Atributos do solo e ciclagem de nutrientes em sistema agrossilvipastoril com adubação nitrogenada

Abstract

Knowing the influence of mineral nitrogen fertilization on agroforestry systems and its effects on soil attributes and forage production, as well as its role in the nutrient cycling process, is essential to promote the efficient use of nutrients in this ecosystem.Thus, the objective was to evaluate the effects of mineral nitrogen fertilization and forestry component on forage production, physical attributes and soil organic carbon fractions, on decomposition and chemical composition of Marandu grass litter and bovine excreta, in an agrosilvopastoral system , in the humid tropics of Maranhão.The experiment was conducted in 2019 and 2020, at the Technological Reference Unit - TRU in ICLF of EMBRAPA Cocais in Pindaré Mirim - MA. A randomized block design was used, with four nitrogen fertilization treatments (0, 100, 200 and 400 kg of N ha-1 year-1) and three replications in a total area of 3 ha divided into twelve equal plots kept under continuous stocking with two “tersters” animals, and with a variable stocking rate with “adjustment” animals, annealed steers with an average body weight of 150 to ±180 kg. For the purpose of evaluating forage production and soil attributes, samples were taken through transects perpendicular to the double rows of eucalyptus in each experimental plot.This was divided into five points: P1 (0 m), P2 (1.75 m), P3 (5.25 m), P4 (8.75 m) and P5 (12.25 m), which corresponded to the distances of the eucalyptus row. For each of these distances, the forage was collected to evaluate the production of dry matter of green forage and dry and then the mini trenches of 30 x 30 x 30 cm were opened to collect undeformed samples in the depths: 0 - 10, 10 - 20, 20 - 30 cm, to determine physical attributes (density, macroporosity, microporosity and total porosity).For the chemical attributes of the soil (organic carbon fractions), deformed samples were collected at the same depths perpendicular to the mini-trenches. To evaluate the decomposition of Marandu grass litter and bovine excreta, samples of senescent leaves and feces were used, which were incubated in nylon bags, in exclusion areas, in 2019 and 2020. The chemical composition of Dry Matter (DM), Organic Matter (OM), Fiber in Neutral Detergent (NDF), Fiber in Acid Detergent (ADF), N, C, and C:N ratio were determined. The results of forage mass production in the green and dry fractions of the marandu grass did not differ (P> 0.05) due to nitrogen fertilization, however the green forage mass was influenced by the distance of the double row of eucalyptus, with less production of 1733 kg DM ha-1, 49% of the total mass, closer to the trees. The organic carbon fractions ranged between 10.32 g kg-1 and 7.56 g kg-1 for total organic carbon (TOC) at the estimated doses of 17.14 and 411.43 kg of N ha-1 year-1, respectively. The particulate organic carbon (POC) contents ranged from 1.94 and 4.13 g kg-1 between 200 and 400 kg of N ha-1 yr-1 respectively, with a greater increase in POC for the estimated dose of 203 kg of N ha-1 yr- 1. There was a decrease in organic carbon associated with minerals (OCAM) at doses up to 278 kg of N ha-1 yr-1. The arboreal component influenced the macro and microporosity of the soil, however without effect for the fractions of organic carbon and density of the soil. In depth, the organic carbon fractions decrease with a significant difference between the 0-10 cm layers and the two underlying layers, with values between 11.24 and 8.07 g kg-1 for TOC, 3.85 and 2.39 g kg-1 for POC and 6.92 to 5.67 g kg-1 for OCAM at depths 0-10 and 20-30 cm. The same happens with the physical attributes, with the exception of microporosity. The attribute values ranged from 0.54 to 0.56 m3 m-3 for total porosity, from 0.12 to 0.15 m3 m-3 for macroporosity and from 1.45 to 1.49 g cm-3 for density of the soil at depths 0-10 and 20-30 cm. The production of green forage was reduced as well as the soil microporosity, and the opposite happens with the soil macroporosity close to the double row of trees in an agrosilvopastoral system. The use of nitrogen fertilization in agrosilvopastoral system modifies the physical fractions of organic carbon in the soil, reducing its levels in depth even during a relatively short period of system implementation. The agrosilvopastoral system, in the period evaluated, contributed to improve the physical quality of the soil, in depth and along the distance from the double rows of trees, however, it depends on the type of management used in the area. Litter decomposition between the years 2019 and 2020 was influenced by nitrogen fertilization, with a triple effect (P<0.0001) between the levels of fertilization x time x year. The average speed of decomposition was lower in the second year, with k = 0.00589925 g g-1day-1 against k = 0.00798675 g g-1day-1 in the first year. The average N mineralization in litter was 74% and 17% in feces, which provided an annual input of 146 kg N ha-1 yr-1 and 5 kg N ha-1 yr-1, respectively. There were no differences in the decomposition and chemical composition of excreta (P>0.05) between doses and incubation time for the analyzed variables. Both materials, litter and excreta, presented a high C:N ratio, above 30:1, revealing the greater difficulty of degradation of the material. The nitrogen levels applied did not influence litter and excreta decomposition. The contribution of nutrients via litter decomposition was faster than in the excreta component, due to higher nutrient concentration and decomposition rate. The incubation time of 128 days was not enough to express the real behavior of the litter decomposition. Thus, there is an increasing need for long-term experiments to assess nutrient decomposition in N-fertilized tropical pastures