Effects of land use and land cover types on soil organic carbon stocks and soil loss due to gully erosion in Olesharo catchment, Narok County, Kenya

Abstract

Land use and land cover changes have posed serious effects on the ecosystem particularly on soil quality and sustainability. This study sought to investigate effects of land use and land cover changes on soil organic carbon (SOC) within the southern ASALs of Kenya. The specific objectives of this study were to evaluate the effects of different land use/ cover types on soil organic carbon stocks (SOCst) and total nitrogen stocks (TNst) in the Olesharo Catchment, Narok County; to assess impact of different LUTs on carbon fractions and carbon management index; and to determine the total carbon and sediment lost from the gully. Using LandSat imageries, four land use types were identified: shrubland (SH), agricultural land (AG), grasslands (GR) and barelands (BL). Disturbed and undisturbed soil samples were taken from 30 x 30 m plots randomly distributed for each of the LUT at 0-15 cm and 15-30 cm depths for the analysis of SOC/ and TN stocks. Similar procedure was taken for the carbon fractions to analyse for total organic carbon (TOC), particulate organic carbon (POC), and mineral organic carbon (MOC). Further, the carbon management index (CMI) for each land use type was calculated using shrublands as the reference land use. For total SOC lost from the gully channel, profile pits were dug adjacent to each channel and undisturbed samples collected from each identified horizon. Dimensions of the channels of the gullies were used to calculate the total sediment lost. The study showed that the means of SOC in land use types were significantly different (P < 0.05). Shrublands registered the highest mean total of SOCst of 31.26 Mg C ha-1 while bareland was the least with 12.85 Mg C ha-1 which were significantly different from each other. Grasslands unexpectedly had significantly lower SOCst compared to AG and this could be attributed to overgrazing in the catchment. Similarly TN stock registered the same results as SOC stock which was validated by corresponding C/N ratios. For carbon fractions, POC was the most sensitive pool, indicating that it can be used as an early indicator for soil degradation. Shrubland had significantly higher (p < 0.05) POC at 7.79 g kg-1 and lower in GR and BL at P < 0.05. The CMI showed that level of degradation in the GR was as severe as that of BL. Therefore efforts aimed at improving SOM within each land use types will improve the soil quality and otherwise reverse degradation within the catchment. The total amount of sediment lost from the gully was high at 313748.71 tons of soil obtained from the gully dimensions. The study further showed that in the sediment area, the SOC recorded was lower in the upper horizons and higher in the lower horizons. This may be due to burial of SOC rich top soils by low carbon sub-layers which purports that erosion is a carbon sink as opposed to a source. Overall the study concluded there is need to improve grazing management strategies in the catchment and it also highlighted the possible climate change mitigation strategies by sustainable management of different land use types.