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).