Forest is the largest ecosystem which has tremendous amount of carbon sequestrated by plants (Franklin et al., 2009; Poorter et al., 2016; Khan et al., 2018). The soil carbon pool, which is the second largest carbon pool after aboveground forest carbon in the world, cannot be ignored (Sedjo, 1993; Lal, 2005). The carbon exchange between the soil and the atmosphere largely affects the global carbon cycle and climate change all the time (Dib et al., 2014; Tian et al., 2016; Gabriel et al., 2018). The soil respiration is the gas exchange between soil and atmosphere (Wei et al., 2010; Goldberg et al., 2017) and at the same time, soil organic carbon is also stored in forest soil in various forms and plays different roles (Tian et al., 2016; Jílková, 2020). Thus, understanding the characteristics of forest soil respiration and carbon sequestration is important for managing forest ecosystem. Through the proper management of forests, processes of greenhouse gas emission and soil organic carbon sequestration may be controlled to a certain extent.
The stand density, which is usually regulated by thinning, is considered as an indispensable influence on the forest production. The principle of thinning maybe forming the local climax plant community as the basis of forest management research (Jens et al., 2000; Ming et al., 2018), which may be also based on the integrity principle of the forest ecosystem. Therefore, the effect of stand density is not only limited to the change of canopy structure and vertical forest structure (Jack and Long, 1991; Liu et al., 2019), but also to the change of microclimate, interspecific competition, and so on (Shao and Shugart, 1997; Ali et al., 2019; Bello et al., 2019; Eldegard et al., 2019; Liu et al., 2019).
In the previous studies, it was considered that soil respiration (RS) is composed of autotrophic respiration (RA) and heterotrophic respiration (RH) whereby decomposition of microorganisms and the turnover of roots in the soil are the main forms of heterotrophic respiration and autotrophic respiration, respectively (Baggs, 2006; Hopkins et al., 2013; Xu and Shang, 2016). This process provides better understanding on the mechanism of soil respiration and the influence of various factors on soil respiration. These studies, which were carried out on the forest soil within two years of thinning of the Carpinus betulus plantation, have reported significantly higher soil microbial respiration of the thinned plots compared to the control plots, and no significant difference was reported between soil respirations among the three thinning intensities (Akburak and Makineci, 2016). In the study of mature Masson pine forests, Leilei reported that thinning could effectively increase the rate of soil respiration in a short period (Lei et al., 2018). When studying the soil respiration of Shanxi Pinus tabulaeformis young forests that have been thinned, it is reported that moderate thinning positively affected soil respiration by changing soil temperature and humidity (Cheng et al., 2015). Similar results are also reported elsewhere (Zhang et al., 2018). Many researchers found that moderate thinning could make the stand microclimate changed, thereby optimizing the living environment of soil microorganisms, and reserve the sufficient soil substrates and soil organic carbon for respiration. Few researchers have also reported different results, for example, thinning could cause the death of plant roots and reduce the autotrophic respiration of the soil and much sparse stands would reduce the activities of soil microorganisms(Park et al., 2009; Mosca et al., 2017).
Soil organic carbon (SOC) is the C contained in soil organic matter (SOM), which is an important indicator of measuring soil carbon sequestration (Lull et al., 2020), and this substantially affected by land use changes, forest management, natural and human interference, and so on. Forest soil organic Carbon is attributed to strong dynamic changes (Cambardella and Elliott, 1992; Liang et al., 1997). However, due to the complexity of the composition, structure, and existence of soil organic matter, the performance of a certain functional characteristic of the soil is often the result of a chemical mixture's simultaneous action with similar chemical components, structural characteristics, and functional groups. It is not the total amount of soil organic carbon that characterizes the soil carbon pool activity, but the soil labile organic carbon.The degree of activity is not only the total amount of soil organic carbon, but also the soil labile organic carbon. The active component of soil organic carbon is the most active and unstable C in the soil, which has the characteristics of availability, easy oxidation, and solubility. Generally, soil carbon may be grouped based on stability of SOC and measure soil dissolved organic carbon (DOC), such as microbial biomass carbon (MBC), light fraction organic carbon (LFOC), and easily oxidizeable organic carbon (ROC). This is considered as the most quantitative expression of soil labile organic carbon (LOC) (Hu et al., 2010).
It has been reported that the decrease of stand density increases soil temperature, humidity, soil respiration, soil SOC, N, P, K, etc. (Wic Baena et al., 2013; Zhang et al., 2018). However, many studies have found that the changes of soil temperature, humidity, and soil respiration by thinning could become stable after certain period of thinning, and even return to the original level (Fernandez et al., 2012; Olajuyigbe et al., 2012; Bai et al., 2016). These studies also reported that the root system or the plant remaining in the stand after thinning acted as the exogenous carbon, which led to the change of soil rather than the effect of stand density. Most of the existing studies focus on the short-term "stress response" of forest soil carbon pool after thinning (Ryu et al., 2009; Bolat, 2013; Zhao et al., 2019). Although thinning directly changes the stand density, the impact of stand density on the soil should be the steady state achieved by biochemical action under the influence of different stand densities. The effect of stand density on the soil carbon pool is caused by the interaction of litter return, root growth and respiration, microbial activity, and mineral turnover after thinning. Our study intends to explore more on the effect of different stand density on the soil respiration and soil labile organic carbon and their mechanism.
This is not clear whether the soil carbon pool of different age plantations would have different responses to different stand densities. This study considers paying enough attention to the factors of forest development and growth in carrying out more effective human intervention and forest management on the forests of all ages. As the substrate of soil respiration, soil organic carbon cannot not be discussed in isolation, the hypothesis presented in this study was that both stand density and stand age are essential factors influencing soil respiration, soil organic C and soil labile organic C, and a close correlation between soil labile organic C and soil respiration. Therefore, objectives of this study were to: 1) Examine whether RS, RH, RA and SOC, MBC, DOC, LFOC, ROC in mineral soil would be significantly affected by stand density and stand age during the studied period (Five years after thinning); 2)Find out the trend of the variables mentioned above with the stand density, and compare the trends for different stand ages; 3༉Identify the crucial factors shaping the RS, RH and RA in soil organic carbon pools.