Response of phosphorus pools in Mollisols to afforestation with Pinus radiata D. Don in the Argentinean Pampa

The introduction of fast-growing coniferous species generates major changes in the edaphic phosphorus (P) dynamics. In general, the literature shows decreases in the organic and total P (Po and Pt) and increases in labile inorganic fractions and contradictory results in the associated enzymatic activity. Therefore, the aim of this study was to quantify the impact produced by the introduction of Pinus radiata on different P pools and to evaluate the enzymatic activity of acid phosphatases with respect to adjacent soils supporting natural grassland vegetation. In the Ventania area, six mature P. radiata stands (Forest treatment: F) and six adjacent natural grassland areas (Grassland treatment: Gr) were selected. On the organic horizons of F and the mineral soil of both treatments (0-90 cm), different P fractions like Pt, Po, inorganic P (Pi), occluded P (Poc), extractable P (Pe), soluble reactive P (SRP) and the acid phosphatase activity (APA) were quantified. The results indicated that the pine afforestation generated a new P reserve constituted by the organic horizons rich in Pt (17.2 g m(-2)), with a high proportion of Po and high concentration of Pe and SRP. In the mineral soil, no differences were detected between treatments for Po, although there were significant increases in Pi and Poc and consequently Pt, under the pines up to a depth of 90 cm. Labile fractions such as Pe and SRP, showed the major alterations associated with the land-use change, with concentrations up to eight times greater for Pe (0-15 cm) and more than10 times greater for SRP (15-60 cm) in relation with the grassland base line. The APA was similar between both ecosystems, however by calculating the APA/OC ratio, a significant decrease in the enzymatic efficiency was detected under the coniferous vegetation. Overall, the results showed that the afforestation with pines on natural grasslands developed on Mollisols produced a significant alteration of the vertical distribution of different pools of P. The increments of different inorganic fractions of P found under the pines, both labile and non-labile, could not be linked to specific extraction zones within the one-meter radius sampled from the trunk. The changes observed in the vertical distribution of different fractions of P indicates a profound alteration in the cycling of this element in afforestation of P. radiata on Mollisols.