Influence of grazing management practices on vegetation composition, soil organic carbon and greenhouse gas emissions in semi arid grasslands of Makueni County, Kenya

Abstract

Rangeland degradation threatens the productivity of rangelands, soil organic carbon storage and offset of greenhouse gasses. A key component for sustaining production in rangeland ecosystems is the maintenance of soil organic matter (SOM), which is strongly influenced by land management. Many management techniques intended to increase forage production may potentially increase SOM, thus sequestering atmospheric carbon (C) and other greenhouse gasses such as nitrous oxide and methane. Effective management activities require an understanding of the impact of livestock grazing on these ecosystems. The purpose of this study was to investigate the influence of grazing management practices on vegetation composition, soil organic carbon and greenhouse gas emissions. The study was conducted on a commercial grazing land. The study followed a pseudo-replication design in which there were three treatments: (1) continual grazed, (2) rotational grazed and (3) and ungrazed Vegetation attributes, soil parameters and GHG fluxes were measured in each of the three sites. Soil samples for measurement of soil organic carbon concentration were taken up to a depth of 1.2m, at an interval of 0-10, 10-20cm, 20-30cm, 30-60cm, 60-90cm and 90-120cm. Greenhouse gas samples were collected using the static chamber method, and carried out for a period of three months covering the dry and wet season as well as a transition period. The herbaceous biomass production was significantly higher (p≤0.5) in rotationally grazed areas compared to both continually grazed and ungrazed areas. Percentage plant cover was significantly higher under rotationally grazed areas compared to both continually grazed and ungrazed areas which were not significantly different. There was a significant difference in plant species richness between the three grazing management practices (p≤0.05). The plant species richness was highest in rotationally grazed areas, followed by continually and ungrazed areas with mean species numbers of, 13.4±1.82, 11.9±1.74 and 9.67±2.24, respectively. Rotationally grazed site had significantly higher species diversity followed by continual and ungrazed area with mean values of 3.08±0.65, 2.88±.072 and 2.43±0.02, respectively. The ungrazed site recorded significantly (p≤0.05) higher soil organic carbon concentrations than rotational and continuously grazed sites for all soil depths up to 1.2m depth. Cumulative soil CO2 fluxes were highest from continual grazing system, followed by rotational grazing and lowest in ungrazed area, having 2357±123.9, 1285±123.9 and 1241±143 CO2 (kg ha-1 3 month), respectively. The N2O and CH4 fluxes were also highest in continual grazing and lowest in ungrazed site, with 677.9±130.1, 208.6±127.3 and 162.2±150.3 (gm ha-1 3 month) and CH4, 232.7±126.6, 173.1±126.6 and 80±46.2 (gm ha-1 3 month), respectively. The rotationally grazed site showed enhanced plant species biomass production, percentage cover and species diversity compared to the continuously grazed area. Soil organic carbon concentrations were significantly higher at the ungrazed site followed by rotationally grazed and continual grazing system respectively. Greenhouse gas emissions were significantly higher in continuously grazed area with ungrazed site having the lowest emissions. From the above findings, study has demonstrated that rotational grazing management system has the potential to significantly improve the range productivity, increase soil organic carbon and reduce emissions of greenhouse gasses.