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Séquestration de carbone et flux de gaz à effet de serre
Comparaison entre semis direct et système conventionnel dans les Cerrados brésiliens

Abstract : Nowadays, around 20 million hectares are cultivated with no tillage systems (NT) in Brazil. These systems are agronomically efficient under tropical conditions. The present study, based on field data, laboratory experiment and modelling, deals with the impact of NT under cover crops on carbon (C) sequestration in comparison with a disc tillage system (offset, OFF) on an oxisol in the Cerrados (Central Brazil).
First of all, we studied the soil carbon stocks and compared our results to initial stocks (1998). NT was found to increase the storage of C in the topsoil layer (0-30 cm) compared to OFF. The difference observed for the 0-10 cm layer under NT represented C sequestration under no-tillage amounting to 0.35 t C ha-1 yr-1 which corresponds to about 10 % of cover crops residues returned to the soil. A particle-size fractionation of soil organic matter (SOM) showed that new C is mainly located in fractions smaller than 50 µm, whose SOM is generally considered as relatively stable. But, as NT increases C-CO2 fixation in soil, it may also increase other greenhouse effect gases (GHG) emissions and in particular N2O and CH4.
Secondly, N2O and CH4 fluxes were determined using a closed-chamber; N2O and CO2 concentrations at 3 depths (10-, 20- and 30 cm) and determinants of soil gas productions and emissions (water content, temperature, mineral nitrogen) were measured during the whole cropping cycle. No significative difference between treatments was observed for both gases. Total annual estimated emissions of N2O correspond to 0.03% of the total N-fertilizer applied which is rather low but in agreement with literature concerning Cerrados soils. CH4 emissions are very low as well. On a C-CO2 basis, N2O and CH4 emissions represent 6.0 and 7.8 kg C-CO2.ha-1.yr-1 for NT and OFF respectively. As a result, the carbon sequestration balance taking into account the CO2 (measured from soil C stocks), CH4 and N2O (measured by the closed chamber method) on a C-CO2 equivalent basis is in favour of NT (351.8 kg C.ha-1.yr-1) considering that this treatment increases carbon storage originated from C-CO2 for the topsoil layer in comparison with OFF treatment.
But, the global GHG balance calculated from field data is weakened by the discontinuity and the high variability of field measurements. Thus, we studied under laboratory conditions the potential for these soils to emit N2O from nitrification and denitrification. The results indicate that our soils emit low amounts of N2O even under optimal conditions. Then we used both laboratory and field measurements to implement a model (NOE: Nitrous Oxide Emissions) to simulate N2O emissions. To evaluate the whole range of N2O emissions, we simulate continuously the water content using another model (PASTIS) on this soil over the whole agricultural cycle. Simulations by NOE showed that (i) our soils emit low amounts of N2O (and CH4) for both systems (NT and OFF), (ii) N2O emissions from nitrification and denitrification cumulate during the whole cropping cycle, (iii) emissions by denitrification correspond to short periods of high N2O emissions (15 times higher than emission by nitrification), (iv) nitrification is not negligible and contributes to 35 and 31 % of the total N2O emissions for NT and OFF respectively and that (v) field measurements of N2O emissions seem to correspond to nitrification only. The new simulated C sequestration balance confirms the previous balance based on field data. As a result, the carbon sequestration balance taking into account the CO2, CH4 and N2O on a C-CO2 equivalent (around 320 kg C.ha-1.yr-1) basis is in favour of NT treatment.
The ability of NT to sequester C is another environmental benefit of this system.
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Submitted on : Thursday, July 20, 2006 - 9:51:17 AM
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Aurélie Metay. Séquestration de carbone et flux de gaz à effet de serre
Comparaison entre semis direct et système conventionnel dans les Cerrados brésiliens. Sciences de la Terre. Institut national agronomique paris-grignon - INA P-G, 2005. Français. ⟨tel-00086830⟩

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