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Soil chemistry differences due to land-use change from native forest to Pinus plantations in sub-tropical coastal lowlands

Timothy E. Smith1,3, John A. Simpson1 and Paul T. Keay2

1Horticulture and Forestry Science, Delivery, Department of Primary Industries and Fisheries, Queensland, Australia. Email:
DPI Forestry, DPI&F, Queensland, Austrlia.


Soil chemistry differences were determined between adjoining native forest and Pinus (P. elliottii and P. caribaea) plantations growing on the same soil type in the coastal lowlands of south east Queensland. There were 10 paired comparison sites selected for sampling and analysis. These represented the dominant soil types under Pinus plantation in this region. The soil types sampled were podosols (3 sites), kurosols (1 site), chromosols (4 sites) and kandosols (2 sites). Soils were sampled to a depth of 30cm in increments of 0-7.5 cm, 7.5-15 cm and 15-30 cm. Pinus plantation age ranged from 13-31 years at the time of sampling.

Soil carbon (C) differences changed with depth. Soil C was lower under Pinus at 0-7.5 cm (13.9%), similar at 7.5-15 cm, and higher at 15-30cm (13.5%), compared to native forest. The changes in C with depth were most probably an artifact of ploughing during site preparation prior to planting of the Pinus. There was no difference in average 0-30cm soil C between the 2 land uses. Although the Pinus sites had substantially more C as litter above the soil surface (10.2 tonnes C ha-1) compared to that under native forest (4.2 tonnes C ha-1). Nitrogen (N) in the 0-30cm soil declined by 2.8% under Pinus sites compared to native forest. However there were no differences in total N between land uses when 0-30cm soil and litter N were combined.

Soil phosphorus (P) measured as total P and acid extractable ‘available’ P increased under Pinus at all sample depths i.e. an average over 0-30cm of 65.7% and 299%, respectively. The increase in total P under Pinus in the 0-30cm soil and litter accounts for 81% of applied P at plantation establishment (60 kg P ha-1 at plantation establishment and 40 kg P ha-1 at age 10 years).

Soil pH increased under Pinus by 2.5% in the 0-7.5cm soil, with no change in lower depth increments. Electrical conductivity (EC) decreased under Pinus by 23.1% and 15.8% at depths of 0-7.5 and 7.5-15 cm, respectively with no change at 15-30cm.

Differences in effective cation exchange capacity (eCEC) due to land use change were closely correlated with changes in organic matter (r2=0.81). The ratio of exchangeable aluminium (Al) over eCEC increased under Pinus plantations growing on 2 of the 3 podosol soils but not on the other representative soil types. Conversely the ratio of magnesium (Mg) over eCEC decreased under Pinus plantations growing on podosol and the kurosol soils but not on the other representative soil types. The ratio of potassium (K) over eCEC declined under Pinus across the range of soils represented.

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