**1. Introduction**

Arid and semiarid regions cover >30% of the earth's land surface in the world [1]. The soil carbon (C) in arid and semiarid forests plays an important role in global carbon storage and alleviates the increase of atmospheric carbon dioxide (CO2) concentration, and its contribution to the global carbon cycle is increasingly significant [2]. Afforestation has occurred globally within the framework of the Kyoto Protocol [3] and has the potential to mitigate the rising atmospheric CO2 concentration caused by anthropogenic emissions [4]. Recent studies also suggested that arid and semiarid ecosystems have strong soil C sequestration potential [5].

Therefore, soil C change in arid and semiarid forest ecosystem is a key process for understanding the global C cycle, assessing the responses of terrestrial ecosystems to climate change and to aid policy makers in making land use/management decisions [6]. In the past, the study of soil carbon cycle was mainly focused on soil organic carbon (SOC), and the behavior of soil inorganic carbon (SIC) was rarely considered. Global "Missing Carbon Sink" reaches 2–3 × 1015 g C, and carbonate and carbon fixation in the arid and semiarid regions account for about 1/3 of global "carbon sinking" [7]. The SIC storage in arid and semiarid regions was huge, and it needs more attention in the soil carbon cycle. In this chapter, we will review the cutting-edge work in forest soil carbon biogeochemistry undertaken in the last three decades. We also attempt to synthesize recent advances in soil carbon biogeochemistry in arid and semiarid regions and discuss future research needs and directions.
