Tillage systems and cover plants change organic fractions of phosphorus in oxisol of eastern Amazonia

Due to the large extent of degraded areas in the Amazon, the use of conservation systems is very important to contain the advance of the agricultural frontier, and may favor the availability of nutrients such as phosphorus (P). This study evaluated effect of tillage systems on and cover plants distribution of organic P fractions (Po) in oxisol under soybean and grasses in crop successions. The experimental design was completely randomized with four replicates and five treatments: (i) conventional tillage (CT) with annual soil tillage; (ii) no-till (NT) in succession with Urochloa ruziziensis (NT1); (iii) NT in succession with U. brizantha (NT2); (iv) NT in succession with Panicum maximum (NT3); and (v) a control consisting of a fragment of native vegetation (NV). The Po fractions were quantified at depths of 0-5, 5-10, and 10-20 cm, before and after soybean cultivation and the P fractions were analyzed. The sample sat a depth 0-5 and 5-10 cm had increased levels of biomass P in the NT1 and NT2. In addition, at depths of 0-5 and 5-10 cm, the treatments in no-tillage systems accumulated the most of the Po labile fractions. These results showed that conservation systems tend to accumulate most of the P fractions in soil through decomposition of organic residues. Thus, no-tillage system was shown to be important for Po supply, where the use of U. brizantha 'Piata' (NT2) favored higher levels of organic P in labile and moderately labile fractions in soil, which was considered the best alternative for Po supply among the species tested.