Análise de fósforo em Cambissolo sob sistema integrado de produção agropecuária tratado com fosfatos de diferentes solubilidades

Abstract

The objective of this study was to evaluate the contributions of phosphorus fractions, changes in phosphate adsorption, and efficiency of methods to predict this nutrient in a clayey Haplic Cambisol, managed in an integrated crop-livestock system (ICLS) and treated for five years with sources and doses annual of phosphates. The experiment was installed in the Castro, Paraná, Brazil in April 2009. It was carried out in an incomplete factorial scheme (3x3+1) using a randomized block design with four replications. The treatments consisted of three P sources (triple superphosphate, rock phosphate Arad and magnesium thermophosphate), three doses of total P2O5 (60, 120, and 180 kg ha-1), and treatment without phosphate fertilization. Phosphate applications occurred annually, distributed on the surface broadcast, without incorporation, at the moment of sowing of the autumn-winter crop. At 60 months after the beginning of the experiment, soil samples were collected at depths from 0 to 5, 5 to 10, 10 to 15, 15 to 20, and 20 to 30 cm. Chemical attributes and phosphorus (P) were evaluated by Mehlich-1 extraction and ion exchange resin, chemical fractionation of P, and total Fe and Al oxides, and poorly crystallinity. The concentrations of accumulated and extracted P in the dry mass, and corn grain yield, referring to the 2013/2014 harvest, were evaluated. The maximum phosphorus adsorption capacity (MPAC) and binding energy were estimated by adjusting the Langmuir isotherm. The annual application of soluble and insoluble phosphates on the broadcast surface did not influence the concentrations of P fractions and MPAC at 60 months of experimentation. The labile P fraction was strongly associated with total organic carbon and Ca concentrations in layers up to 15 cm. Phosphate fertilizer doses above 90 kg ha-1 of total P2O5, anticipate applied to broadcast surface and, high concentrations of Ca and sulfate in the soil solution increased the fractions of moderately labile and non-labile P in ICLS after 60 months of study. Total organic carbon concentrations interfered with MPAC after five years, related to increased P adsorption in layers up to 5 cm and reduced adsorption in layers from 5 to 10 cm. Exchangeable Ca and available P concentrations were strongly associated in the layer up to 10 cm, with lower binding energy. The concentrations of P extracted by Mehlich-1 were influenced by the phosphates applied on the broadcast surface, but this method showed a positive relationship with the labile organic fractions of P and grain yield in the 0 to 20 cm layer. The P concentrations obtained by the ion exchange resin method showed associations with readily available P fractions for the plant and moderately labile organic fractions. The complexity of the soil-plant-animal system in maintaining soil phosphorus levels, especially in a Cambisol under ICLS, becomes an intricate process of phosphorus release/adsorption associated with soil chemical attributes, seeking to maintain adequate levels for plant absorption. Furthermore, P extraction methods show positive relationships with different P fractions in the soil. However, it is necessary to go deeper into the P extraction methods that reflect the transformations of this nutrient in the solution for more assertive interpretations.