The structure and development of Loess Critical Zone and its soil carbon cycle

The Loess Critical Zone (LCZ) is located in the intersection of bedrock, groundwater, pedosphere, atmosphere and biosphere. It is a key issue to understand the structural characteristics and soil carbon (C) cycle of the LCZ. We summarized the mechanisms of C exchange between rocks and the atmosphere, and discussed the mechanisms of C stabilization and persistence of the LCZ. Due to the deep layer, C stocks of the CLZ could be underestimated. In light of the recent theory of microbial C pump, soil microorganisms play an important role in C cycle, however, the microbial function is not widely considered in C cycling model of the LCZ. For future studies, it is suggested to systematically study the C cycling process from plant canopy to bedrock by the framework system of the LCZ. A variety of techniques and methods are integrated to combine short-term and high-frequency observations with long-term positioning observations, and pay attention to the response and feedback mechanisms of soil organic C (SOC) cycling to global changes and human activities, especially the migration and transformation of SOC in each circle and interface of the LCZ. We also recommend the necessity for intensive and long-term C monitoring in LCZ over broad geographic scale, to improve microbial C model for accurately evaluating terrestrial C budget and its dynamics. Altogether, this is the first review of C cycling, spanning from the land surface down to the bedrock in the LCZ, which is significant implications for biogeochemical cycling of C in surface and deep layers down to the bedrock.