Soil organic matter (SOM) is the largest reservoir of carbon (C) storage in terrestrial ecosystems and its mineralization produces a major CO2 flux into the atmosphere (Lal, 2008). The C storage is largely regulated by the plant-derived C incorporation into SOM and the release of C from microbial mineralization of SOM, and even small changes in this balance might largely affect the atmospheric CO2 concentration and global C cycle (Davidson and Janssens, 2006; Lal, 2008). The priming effect (PE), which is defined as the alteration of fresh organic substrate inputs on the decomposition of native SOM, can significantly influence SOM decomposition and nutrient (especially nitrogen, N) cycling (Kuzyakov, 2010). Although it is generally recognized that the PE has a large effect on the global C cycle and climate change (Perveen et al., 2014; Vestergård et al., 2016; Guenet et al., 2018), there remain large uncertainties and controversies associated with both the direction and the magnitude of PE (Cardinael et al., 2015; Zhang et al., 2017). A major cause of these uncertainties is the lack of understanding on the PE of the relatively stabilized SOM (Guenet et al., 2012; Cheng et al., 2014).
SOM can be divided into three C pools with different stabilities and turnover times: a labile pool (turnover time of several years), an intermediate pool (turnover time of several decades) and a stable C pool (turnover time of several centuries or more) (Davidson and Janssens, 2006; Barré et al., 2010). The intermediate and stable C pools can be collectively described as a relatively stabilized C pool with a turnover time from decades to centuries or more (Anderson and Paul, 1984; Jenkinson and Rayner, 1977; Trumbore et al., 1989). This relatively stabilized C pool is the major component of SOM stocks (Davidson and Janssens, 2006) and its vulnerability to the PE can significantly impact the global C cycle (Derrien et al., 2014). Earlier, it was suggested that even the most stable SOM could be rapidly mineralized in the presence of fresh C, such as SOM in deep soil layers (Fontaine et al., 2007) or charcoal (Kuzyakov et al., 2009; Nocentini et al., 2010). Recently, it was shown that the stable SOM is as vulnerable to the PE as is the more labile SOM (Guenet et al., 2012). Later, it was further demonstrated that the PE of relatively stable SOM was more sensitive to elevated atmospheric CO2 (Vestergård et al., 2016), suggesting that the PE has a huge potential impact on the relatively stabilized SOM. Assessing the role of PE on the dynamics of the relatively stabilized SOM under climate warming is yet limited, because to date no study has compared the temperature effect on PE of the relatively stabilized SOM pool with that of the total SOM (i.e. a mixture of the relatively stabilized and labile SOM pools). Moreover, the absence of an adequate understanding on the PE of the relatively stabilized SOM leads to a debate on the temperature sensitivity of PE (Ghee et al., 2013; Frøseth and Bleken, 2015; Lyu et al., 2019).
It is well known that PE correlates with microbial biomass (Thiessen et al., 2013), substrate availability (Chen et al., 2013; Hopkins et al., 2014) and nutrient status (Chen et al., 2013; Feng et al., 2021). Some studies suggested that the response of PE to elevated temperature was short-lived or weak because no changes in substrate use efficiency or active microbial community (Thiessen et al., 2013) were observed under elevated temperture (Ghee et al., 2013; Hopkins et al., 2014). However, this may be different in soils dominated by the relatively stabilized SOM. The stabilized SOM has been widely accepted to be more vulnerable to temperature increase due to its higher activation energy than the labile fraction (Conant et al., 2008; Hopkins et al., 2012; Lin et al., 2015). However, some studies suggested that the temperature sensitivity of the stabilized SOM decomposition may be suppressed because of the accelerated depeltion of available C at elevated temperature (Kirschbaum, 2004; Erhagen et al., 2015). If exogenous labile C input can compensate this available C depletion due to temperture increase, the activated microbes may accelerate native SOM decomposition through co-metabolism in these soils (Fontaine et al., 2007), which was supported by Zhu and Cheng (2011). Meanwhile, the presence of labile C may also compensate energy for microorgnisms that specialize in decomposing N-rich SOM to acquire N (Craine et al., 2007; Fontaine et al., 2011). Liu et al. (2019) demonstrated that temperature increase significantly increased the N-acquiring enzymes in labile substrate amended soil, which emphasized the role of microbial community in mediating the response of PE to temperature increase. Therefore, considering the changes in microbial growth and activity and nutrient status may provide mechanistic insights into the temperature sensitivity of PE on the relatively stabilized SOM.
Separating stabilized SOM from bulk soil is method-limited, but long-term bare fallow treatment provides such an opportunity (Guenet et al., 2012). In the soils without fresh C inputs, the most labile C are prograssively depleted (Barré et al., 2010; Nunan et al., 2015). Comparing paired bare fallow and old field soils would provide insight into the relative vulnerality of the relatively stabilized vs. the total SOM (i.e. a mixture of the relatively stabilized and labile SOM pools) to perturbations such as substrate input and/or temperature increase (Guenet et al., 2012; Lefèvre et al., 2014; Zhang et al., 2017).
In this study, we aimed to test how elevated temperature affects the magnitude of the PE of the relatively stabilized SOM. To differentiate the relatively stabilized SOM from the labile fraction, we used soils from a bare fallow plot which has been deprived of fresh C inputs for 23 years. We compared the dynamics of the PE under low and higher temperatures in this soil where all SOM was older than 23 years to that in an adjacent old field soil where both the relatively stabilized and labile SOM exist under the same pedo-climatic conditions. We hypothesize that (ⅰ) plant inputs may produce positive PE of SOM decomposition due to the activation of the dormant microorganisms by labile substrate input; (ⅱ) PE of the relatively stabilized SOM (i.e. bare fallow soil) would be more sensitive to temperature than that of the bulk SOM pool (i.e. old field soil) due to the higher resistence of the relatively stabilized SOM for decomposition under conditions of low substrate availability or temperature. To test these hypotheses, we incubated bare fallow and old field soils at 10 and 20 ℃ for 815 days, and applied the natural 13C tracer method to distinguish the CO2 source of native soil and added substrate. Moreover, we measured net N mineralization, dissolved organic C and microbial biomass C to investigate the mechanisms that controlling the response of priming process to temperature.