Soil hydric properties and carbon stock in a semi-arid region of Iraqi Kurdistan: The importance of historical pedogenesis, climate and locality

Abstract Semi-arid ecosystems are increasingly being recognized as key players in the carbon cycle, with a huge capacity for storing carbon when hydrology improves, but also with the downside of decarbonization during drier periods. It is therefore important to understand the hydrological and carbon relationships of these ecosystems, and to encompass the huge inter-regional variation, while factoring in aspects of the historical pedogenesis that underpin much of the observed soil behaviour. This work reports on a soil survey down to 2 m depth, examining the water availability and infiltration, soil taxonomy and carbon pools (organic (TOC) and inorganic carbon (TIC) separately), in a semi-arid region in Iraqi Kurdistan (20,000 ha in total), providing insight into the geosphere-hydrosphere-atmosphere nexus. Soil type diversity was high, including Alfisols. We identified 5 soil orders in total, mostly with typical soil profile development, soil processes and pedogenesis, but with some peculiarities. Available water capacity varied widely across soil taxons, ranging from levels of 100–280 l m−2 to levels of 600 l m−2, with the narrowest range in Aridisols and the widest range in Inceptisols and Alfisols, while the base infiltration rate, essential in distributing the incident water through the profile, were generally low (less than 2 cm h−1 in Aridisols and in some Inceptisols), except for Mollisols (ca 8 times higher). The highest carbon concentrations were found in Mollisols for TOC and in Alfisols for TIC. However, the Inceptisols held the highest carbon stocks due to the occupied area (60%) (almost 13,000 Gg within the area, of which ca 1800 Gg represented organic carbon and 11,100 Gg represented inorganic carbon). This investigation has highlighted the importance of detailed and regional study of hydrological and carbon characteristics of semi-arid soils as key elements in terms any global understanding, given the significance of local climatic, pedogenic and historical factors, the implications for local and regional management decision-making (rather than a one-size-fits-all approach) and their involvement in global water and carbon cycles. While increased precipitation followed by drier periods may well lead to decarbonization of the newly enlarged stores, on the other hand, currently dry regions may have significant capacity to store additional carbon. Soil water regime and carbon stock should be included in landscape conservation and sustainability strategies.