Influence of Grazing Management Practices and Topographic Positions on Vegetation Attributes, Soil Organic Carbon and Greenhouse Gas Emissions in Semi-arid Rangelands of Laikipia County, Kenya

Abstract

Semi-arid rangelands of Kenya have been managed through grazing for many years. This has influenced the landscape in terms of vegetation attributes such as species diversity, richness, composition, and abundance. Furthermore, a few studies have been done on how different land cover types under different grazing management practices as well as topographical positions influence soil organic carbon fraction content in the soil. Also, the aspect of greenhouse gas emissions under different land cover types and grazing management influence is poorly understood for semi-arid rangelands. The effect of grazing management practices on vegetation attributes, soil organic carbon fractions, and greenhouse gases was assessed for semi-arid rangelands of Laikipia County, Kenya. An experiment with a complete randomized design was set up to determine; (1) the effect of grazing practices and topographic positions on vegetation attributes, (2) effect of grazing practices, topographic positions, and land cover on soil mineral-associated carbon (MAOC) and particulate organic carbon (POC) and (3) effect of grazing management practices and land cover types on CO2, CH4 and N2O emissions. The treatments were grazing management practices, continuous grazing and controlled grazing; and the topographic positions; mid-slope, foot slope, and bottomland. The land cover types, bare ground, patches of grasses, and mosaics of trees, were selected randomly and replicated three times. Both grazing practices and topographical positions had a significant (p<0.05) effect on the relative abundance of trees as well as grasses while it was highly significant (p<0.001) on herbs and forbs. Effect of grazing practices and topographical positions on species diversity was significant (p<0.01) on herbs cover. Tree density responded to grazing practices significantly (p<0.05). Under controlled grazed zones, bottomland recorded high species composition. Grazing practices had a significant effect (p<0.001) on POC. Controlled grazing (POC= 0.887%, CC SD=0.49) zones was significantly different compared to zones under continuous grazing (POC = 0.718% CC SD=0.3). Mosaic of trees (POC =1.15% CC, SD = xi 0.22) recorded the highest concentration of carbon followed by patches of grass (POC = 0.87% CC, SD= 0.37) whereas bare ground (POC = 0.38% CC SD = 0.12) had the least. Topographical position had an increasing trend of carbon concentration down the gradient that is mid-slope (POC = 0.59% CC, SD =0.4)< foot slope (POC = 0.84% CC, SD =0.34) < bottomland (POC = 0.98% CC, SD = 0.41). Grazing practices significantly (p<0.001) influenced CO2 and N2O fluxes though it was insignificant (p>0.05) on CH4 flux. Controlled grazing produced 74.16% CO2-C and 82.47% N2O-N fluxes compared to continuous grazing which was observed to be 21.5 mg.m-2.h-1 CO2- C and 3.4μg.m2h1 N2O- N flux. Under different land covers, there was a significant (p<0.05) effect on CO2 and N2O but insignificant in CH4. Bare ground had 27.86mg.m-2.h-1 CO2- C flux, 0.007 mg.m-2.h-1 CH4-C flux and 3.71μg.m2h1 N2O- N fluxes.Tree had 66.57 mg.m-2.h-1 CO2- C emission -0.005 mg.m-2.h-1 CH4-C fluxes and 11.43 μg.m2h1 N2O- N fluxes. Grass exhibited 61.88 mg.m-2.h-1 CO2- C fluxes, -0.034 mg.m-2.h-1 CH4-C fluxes and 18.81 μg.m2h1 N2O- N fluxes. Continuous grazing accelerated species richness of unpalatable species and increased bare ground, unlike controlled grazed zones. Controlled grazing management enhanced species relative abundance and led to less loss of POC but emitted more N2O-N and CO2-C fluxes, unlike continuous grazing. Controlled grazing should be used to increase species relative abundance and composition and reduce further loss of POC fraction. Destocking should be carried out under continuous grazing management to curb further loss of vegetation cover and enable vegetation to recover from intensive grazing. Grazing activity should be only in the bottomlands to avoid the further decrease of POC in mid-slope and foot slope which recorded less POC content. Keywords: grazing management, land cover types, greenhouse gas emissions, topographical positions, particulate organic carbon,