Carbon sequestration in the pastoral area of Chepareria, western Kenya : a comparison between open-grazing, fenced pastures and maize cultivations

Sammanfattning: Carbon sequestration through restoration of degraded pastoral soils is an advocated way of mitigating global warming, and simultaneously alleviating poverty. An often proposed rehabilitation strategy is fencing of pastures, a method that was introduced to the farmers of Chepareria by the Vi-Agroforestry organization in 1987. The landscape of Chepareria changed from eroded, over-grazed grasslands, to a mixture of open-grazed commons, pastoral enclosures and cultivations. The aim of this study was to investigate (1) if the soil organic carbon (SOC) is higher inside the enclosures than on the open-grazed commons, (2) if SOC is affected by duration of fencing and (3) what effect cultivation of pastures has on the SOC. Estimations of vegetation cover and deep profile (100cm) soil sampling was performed on six clusters containing; (1) open-grazing (OG) (2) 1-5 years of fencing (FENCED(1)), (3) 7-10 years of fencing (FENCED(2)), (4) 15-23 years of fencing (FENCED(3)), (5) maize from OG (A(OG)), (6) maize from fenced pasture (A(FENCED)). Spectrometric analysis of SOC was performed and the results were statistically tested with correlations and ANOVA’s. The average mass of SOC in the 100cm soil profile was 77,76 ± 22,73 t/ha, or 0,68 ± 0,13%, ranging from 61,12 ±15,44 t/ha (0,55 ± 0,08%) on OG to 87,21 ±29,77 t/ha (0,78 ± 0,16%) on FENCED(2). A significant difference in SOC(%) could be distinguished on 0-20 cm were FENCED(1) and FENCED(2) exceeded the OG, and on 20-40 cm where FENCED(2) > OG. No significant difference was found when comparing SOC(t/ha). This was explained by high variation of SOC and BD, deriving from diverse management and environment. The SOC(%) in FENCED(3) was (insignificantly) lower than in FENCED(1) and FENCED(2). This was proposed as original differences in soil conditions, due to a consistent (insignificant) pattern of SOC and BD throughout the 100cm profile, with OG ≤ FENCED(3) ≤ FENCED(1) ≤ FENCED(2). The ground vegetation cover increased significantly from OG to FENCED(2) and FENCED(3). The maize-cultivations contained similar levels of SOC as the fenced pastures, e.g. more than OG. Fertilization, and farming of soils with initially high SOC contents, was used as explanations to why SOC did not decrease after plowing and harvest. The study area and method were considered as suitable for analysis of carbon sequestration on rehabilitated land, but more information about present and previous management, soil properties and vegetation is requested.