Soil organic carbon (SOC) is key indicator of soil quality and health. It has quite a lot of benefits to the ecosystem. Information on the magnitude of carbon pool under field scale of subsistence farms is scanty. This study was aimed to assess SOC storage, CO2 emissions and soil quality, under different fields in smallholder farms of southern Ethiopia. Five fields within a farm were investigated viz. coffee (Coffea arabica L.), enset (Ensete ventricosum), root and tuber crop field (RTC), cereal and pulse field (main field) and fallow land. For each field, eight representative farms that make a total of 40 farms were randomly selected. Surface soil samples were collected, and analyzed to determine soil physico-chemical properties. SOC stock (t ha-1), C-sequestration, and CO2 emission were also computed.
The result showed that the field that sequestered more OC and emitted less CO2 was Coffee > Enset > RTC > main field > fallow land. C-stock and sequestration (t/ha) magnitude was being: coffee (81.4, 298.5), enset (75.5, 277.0), RTC (68.8, 252.6), main field (57.5, 211.0) and fallow (43.3, 159). Using coffee field as least CO2 emitter and as the base value for comparison, the percentage increment in CO2 emission out of the sequestered carbon was 7.2% (enset), 15.4% (RTC), 29.3% (main field) and 46.7% (Fallow).The result regarding soil quality further revealed significant differences in almost all of investigated parameters. The minimum and maximum values being recorded in all fields were: bulk density (BD) (1.05, 1.29 gm cm-3), pH (6.1, 7.0), SOC (1.1, 2.48%), TN (0.09-0.19%), available P (1.1, 70.9 mg/kg), total exchangeable bases (9.5, 20.5 Cmolc/kg), K/Mg (0.37,1.02), B (0.4, 1.2 mg/kg) Cu (0.32, 0.91 mg/kg), Zn (5, 20.5 mg/kg), Fe (105.5, 133.8 mg/kg) and CEC (18.6, 27.5 mg/kg). Fields in the backyard (enset and coffee) showed lowest BD and maximum values of soil chemical properties. On contrary, highest BD and lowest values of chemical parameter were observed in main and fallow fields. Soil deterioration index (%) relative to coffee field for OC, N, P, and K in their order was as follows: enset [-2.8, 0, 254.5, 23.1], RTC [-17.7, -31.6, -59.5, -34.6], main field [-35, -47.4,-79, -46] and fallow [-55.7, -52.6, -94.5,and -76.9].
Different fields within smallholder farms exhibited significant variation in amount of carbon sequestered, CO2 emission, soil degradation and soil quality. Thus, climate smart soil management practices that would enhance SOC and simultaneously increasing soil quality is suggested e.g., integrated nutrient management.