Re-evaluation of organic carbon pool from land surface down to bedrock on China's Loess Plateau

Large reservoirs of organic carbon (OC) store in deep soils (>1 m below land surface) are not usually included in regional and global terrestrial C inventories. Chinaös Loess Plateau (CLP), which has the worldös deepest loess deposit and has experienced long-term, intensive agricultural and revegetation activities, could contain large stores of OC. In this study, the distribution of OC concentration and stock across the entire loess profile from the ground surface down to the bedrock (56–205 m) was assessed at five sites (Yangling, YL; Changwu, CW; Fuxian, FX; Anösai, AS; and Shenmu, SM) under three land use types (farmland, grassland and shrubland). There was pronounced decrease in mean OC concentration with increasing depth along loess profiles at all the investigated sites. OC concentration in the topmost 20 m of the loess was much higher and fluctuated more significantly than that in the deeper layers at YL, CW, FX and AS, where mean annual precipitation was >550 mm. In contrast, OC concentration was low and stable at SM with mean annual precipitation <450 mm. The restoration of vegetation with deep roots following the abandonment of farmlands resulted in deep OC accumulation at YL and AS, whereas natural grassland recovery did not result in the accumulation of OC at SM. Our results suggested that land use change could alter OC distribution in deep soils, of which degree depended on climatic condition and vegetation type. The estimated OC storage varied significantly across the sites (859–5044 Mg ha^–1), which mainly depended on the thickness of loess deposit. OC store on the CLP and other areas around the world with deep soils or sediments could be underestimated and need re-consideration in future C budget studies. This is the first estimate of regional OC stock across loess profile, spanning from the land surface down to the bedrock. The findings could have significant implications for biogeochemical cycling of C in surface and deep soil layers down to the bedrock.