Labile and Non-Labile Fractions of Phosphorus and Its Transformations in Soil under Eucalyptus Plantations, Brazil

Phosphorus (P) is the main limiting nutritional factor in the cultivation of Eucalyptus in Brazil. Despite this, little is known about the P fractions and dynamics in Eucalyptus soils in tropical regions. The aims of this study were (1) to evaluate the distribution of labile and non-labile fractions of P in soil in 10 sites of Eucalyptus in the state of São Paulo, Brazil and (2) to determine the interactions among the P fractions and to identify which fractions act as sinks or sources for the available P fraction. The P fractions were separated using the Hedley sequential extraction method, and the P transformation processes were evaluated via structural equation modeling. The labile P (resin-Pi + Pi and Po in NaHCO3) varied between 6% and 16%, the moderately labile P (Pi + Po in NaOH) varied between 28% and 40%, the occluded P (Pi + Po in Sonicate + NaOH) varied between 6% and 19%, the P associated with Ca (HCl-Pi) varied between 0.4% and 3% and the residual-P varied between 35% and 47%. The hypothetical structural model of soil P cycle was composed of four P pools (latent variables): the organic pool, occluded pool, inorganic pool, and available pool. The structural model was adequate to identify functional pools of P in soil, and to identify transformation processes of P based on the source-sink relationships. The available P pool was highly dependent on inorganic and occluded pools and the organic pool acted predominantly as a sink of P on available and inorganic pools. The results reinforce the high level of recalcitrance of the organic pool and the fact that Eucalyptus plants must access pools of limited-availability P to meet their nutritional demands.