Decomposição de resíduos culturais de plantas de cobertura de solo e emissões de óxido nitroso

Abstract

The inclusion of summer cover crops (SCC) in the no-till system is an interesting alternative to promote the addition of elevated quantities of plant biomass and nitrogen (N) to the soil. The C and N dynamics during the SCC residue decomposition can control the nitrous oxide (N2O) fluxes to the atmosphere. This study was carried out to quantify the SCC residue (shoot and root) decomposition, N release and N2O production after the SCC management, in order to select species that combine high biomass and N addition to the soil with low N2O emissions. The study was conducted in 2010 and 2011 from April to September, on a Typic Hapludalf, in the central region of the Rio Grande do Sul state. The SCC used in the experiment were: velvet bean (Mucuna aterrima); pearl millet (Pennisetum americanum); pigeon pea (Cajanus cajan); sunn hemp (Crotalaria juncea); crotalaria (Crotalaria spectabilis) and jack bean (Canavalia ensiformis). In addition, there were two fallow treatments. After the SCC management white oat (Avena sativa) and black oat (Avena strigosa) were sowed in 2010 and 2011, respectively. Each year the N fertilizer was applied only in one of the two fallow treatments. In both years the SCC residue decomposition and N release was evaluated through the mesh-bag technique and the N2O fluxes were evaluated through the static chamber technique. The C and N release dynamics followed same pattern for both the SCC shoots and roots. The highest C and N release was observed from shoot residues during first 30 days after SCC management. After 140 days, lowest C and N release from both shoot and root residues was observed from jack bean while highest was recorded for pearl millet. During this period, roots showed higher values for the remaining C and N, indicating that they can contribute relatively more significantly for the C and N accumulation in the soil. The highest N2O fluxes occurred during first 40 days after SCC management, followed by rainfall events. The C availability seems to control the N2O fluxes to the atmosphere, working directly as the energy source for the denitrifier bacteria or indirectly on the reduction of the O2 availability. The highest N2O cumulative emissions were observed during the decomposition of the crotalaria residue. The N losses as N2O were not proportional to the N addition, reaching 0.8 % of the N added by the SCC. Among SCC, the sunn hemp showed lowest values in terms of N2O emission with relation to added biomass (90 and 176 g of N-N2O Mg-1). Therefore, it can be concluded that among SCC, sunn hemp can be used to combining C and N addition to soil with low N2O emissions.