Quantificação das emissões de N2O resultantes de fertilizante, fixação de N2 e resíduos de colheita em Argissolo Vermelho-Amarelo

Abstract

Nitrous oxide (N2O) is one of the gases responsible for the greenhouse effect, and soils are the main source of this gas to the atmosphere. The N availability in the soil is the main factor influencing their emissions. Thus, cropping systems that use nitrogen fertilizers, along with the decomposition of harvesting residues can enhance N2O emissions, exacerbating the greenhouse effect. Many studies have being made about the effect of increasing available N in soil on the N2O emission in temperate climate conditions, however there is still few studies in tropical conditions. This study aimed to quantify the N2O emissions in different conditions, and levels of N fertilizer applied to a maize crop, from the biological nitrogen fixation process in soybean, and from addition of different crop residues onto soil. The study was carried out at Embrapa Agrobiologia in Seropédica-RJ, in an Argisol (Typic Hapludult). The maize and soybean crops were grown in the period of December 2007 to April 2008, under the conventional soil preparation system. After harvesting, crop residues were laid on the soil for the assessment of N2O emissions from them, in the period of May to August 2008. The experimental design applied to all studies was of randomized blocks. In the maize crop, the treatments were different rates of urea-N (0, 50, 100 and 150 kg N ha-1), applied on soil surface. The two highest urea rates were split in two half applications. For the soybean crop, a maize control plot was used, without N fertilizer. The study of the effects of crop residues on N2O emissions were performed with maize, and soybean residues, and using bare soil plot as control. The N2O emission was evaluated using a static closed chamber technique. The largest N2O fluxes were observed in maize after the N fertilization. The largest emission of N2O, of 1.055 kg N ha-1, was obtained in the treatment with the N rate of 150 kg ha-1, whereas in the treatments with 0, 50 and 100 kg N ha-1 emissions were of 0.509, 0.529, and 0.850 kg N2O-N ha-1, respectively. For the BNF experiment N2O emissions under soybean were of 0.633 kg N ha-1 whereas under maize it was 0.509 kg N ha-1. Soybean residues caused the largest N2O emissions, reaching 0.813 kg N ha-1, while maize residues and bare soil accumulated, respectively, emissions of 0.55 and 0.324 kg N ha-1. Considering that 40% of the N was lost from the soil system, and that fertilizer application is a quantitative variable, direct N2O emissions from fertilizers allowed to calculate an emission factor of 0.52%. The emission factors for the soybean and corn residues were similar, ranging from 0.44 to 0.42%, respectively. The emission of N2O in conventionally cropped maize increased with the application of N fertilizer, while emissions from the N2 biological fixation were negligible. The residues quality influenced the N2O emissions, with low C:N residues presenting the highest emissions. The emission factors were smaller than the average value (1%) proposed by IPCC; however, they still are within the uncertainty range of this number (0.3 to 3%).