N2O and CH4 emissions from soils under conventional and no-till management practices in Goiania (Cerrados, Brazil)

Nitrous oxide (N2O) and methane (CH4) are important greenhouse gases (GHG) produced respectively by the naturally occurring microbial processes of incomplete denitrification or nitrification and methanogenesis. Tillage practices and climate affect the release of GHGs. No tillage (DMC) systems may increase CO2-C fixation in soil but also N2O and CH4 emissions. The aim of this article is to question whether the positive effect of a DMC system observed on carbon storage for the topsoil layer in Cerrado soils is offset or not by the N2O and/or CH4 emissions. Two 5-year-old systems, tillage (disc on the first 15 cm called offset: OFF) and a direct-sowing mulch-based crop system (DMC) with an additional cover crop were studied during a cropping cycle. N2O and CH4 fluxes are determined using a closed-chamber, N2O and CO2 concentrations are measured at 3 depths (10-, 20- and 30 cm). No significant difference between treatments was observed for both gases (for emissions and concentrations). Soil NO contents increase from surface to depth (30 cm) and range from 300 ppbv to 3 ppm for both treatments. Total annual estimated emissions of N2O range from 31 to 35 g N2O-N ha(-1) year(-1) for DMC and OFF respectively which is low and corresponds only to 0.03% of he total N-fertilizer applied. Monthly means N2O emissions were strongly correlated to monthly means of N2O content at 10 cm depth (R-2 = -0.66) and seem to increase exponentially with monthly mean Water Filled Pore Space WFPS (0-10 cm layer) (R-2 = 0.33). CH4 fluxes were very low as well: both treatment act as source of CH4 (245 and 403 g CH4-C ha(-1) year(-1) for DMC and OFF respectively. On a CO-C equivalent basis these results correspond to 4.1 and 4.7 kg CO2-C ha(-1) year(-1) for NO and to 1.9 and 3.1 kg CO2-C ha(-1) year(-1) for CH4 for DMC and OFF respectively. As a result, the carbon sequestration balance taking into account the CO2, CH4 and N2O on a CO2-C equivalent basis is in favour of DMC treatment considering that this treatment increases carbon storage originated from CO-C for the topsoil (0-10 cm) layer (350 kg C ha(-1) year(-1)) in comparison with OFF treatment. (C) 2007 Elsevier B.V. All rights reserved.