The paradox of increased runoff with decreased soil loss with growth of Acacia decurrens plantations in the highlands of Ethiopia

Acacia decurrens (AD) plantations are increasingly used as an agroforestry practice in the humid Ethiopian highlands, mainly for their economic and ecological benefits. However, their effects on runoff response and soil loss with their growth are poorly understood. The main objective of this study was to examine the responses of runoff and soil loss with AD plantation age in a highland agroecological setting of Ethiopia. A field experiment was conducted during the rainy season in 2019 and 2020 using bounded runoff plots (30 m long x 3 m wide) installed on control and AD plantations of 1 to 4 years. The result showed that soil chemical properties (TN, SOC, and av. P) were improved in AD plots compared to the control. Surface runoff loss increased with the age of the AD plantation. Seasonal runoff was significantly higher (P < 0.01) in year 3 (456 mm) and year 4 (539 mm) AD plots in 2019 and 2020, respectively. This was mainly on account of decreases in understory vegetation biomass and soil infiltration rate and an increase in soil bulk density with AD plantation age. But at all growth stages of AD plantations, sediment concentration and soil loss were significantly reduced compared to the control (P <0.01). Our results imply a need to use suitable soil and water conservation measures such as trenches combined with AD plantations to reduce surface runoff loss and related effects downstream. However, the effects of AD on soil water content need to be revealed by further research.