Dispersed trees on smallholder farms enhance soil fertility in semi-arid Ethiopia

Introduction Dispersed trees such as Oxytenanthera abyssinica (A. Rich.) and Dalbergia melanoxylon (Guill. & Perr.) which are objectively maintained or planted on farmland provide a significant contribution to soil fertility improvement. However, there was no quantitative information on the level of soil nutrient additions of these trees to the soil system. Methods This study was conducted on the farmers' fields in Kafta Humera district, Tigray region (northern Ethiopia), where mature stands of O. abyssinica and D. melanoxylon trees exist. Radial distance-based soil sampling (under the canopy, near to canopy, and far from canopy) was adopted to quantify the role of these trees on soil fertility improvement. Soil parameters tested were soil reaction (pH), total nitrogen (TN), available phosphorus (AvP), electrical conductivity (EC), cation exchange capacity (CEC), and organic carbon (OC). Results There was a negative linear relationship between the radial distance of the O. abyssinica tree trunk and soil TN, OC, CEC, and AvP contents but not for pH. Similarly, negative linear relationship between distance from D. melanoxylon and TN, OC, and AvP was obtained. The average total nitrogen (0.26% and 0.13%), available phosphorus (7.21 ppm and 6.37 ppm), and organic carbon (1.73% and 1.02%) contents were respectively higher under the tree canopies of O. abyssinica and D. melanoxylon compared with the adjacent open canopies. The amount of soil OC, TN, AvP, and CEC under O. abyssinica tree species was also significantly higher by 69%, 100%, 13%, and 42% compared to that of D. melanoxylon tree species. However, the amount of EC and soil pH was significantly lower by 57% and 19%, respectively. Conclusion In general, O. abyssinica and D. melanoxylon added a significant amount of nutrients to the soil. Thus, retaining these important tree species on farmland played a positive role in replenishing soil fertility for resource-constrained households so as to reduce chemical fertilizer amendments.