Establishment Of Pasture Enclosures Restores Soil Organic Carbon And Increases Greenhouse Gas Emissions In A Semi-Arid Rangeland, Kenya

Abstract

Communal grazing system can cause degradation of soil and vegetation in arid and semi-arid rangelands thereby impacting soil organic carbon (SOC) stocks and greenhouse gas (GHG) emissions. A field study was conducted to assess the influence of pasture grazing enclosures on pasture rehabilitation, SOC and emission of GHGs. The study was carried out in the semi-arid rangeland of Chepareria Ward in West Pokot County, Kenya. The objectives of the study were to determine the influence of pasture enclosure and its age on: (1) herbaceous vegetation cover, species composition and diversity, and aboveground biomass production; (2) total soil organic carbon, particulate organic carbon, and microbial biomass; and (3) flux rates of soil CO2, CH4 and N2O. Completely randomized design was used for this study. Two types of pasture enclosures, namely contractual grazing enclosures (CGE) and grazing dominated enclosure (GDE) with differing grazing utilization strategies, were assessed based on years since establishment, hereby referred to as age class, as follows: 3-10, 11-20 and >20 years since establishment. Three enclosures were selected in each age/enclosure type combination (n = 3). The adjacent open grazing rangeland (OGR) was used as control. Herbaceous vegetation cover, species diversity as well as above ground, biomass differed significantly among grazing management systems and were consistently higher in enclosures and lower in the OGR. On average, herbaceous species diversity, vegetation cover, and aboveground biomass were 1.47, 1.83 and 7.25 times higher in the enclosures compared to OGR, respectively. Perennial grass cover, species diversity and aboveground biomass were considerably higher in the middle age enclosures (10-20 years) than in the newly established (3-10 years) and older (>20 years) enclosures. The SOC significantly increased from (mean ± SD) 4.72 ± 0.73 g kg-1 in OGR to 5.88 ± 0.72 and 6.12±1.00 g kg-1 in CGE and GDE respectively (P < 0.001), with age exhibiting non significant influence (P > 0.05). Significantly higher POC and MBC were observed in GDE than in CGE (P < 0.05). The concentration of MBC ranged from 32.05 ± 7.25 to 96.63 ± 5.31 μg C/g of soil in all grazing systems. Total SOC exhibited significant (P < 0.001) positive correlation with POC and MBC, suggesting that POC and MBC would account for the dynamics of soil organic carbon and soil biological status in the area. Soil CO2 emission rate was higher in GDE (224.4 mg C m-2 h-1) and CGE (239.9 mg C m-2 h-1) relative to OGR (102.4 mg C m-2 h-1) (P < 0.001). Soil moisture was significantly and positively correlated with CO2, CH4, and N2O flux rates (P < 0.001); with peak emission rates observed at soil moisture content between 15 and 25% (v/v). This suggested that soil moisture is the critical factor that influences the emission of CO2, CH4, and N2O from soil in the semi-arid rangeland. These results demonstrated that pasture enclosures were important to restore the degraded communal grazing lands in terms of herbaceous vegetation cover, diversity, aboveground biomass, and soil organic carbon and microbial biomass contents. The higher emission of soil CO2 in the enclosures was as a result of the improved soil and vegetation conditions which enhanced microbial respiration Overall, the higher soil-atmosphere CO2 emission in enclosure systems could be offset by the higher aboveground biomass and the SOC sequestered in the soil. Future research in the area should focus on carrying capacity of the enclosures, and include landscapes such as water points and settlements to assess the ecosystem SOC dynamics and GHG emissions.