Microestrutura de um argissolo com aplicações de biochar de arroz e soja

Abstract

The addition of organic compounds to the soil causes different effects on its structure, either at the macro or microscale. But the extent of these effects is closely related to the molecular structure of the added organic material and soil mineralogy. The use of biochar has been reported as promising. Among several uses, most known is as a soil conditioner, improving soil physical and chemical conditions for plant growth and development. However, few studies have dealt with the effects of biochar on soil mechanical behavior, especially the effects on more weathered soils on a particle scale. In this study, the impacts caused by the addition of biochar on the stability and resilience of the microstructure of a weathered soil were investigated, using rheometry as a tool. Thus, two studies were carried out in a Typic Hapludult with different objectives: ii) To investigate if there are structural effects, under oscillatory shear, of the addition of different biochar’s in weathered soil, in the short term; ii) and if there are effects on the resilience and stability of the microstructure of a weathered soil, after successive cultivation with addition of biochar. The studies were conducted at the Federal University of Santa Maria and two types of biochar were tested in both studies: Biochar from rice straw and soybean. In study I, the incubation was performed in plastic boxes, with biochar being incorporated in the dose of 10 t ha-1. The moisture content was maintained constant, and after 15, 30, 45 and 60 days of incubation, disturbed soil samples were collected for conduction of the amplitude sweep test with controlled deformation and analyses of the chemical properties. Study II was conducted in a greenhouse, with the use of cylinders with soil (preserved structure), two crops (wheat/soybean) were grown, with application of biochar on the surface, in two doses and two types of biochar. After the harvest of the cultures, deformed samples were collected in different soil layers, and amplitude sweep tests, thixotropy tests and analyses of the chemical properties of the soil were performed. From the amplitude sweep tests, the rheological properties that characterize the degradation of the stiffness and elasticity of the soil microstructure were obtained. The results indicated that the rheological properties of shear and viscoelasticity were affected by the addition of biochar in the soil. In study I, the shear properties were altered, and reduced by the addition of biochar, regardless of the biochar type. In turn, the properties of viscoelasticity were not altered by the addition of biochar. In study II, regardless of the amount or type applied to the soil, biochar reduced the viscoelasticity of the structure. The different effects on shear properties and viscoelasticity in studies I and II are probably the result of different time of contact with biochar (2 vs 6 months), and of the additional effects of the cultures, soil - plant relationship. The impacts of biochar in study II were restricted to the superficial layer, since, the biochar was not incorporated in the soil. The results suggest that biochar affects the interaction between the particles, weakening the mineral particles contact, thus weakening the cohesion of the soil, its stiffness and microstructural viscoelasticity. Regarding the resilience and the resistance of the soil to high strain, we observed that the addition of biochar, independently of quantity and type, did not affect the resilience, but it reduced the resistance of the soil. Both studies indicated that biochar affects the mechanical behavior of a Typic Hapludult under oscillatory stress. The shear strength (study I) and the viscoelasticity (study II) were reduced after application of biochar in the soil. However, further studies are needed, involving more types of biochar, higher temporal resolution, more field repetitions, and greater control of normal force in amplitude sweep tests.