Background Soil organic carbon (SOC) is key indicator of soil quality and health. It has substantial benefits to the ecosystem. Information on the magnitude of carbon pools under field scale of subsistence farms is scanty. This study aimed to assess carbon stock, sequestration and soil properties among different fields in smallholder farms of southern Ethiopia. Five field types within a farm were investigated viz. coffee (Coffea arabica L.), enset (Ensete ventricosum), root and tuber crop field (RTC), crop field used for growing cereals and pulses, and fallow land. For each field, eight representative fields as a replication were selected. Surface soil samples were collected, and analyzed to determine soil physico-chemical properties. In addition, C stock, C-sequestration, and CO2 emission were also evaluated.
Results The result regarding soil properties revealed significant differences in almost all of investigated parameters. The minimum and maximum values being recorded 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 parameters were observed in crop and fallow fields. The result further indicated that the field that stocked and sequestered more carbon, and emitted less CO2 was coffee > enset > RTC > crop 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), crop 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% (crop field) and 46.7% (fallow).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], crop field [-35, -47.4,-79, -46] and fallow [-55.7, -52.6, -94.5,and -76.9].
Conclusion Different fields within smallholder farms exhibited significant variation in amount of carbon sequestered, CO2 emission, soil degradation and soil properties. Thus, climate smart soil management practices that would enhance carbon pool and simultaneously increasing soil quality are suggested e.g., integrated nutrient management.