Forest conversion to pasture affects soil phosphorus dynamics and nutritional status in Brazilian Amazon

Understanding the pathways of soil phosphorus (P) transformations and determining the factors related to P nutritional status of soils when land use changes is critical for a better management, especially in Amazon region. We investigated different P fractions and their transformations in different land uses (primary forest and pasture) and soil textures (clayey and sandy) in Amazonian Oxisols using path analysis. Besides P fractionation, phosphatase activity and its correlation with soil carbon (C):organic P (Po) ratio was evaluated to correlate it with soil P nutritional status. After 15 years from forest slashing and burning, total P in pasture reaches to the forest levels in Amazonian soils, regardless of soil texture. Path analysis showed that land use conversion from forest to pasture decreased the diversity of the contribution of P pools to buffer P extracted by anion exchange resin. However, Po accounted for one-fourth of total P in our sites, it plays an important role as source of plant available P and contributed more in pasture compared to forest. Our results from P fractionation and C:Po ratio revealed that Amazonian pastures and forests are not P deficient. We also showed that with increasing C:Po ratio, plant-available P content became more dependent on P mineralization. Soil acid phosphatase activity can be used as an indicator for evaluating soil P nutritional status; however, its range changes according to the land use. (AU)