Quantification and Simulation of Organic Carbon Stocks and Fractions under Different Tillage Systems and Sugarcane Plantation In Soils Of Western Kenya
Ouma, Ojuka David
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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: A- TTS with residue retention, B- OTS with residue retention and C- TTS with residue burning. The predicted results showed that the rate of SOC stock (t/ha) change was positively higher under tractor tillage system in comparison to oxen tillage system except where burning occurred in TTS, for the modelling period of 2017-2067.Yearly estimate of carbon storage varied from 0.1 to 0.14 t ha-1 yr-1 (0 ̶ 10 years), 0.1 to 0.2 t ha-1 yr-1(11 ̶ 20 years) and 0.16 to 0.21 t ha-1 yr-1in (more than 20 years) for OTS. TTS with residue retention recorded 0.14 to 0.16 t ha-1 yr-1 (0 ̶ 10 years), 0.15 to 0.18 t ha-1 yr-1 (11 ̶ 20 years), 0.16 to 0.24 t ha-1 yr-1 (more than 20 years) whereas TTS with residue burning recorded annual carbon loss between 0.7 - 0.2 t ha-1 yr-1over the period 2017-2067. These results assert the potential role of sugar-cane residue management on enhanced SOM sequestration in sugar cane growing belts, besides, linking Roth C model with the measured soil details, will be important for predicting the capacity of sugarcane soils to sequester organic carbon as affected by the current land-management practices in Western Kenya sugar belt.