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

Abstract

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); pHw; and electrical conductivity. Of these, pHw showed a significant response to time of clearing for all 3 tree communities. Soil pHw 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 pHw and exchangeable cations was more evident at >0.30 m soil depth. The increase in soil pHw 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.

Pasture production in cleared and uncleared grazing systems of central Queensland, Australia

“…Soil pH showed a notable increase with age of clearing, to such an extent that it adversely affected the availability of soil nutrients (Sangha et al 2005). There was a loss of microbial biomass with clearing that would most likely lead to less mineralisation of soil organic matter, and hence lower availability of nutrients for plant growth and the noted decline in pasture production over time as reported here.…”

“…There was a loss of microbial biomass with clearing that would most likely lead to less mineralisation of soil organic matter, and hence lower availability of nutrients for plant growth and the noted decline in pasture production over time as reported here. The changes in soil pH or soil microbial biomass occur mainly due to change in soil processes with clearing (Sangha et al 2005). But, over a longer term, such effects of soil pH or microbial biomass on nutrient availability will possibly become more apparent.…”

Effect of woody vegetation clearing on nutrient and carbon relations of semi-arid dystrophic savanna

“…Old soils, such as the Kandosols studied here, support vegetation characterised by the ability to efficiently access and use soil nutrients, and among woody species, Proteaceae which are ubiquitous in Australian ecosystems, are prominent P mobilisers (Lambers et al 2008). In E. melanophloia savanna, extractable P levels in the top 30 cm of the soil were elevated 10 years after clearing but declined to below preclearing levels 30 years after clearing (Sangha et al 2005). A more detailed investigation of longer term (>30 years) cleared sites has to determine the generality of these findings.…”

“…In contrast, soil-extractable ammonium and nitrate levels were similar across cleared and uncleared sites. Similarly, nitrate levels following soil-extraction were elevated at only one out of three cleared sites in E. melanophloia savanna (Sangha et al 2005), which suggests that in situ resin incubation is the more appropriate technique for detecting differences in time-integrated N availability (Vourlitis et al 2007). Higher levels of ammonium and nitrate are in agreement with the notion that disturbance increases N mineralisation and associated net nitrification (Bradley 2001).…”

“…Soil N relations were examined because N is likely to (co)-limit plant productivity in the dystrophic soils of the Desert Upland, and because N is linked to soil C storage (Neff et al 2002). We measured macronutrient content of grasses to determine if soil nutrient availability is mirrored by vegetation, since availability of soil macronutrients increased in the first decade after clearing but decreased in the longer term (Sangha et al 2005). Nitrogen composition of xylem sap was analysed to discern the role of trees for nutrient capture.…”