Quantification and Simulation of Organic Carbon Stocks and Fractions under Different Tillage Systems and Sugarcane Plantation In Soils Of Western Kenya

A research Thesis submitted in partial fulfilment of the requirements for the award of the degree of Master of Science in Land and Water Management in the School of Agriculture Development and Enterprise, Kenyatta University. September, 2020 ; Sugarcane farming systems can adversely affect soil properties especially total soil organic carbon stocks (TSOCst) and fractions. This study was carried out in Western Kenya sugar zones to assess impacts of tillage systems (Tractor and Oxen ploughing), and plantation ages (continuous mono-cropping system, more than 20 years, 11 ̶ 20 years, 0 ̶ 10 years) on TSOCst and fractions (particulate soil organic carbon, POC, and the mineral associated soil organic carbon, MOC) in three different Agro-ecological zones (AEZs). Consequently, their effects on soil organic carbon quantities were simulated using Rothamsted organic carbon turnover model (Roth C Model). Undisturbed and disturbed soil samples were sampled from the sugarcane plantations in the three identified Western Kenya sugar zones (Mumias, South Nyanza (SONY) and Chemelil sugar zones) at top 0.6m. Soil Organic Matter was fractionated into labile (>250 μm) and stable (<53 μm) fractions. Cumulative crop residue was greater with tractor tillage systems (TTS) than with the oxen tillage systems (OTS) in all the zones expect in Mumias tractor tillage system where residues were burnt. Regarding plantation age, high SOCst were recorded in the older farms (more than 20years) with SONY sugar tractor tillage system registering the highest average SOCst (80.20 t C/ha). Mean SOCst and fractions differed significantly between the tillage systems and responded to plantation ages, depth and AEZs (P <0.05). The MOC increased with increasing plantation age both in TTS and OTS, which was in contrast to POC which generally decreased with increasing plantation age. In the simulation of Organic carbon, after verifying the genesis soil organic carbon (SOC) content a fifty (50) years prediction was run for three management practices: ...