Impact of tree clearing on soil pH and nutrient availability in grazing systems of central Queensland, Australia

In Queensland, land is cleared at high rates to develop pastures for enhanced production and the associated monetary gains. However, pasture production declines over time in cleared pastures until a new equilibrium is reached. The present study focussed on elucidating the reasons for decline in pasture production and finding the key soil properties that are affected due to clearing. Paired sites for cleared and uncleared pastures were selected to represent 3 dominant tree communities of the semi-arid tropics in central Queensland, i.e. Eucalyptus populnea, E. melanophloia, and Acacia harpophylla. The cleared pastures were chosen to represent 3 different durations of time since clearing (5, 11–13, and 33 years) to evaluate the temporal impact of clearing on soil properties. Various soil parameters were studied: macronutrients—available N (NH₄⁺and NO₃^–), total N, and available P; micronutrients—Cu, Fe, Zn, and Mn; exchangeable cations—Ca, Mg, Na, and K (also macronutrients); pH_w; and electrical conductivity. Of these, pH_w showed a significant response to time of clearing for all 3 tree communities. Soil pH_w increased significantly at cleared sites relative to uncleared (native woodland) pastures, and the increase was highly correlated with concentrations of exchangeable Ca, Mg, and Na. The change in soil pH_w and exchangeable cations was more evident at >0.30 m soil depth. The increase in soil pH_w in cleared pastures decreased the availability of soil nutrients for plant growth and, hence, pasture productivity. The interactions of different soil properties down the profile as a result of changes caused by clearing are important when interpreting the effects of clearing on soil properties.