Disponibilidade e nível crítico ambiental de cobre, zinco e fósforo no solo, associado ao uso contínuo de dejetos líquidos suínos em microbacia

Abstract

The use of swine liquid manure (SLD) as an organic fertilizer in agricultural areas is a sustainable and profitable way of disposing of this environmental liability. In its composition it has elements such as phosphorus, nitrogen, copper, zinc, iron and manganese and also organic matter that are attractive because they are essential macro and micronutrients for the fertilization of the most diverse cultures. Both phosphorus, copper and zinc, in addition to nutrients, when applied in excess to the soil, can act as potential contaminants, causing eutrophication in water bodies and even causing toxicity to plants. Also, when used by successive DLS applications, they increase their levels available in the soil, sometimes saturating this environment. In view of this, the present study was carried out in 67 plots of 15 swine farms in the Lajeado Caldeirão watershed, Palmitinho-RS, which aimed to evaluate the availability levels of phosphorus, copper and zinc in order to verify their agronomic potential, as well as verifying their contaminating potential, evaluating them based on the Environmental Critical Limit of Phosphorus (LCA-P) and the Quality Reference Values (QVR) for the pseudototal copper and zinc contents. As a methodology, field research, collection and sampling of soil was adopted, analyzed by the Laboratory of Soil Chemical Analysis at the Federal University of Santa Maria, Frederico Westphalen campus. For sampling of available phosphorus, the USEPA method (SW846-3050B) was used. The available copper and zinc contents were extracted using the Mehlich-1 solution and quantified by atomic absorption spectrophotometry. For pseudo-total copper and zinc contents, the USEPA method (SW846- 3050B) was used. For the leveling of the values, 55 areas with use of DLS (ADS), 5 of native forest (AMN) and 7 with exclusive use of chemical fertilization (AAQ) were used. As a result, the supply of DLS for the sum of the plots of each pig unit proved to be insufficient for 93% of the enterprises. Regarding the agronomic potential of the areas, most ADS and AAQ had high to very high availability of phosphorus, indicating great agronomic potential. For copper and zinc, the high levels of availability found in ADS increased loads up to 8.3 and 5.3 times, respectively, the average levels found in the reference areas (AMN). As for the environmental limits of the elements and VRQs, 14 plots (20.8%) of the studied areas presented critical levels in the soil, being above the LCA-P (5), above the VRQ+50% for copper (10) and above the Prevention Values (PV) for zinc (4). Furthermore, the pseudo-total copper and zinc contents of most AMNs were found to be above the VRQs established by law. Finally, it was evidenced that the management with DLS in the study area is optimized, referring to its agronomic potential, however, it is necessary to monitor the plots that presented critical levels because they are potential soil and water contaminants, still that the QRQs are below the natural grades of the watershed.