plants species’
dominance in community, in particular the grass Elymus nutans (Li
et al., 2014; Li et al., 2018b). These modifications can change the
availability of organic C and the decomposition of SOM, thus reduce SOC
content (Li et al., 2018a; Li et al., 2018b; Luo et al., 2019; Luo et
al., 2020), with the lowest SOC content under P additions, as compared
to N and NP additions (Li et al., 2018a). However, the mechanisms of
this SOC decline induced by nutrient addition are unclear. Furthermore,
little is known about how N and P additions affect soil stoichiometric
characteristics and their effects on C decomposition, including
plant-derived FOM decomposition and recalcitrant SOC decomposition which
plays an important role in SOC sequestration and soil C pool (Li et al.,
2017). FOM decomposition and priming of SOC decomposition caused by
increased litter input due to nutrient addition is a potential mechanism
that can explain the SOC declines and their differences between N and P
addition. In this study, we examined soil and microbial stoichiometric
characteristics after long-term field N and P addition to alpine meadows
and C decomposition by adding 13C labeled substrate
(glucose or vanillin). We hypothesized that: (1) N and P additions
modify soil N or P limitation status and thus differentially affect PEs
and exogenous C substrates decomposition (Blagodatskaya & Kuzyakov,
2008; Chen et al., 2014; Zhu et al., 2018). (2) Glucose induces positive
PEs mainly by “nutrient mining” while vanillin generates negative PEs
mainly by “stoichiometric decomposition” due to the differences in
substrates’ decomposability (Aye et al., 2018; Di Lonardo et al., 2017;
Nottingham et al., 2009; Wang et al., 2015) and in ecological
stoichiometric properties of soil and microorganisms resulted from N and
P additions (Blagodatskaya et al., 2014; Craine et al., 2007; Kuzyakov
et al., 2000; Razanamalala et al., 2018). (3) N and P additions-induced
higher SOC decomposition and lower microbial CUE lead to lower C
accumulation and lower SOC content, with the highest C decomposition in
P addition compared to N and NP additions due to P addition-induced
severe N limitation, i.e., lower soil and microbial N:P (Blagodatskaya
et al., 2014; Craine et al., 2007; Kuzyakov et al., 2000; Razanamalala
et al., 2018).