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

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.