Landscape and topography effects on phosphorus fractions in Mollisols of the Argentinean Pampas

Especially in semiarid regions and for calcareous soils there is a need for better understanding of the processes involved in P cycling in soils and the development of better P fertility management to mobilize the recalcitrant P pools for crop uptake. The objective of the present study was to understand the effect of landscape, slope, soil depth and land use on P-availability in Calcareous and Sandy Plain (CP and SP, respectively) landscapes of the central Argentinean Pampas. Soil profiles were described in pits dug at top, mid and footslope positions: soils were classified according to Soil Taxonomy, and samples were taken at fixed depth (0–20, 20–40, 40–60 cm) for each slope position in agricultural soil (AG) and soil under natural vegetation (NV) for physicochemical analyses and the Hedley P-fractionation. The main differences among the landscapes were in calcium carbonate, pH, CEC, exchangeable Ca, and Ca:FeAl ratio. The largest P-fraction was HCl-P in both landscapes and in general, organic P represented about 20% of total P. The most available P-forms were higher in SP. In CP, Anion exchange membrane-P (AEM-P) was defined by organic carbon and exchangeable Ca, while this was not the case for SP. In CP, AEM-P was positively correlated to NaOH-Pi and Po and NaHCO 3 -Pi and Po, whereas in SP, AEM-P was positively affected by HCl-P and NaHCO 3 -Pi. AEM-P and NaHCO 3 -Pi and Po increased towards the footslope position for both CP and SP mainly at 0–20 cm, while deeper strata showed less differences. Also, NV land use showed a higher proportion of available P fractions while AG presented greater differences between slope positions than NV. The knowledge about soil characteristics, the distribution of soil P-fractions and their interaction affecting the P availability facilitates more efficient fertility management, unique to each landscape and position. • The effect of land use, landscape, slope position, soil depth on P availability was studied. • Agriculture soil had lower contents of labile P fractions and higher calcium bound P than natural vegetation soil. • Landscapes differed in weathering of loess, soil profile depth to underlying calcrete, and exchangeable calcium contents. • The differences determined that P was less available in the semiarid landscape, being fixed by abundant calcium. • Slope position affected P availability through pedogenetic differences: pH, organic matter contents, and soil profile depth.