1. Introduction
Climate system and the global carbon cycle have formed a positive feedback loop to reinforce each other(Friedlingstein, Dufresne, Cox, & Rayner, 2003). Carbon as an important influence factor of the greenhouse effect has become a research hotspot in the scientific community. About 1,500 Gt of carbon is stored in the soil in the form of organic matter, which is 2 to 3 times as much as the carbon pool of terrestrial vegetation or global atmospheric carbon pool. Subtle changes in soil organic carbon pool may cause large fluctuations in atmospheric CO2 concentration, which in turn will affect the greenhouse effect and global climate change (Davidson, Trumbore, & Amundson, 2000). Therefore, study of the stabilization mechanism of organic carbon and its controlling factors is essential for a correct understanding of the biosphere carbon cycle, the importance of soil organic carbon to ecosystem processes and the feedback to climate change(Jobbagy & Jackson, 2000).
Changes of land use and vegetation cover pattern have key influence on the carbon cycle of terrestrial ecosystems (Willcock et al., 2016), All of them affect greenhouse gas production and emission by altering the soil microenvironmen, soil physicochemical processes and microbial activities(Ball, McTaggart, & Watson, 2002) (Flechard et al., 2007). Due to the character of fast growing,drought resistance, thinness resistance and hardwood propertie, Black locust has become an important tree for afforestation and plays an important role in the carbon cycle of ecosystem on the hilly losee platea of china. The stabilization mechanism of soil organic carbon in Robinia pseudoacaciaplantation on the Loess Plateau is of great significance for further understanding of regional climate changes caused by pattern changes in the organic carbon under conversion of cropland to forest.
The stability of organic carbon in the soil depends on soil properties, environmental factors and influence of human activities. Most of the current understanding of the process of soil carbon storage and release comes from the study of the main surface soil (Richter & Mobley, 2009; Zabowski, Whitney, Gurung, & Hatten, 2011). However, deep soil serves as a huge organic carbon pool in terrestrial ecosystems, its carbon storage is three times as much as that of surface soil (Fontaine, Barot et al. 2007). A little change in the organic carbon (SOC) cycle of deep soils may cause the release of large amounts of CO2 and have a significant impact on the global carbon cycle (X. Wang et al., 2014).
Although the vertical distribution of soil organic carbon decreasing with the increase of soil profile depth has already been recognized, people pay less attention to the dynamics of deep soil carbon due to the low content of deep soil organic carbon (Davidson & Ackerman, 1993; Post & Kwon, 2000). Most studies focus on spatial and temporal variation in organic carbon mineralization and soil CO2flux at soil surface layer (ArchMiller, Samuelson, & Li, 2016; Mande, Abdullah, Aris, & Ainuddin, 2015; Pang, Bao, Zhu, & Cheng, 2013), reports on CO2 flux in deep soil layers are very rare (Jassal R, 2005; Liang N S, 2003; Wiaux F, 2015). In addition, since the CO2 flux at the soil-atmosphere interface is a superposition of CO2 emissions caused by organic carbon mineralization from different depths of soil layers. For lack of comprehensive understanding of contribution of soil resources at different depths to SOC mineralization, it will produce larger errors to predict the overall soil carbon pool variation characteristics based on soil surface layers (Takahashi, Hiyama, Takahashi, & Fukushima, 2004). Therefore, distinguishing the characteristics of SOC mineralization variation at different depths, especially in deep layers, will help to improve the accuracy of the soil carbon emission prediction model and further understand the carbon emission mechanism (Subke, Reichstein, & Tenhunen, 2003; Takahashi et al., 2004).
In summary, understanding the stability of soil organic carbon is the key to the study of soil carbon sequestration in plantation forests. How does the process of soil organic carbon mineralization change with soil depth in the Robinia pseudoacacia plantation ecosystem on the Loess Plateau? Which characteristics of carbon fluxes changes in different sections of deep layers and which factors affecte them? All of these issues need to be further studied. In this study, we monitored the CO2 concentration and soil physico-chemical environment at different soil profiles in the artificial Robinia pseudoacaciaof different stand ages and crop land on the hilly losee platea. The aims of this research were: (1) to determine distribution of soil CO2 flux at different profiles based on Fick’s first law; (2) to understand the response of soil CO2 flux to soil temperature ,moisture and soil organic carbon(SOC) at different profiles; (3) to define the contribution of CO2 flux in deep layers to soil-atmosphere interface; (4) to explore multi-factor coordination and construct prediction model. The results of this study contribute to clarify the production process of soil CO2in deep layers and the response mechanism of CO2 flux at different profils for scientifically assessing soil carbon emission effects of vegetation restoration, and to provide a theoretical basis for further clarifying the stability of soil carbon and the dynamic change of soil carbon pool in Robinia pseudoacacia on the hilly losee platea.