Biocarvão e cinzas de casca de arroz: amenizantes da toxidez de ferro em cultivos de arroz irrigado

Abstract

The state of Rio Grande do Sul is the largest rice producer in Brazil and the main cultivation system is irrigation by flooding. In this system, under anaerobic conditions, iron (Fe3+) present in the oxides is reduced to Fe2+, a soluble form and, consequently, more available to plants. The greater amount of Fe2+ in the soil solution may become toxic to the rice, causing damage to the producers. A common practice in the treatment of rice is the burning of rice husks in the generation of heat for the drying of the grains, generating large amounts of ashes at each harvest. Another material, also produced from the burning of plant residues under controlled conditions of time and temperature, is biochar. The biochar, as well as the ashes, have adsorption capacity for metal species and could be used as adsorbents in rice fields where high Fe2+ concentrations are found after flooding. In this study, a rice husk biochar (RHB) and two ashes (RHA1 and RHA2) were characterized by several chemical and physical parameters as well as their Fe2+ retention capacity in solution. Although the difference in carbon (C) content of biochar (46%) and ash (16% and 0.93%) was large, as well as other surface characteristics, RHB had an average adsorption capacity of 5.53 mg Fe2+ g-1 and the ash between 6.74 and 7.22 mg Fe2+ g-1. Subsequently, these materials were applied in the cultivation of rice in greenhouse at doses equivalent to 5 Mg ha-1, 10 Mg ha-1 and 15 Mg ha-1. With the application, there was a significant increase in Fe contents after 36 days of flooding. N, K, Ca and Mg nutrients also had a significant increase in soil solution. In the plant tissue, no change in Fe content was observed and the contents of K increased, and Ca and Zn reduced slightly. The application of biochar and ash at the doses used was not able to produce significant positive effects in reducing the viability of iron in flooded rice cultivation.