Atributos do solo em sistema agroflorestal com adubação nitrogenada

Abstract

Agrosilvopastoral systems, when properly managed, provide diverse ecosystem services. The objective of this study was to evaluate the effects of nitrogen fertilization and the forest component on the physical attributes and organic carbon fractions of the soil, and the agrosilvopastoral system, in the humid tropics of Maranhão. The experiment was conducted in 2019, at the Technological Reference Unit - URT of ILPF of EMBRAPA Cocais, in Pindaré Mirim MA. The design was randomized blocks, with four treatments and three replicates, whose plots corresponded to the nitrogen fertilization levels (0, 100, 200 and 400 kg of N ha year) and the subplots, the distances from the forest component (0; 3.5; 7.0; 10.5 and 14.0 m) in a total area of ​​3 ha divided into two equal plots, maintained under continuous stocking with two "test" animals, and with variable stocking rate with "adjustment" animals, steers with an average live weight of 180 kg. To evaluate the soil attributes, sampling was done by means of transects at different distances from the eucalyptus row. Mini-trenches of 30x30x30 cm were dug to collect undisturbed soil samples at depths of 0-10, 10-20, 20-30 cm to determine physical attributes (density, macroporosity, microporosity and total porosity). For soil organic carbon fractions (Cot, Cop and Coum), disturbed samples were collected at the same depths in a perpendicular direction to the mini-trenches. Statistical analyses were performed using the Procidixed program of SAS. The means of each treatment were compared using the Tukey test at the 5% probability level. The organic carbon fractions varied between 10.32 g kg and 7.50 g kg for total organic carbon (Cot) at the estimated doses of 17.14 and 411.43 kg of N ha year, respectively. The paniculate organic carbon (Cop) concentrations ranged from 1.94 to 4.13 g kg-¹ between 200 and 400 kg N ha¹ year¹, respectively, with the greatest increase in Cop for the estimated dose of 203 kg N ha¹ year-¹, and the smallest decrease in mineral-associated organic carbon (Coam) for doses up to 278 kg N ha¹ year¹. At depth, the organic carbon fractions decreased with significant differences between the 0-10 cm layers and the two subsequent layers, with values ​​between 11.24 and 8.07 g kg-¹ for Cot, 3.85 and 2.39 g kg-¹ for Cop, and 6.92 to 5.67 g kg-¹ for Coam at depths of 0-10 and 20-30 cm. The same occurs with the physical attributes, with the exception of microporosity, with values ​​of 0.54 to 0.56 m³ m³ for total porosity, 0.12 to 0.15 m³ m³ for macroporosity and 1.45 to 1.49 g cm³ for density only at depths of 0-10 and 20-30 cm. Microporosity is reduced and the opposite occurs with macroporosity only near the double row of trees in the agroforestry system. The use of nitrogen supplementation in agroforestry systems reduces organic carbon levels only at depth, even during a relatively short period of system implementation. The macroporosity of the tree component is greater than that of the tree component and microporosity is reduced with this approach. The agroforestry system, in the evaluated period, contributed to improving the physical quality of both the depth and the length of the spacing of the double rows of trees, however, it depends on the type of management used in the area