Impact of land use change and soil type on total phosphorus and its fractions in soil aggregates

Conversion of land from native vegetation to long-term agricultural cropping adversely affects soil phosphorus (P) reserves. We examined surface soils (0-0.1 m) from three paired sites from four land uses (native vegetation, cropping, pasture, and plantation, up to 115 years) from subtropical Australia (a Vertisol from Warra, an Oxisol from Lamington, and an Oxisol from Kingaroy) for the changes in the total P concentrations as well as the fractionation of P in bulk soil and in aggregate fractions. Among different land uses and soil types, the conversion of the Kingaroy Oxisol from native vegetation to cropping decreased soil aggregation markedly. Across all three soils, land use change from native vegetation to cropping decreased total organic C (TOC) concentrations by 68-85% and organic P (P-o) by 55-63%. The corresponding decreases in P-o upon conversion to pasture and plantation were lower, being 12-23%. These decreases in P-o were primarily associated with a decrease in the NaOH-P-o fraction (relatively available P). Although land use change to cropping reduced the proportion of large macroaggregates, the P concentration, including NaOH-P-o fraction was similar across all aggregate sizes. Finally, although the overall contribution of the silt+clay fraction was small, the C and P-o reserves in this fraction tended to be more resilient and the C/P-o ratio remained relatively constant. This study has demonstrated the impacts of cropping on soil P reserves, but that conversion of cropping to pasture or plantation can largely reverse these adverse effects.