Terrestrial carbon stocks following 15 years of integrated watershed management intervention in semi-arid Ethiopia

•Ecosystem carbon status signified the success of forest restoration in exclosures.•Cropland SOC requires urgent rehabilitation.•Subsoils comprised as much as 40% of total SOC stock.•SOC stock is negatively related to rock fragment content.•Land-use system and soil order indicate priority areas for rehabilitation. Our study investigated the total terrestrial stock of organic carbon and its controlling factors in prevalent land‐use systems in semi-arid Ethiopia (610 mm of annual rainfall), as part of the impact assessment of the national Integrated Watershed Management (IWM) program. Above- and below-ground biomass and soil organic carbon (SOC) stocks of major land-use systems (i.e., exclosure, cropland, rangeland, and bare land) were quantified after field sampling along a topographic gradient. We found that aboveground carbon stocks peaked in the 15-year-old exclosures (9.08 ± 1.44 Mg ha−1) owing to intact woody and grass vegetation as well as substantial litter cover (>20% of the total biomass). Croplands cultivated with wheat and rangelands vegetated with perennial grasses showed average aboveground carbon stocks of 3.16 ± 0.24 and 1.45 ± 0.19 Mg ha−1, respectively. The belowground biomass carbon stock was particularly low in croplands (0.76 ± 0.09 Mg ha−1), exceeded by that in both exclosures and rangelands, where values averaged 3.67 ± 0.06 and 3.04 ± 0.42 Mg ha−1, respectively. The topsoil (0–30 cm) SOC stocks also varied with land-use systems but showed a different order, peaking in rangelands (53.9 ± 10.1 Mg ha−1) and exclosures (41.4 ± 8.1 Mg ha−1), followed by bare lands (29.0 ± 11.5 Mg ha−1) and croplands (26.4 ± 4.6 Mg ha−1). The sub-soils (30–100 cm) added 40% to this SOC storage. The greatest total SOC stock identified in exclosures that had been established primarily on degraded hillslopes may signify a successful restoration effort under the IWM program. However, croplands exhibited the lowest SOC stock, which implies the need for urgent interventions to improve the soil fertility.