Impacts of land-use change on nitrogen status and mineralization in the Mulga Lands of Southern Queensland

In the semiarid Mulga Lands of southern Queensland soil nitrogen (N) levels have declined after clearance of the native mulga (Acacia aneura F. Muell. ex Benth.) and conversion to grazed buffel grass (Cenchrus ciliaris) pasture. At three mulga sites, declines in soil total N ranged from 14% to 28% in the surface 10 cm of soil. In situ net N mineralization from December 2003 until November 2004 in the surface 10 cm was 49.5 kg N ha(-1) year(-1) in the mulga woodland, 48.2 kg N ha(-1) year(-1) in the young (< 5 years old) buffel pasture (previously sown to wheat (Triticum aestivum L.) and 34.6 kg N ha(-1) year(-1) in the old buffel pasture (> 20 years). Ammonium-N was the dominant N pool under mulga in the top 30 cm, while nitrate-N was dominant under the buffel pastures. Although ammonium-N under mulga was significantly different to that for 21-year-old buffel pasture at all depths, nitrification and net N mineralization were not different between the three land uses at any depth or in the entire 90 cm profile. The Soil Nitrogen Availability Predictor model was used to predict field N mineralization rates for the mulga woodland and 21-year-old buffel pasture by using a medium-term (6-week) laboratory incubation to establish basal rates of N mineralization. The Soil Nitrogen Availability Predictor overestimated annual net N mineralization in the 0-30 cm depth of mulga by 9% and underestimated it by 28% for the old buffel pasture. The Soil Nitrogen Availability Predictor could be modified further to accurately predict net N mineralization for the mulga woodlands.