Effects of Uprooting and Impala Herbicide Control of Black Wattle on Soil Physicochemical Properties in Nyika National-Park

Black wattle (Acacia mearnsii) threatens montane grasslands and soil functioning in Nyika National Park, Malawi, but comparative evidence on the soil effects of alternative control methods is limited. This four-year study evaluated the effects of mechanical uprooting and cutting followed by Impala herbicide application on selected soil physicochemical properties, using untreated invaded plots as controls in a randomised complete block design. Soil samples were collected at a depth of 0–15 cm before and after treatment and analysed for pH, soil organic carbon (SOC) stocks, total nitrogen (TN), available phosphorus (P), exchangeable sodium percentage (ESP), exchangeable calcium (Ca), exchangeable magnesium (Mg), and electrical conductivity (EC). Linear Mixed-Effects Model (LMEM) was then performed in Minitab (version 21) to test for significant differences among treatments (α=0.05). Uprooting increased pH by 7.3% (4.1 to 4.4) and SOC stocks from 68.88 to 74.95 Mg C ha⁻¹, whereas cutting/Impala increased SOC by 43.8% (64.82 to 93.18 Mg C ha⁻¹). TN declined from 2.52% to 0.61% after uprooting and from 2.51% to 0.62% after cutting/Impala. Available P increased most under cutting/Impala, from 4.3 to 45.3 mg kg⁻¹, while EC declined by 59.4% (1.92 to 0.78 mmho cm⁻¹) after uprooting. Treatment effects were significant for TN (P = 0.014), exchangeable Ca (P = 0.045), exchangeable Mg (P = 0.028), and EC (P = 0.001). ESP declined by 60.5% (3.8% to 1.5%) following mechanical uprooting. Therefore, both methods altered soil properties, indicating that black-wattle control should integrate soil monitoring with vegetation restoration.