Brüggemann, N.

Brümmer, C.

Falk, U.

Papen, H.

Szarzynski, J.

Wassmann, R.

KITopen

C. Brümmer(1), N. Brüggemann(1), R. Wassmann(1), U. Falk(2), J. Szarzynski(3) and H. Papen(1)*Soil-atmosphere exchange of N2O, NO, CH4 and CO2 in natural savannah and savannah converted to agricultural land in Burkina Faso (W. Africa)Funding: Helmholtz GemeinschaftVirtual Institute: “Sustainableuse of near-natural landscapes”(VH-VI-001)Forschungszentrum Karlsruhe in der Helmholtz-GemeinschaftInstitute for Meteorology and Climate ResearchAtmospheric Environmental Research, IMK-IFU(1)*Corresponding author:Address: Kreuzeckbahnstr.1982467 Garmisch-Partenkirchen, GermanyPhone: +49-8821-183130E-mail: hans.papen@imk.fzk.deIntroductionApproximately 65% of the African continent could be considered as native savannah in the past, but large portions of this area have been converted to agricultural land, namely rangeland and low intensity cropping systems. Land conversion triggers mineralization processes and a rapid reduction of soil carbon and nitrogen. It can be hypothesized that the conversion is accompanied by an increased loss of gaseous C and N compounds, which could in turn affect the carbon and nitrogen balance from the landscape to sub-continental scale.DanoBontioliSite characteristicsI Bontioli Réserve - natural park, no farming, no tillage, subtropical brown soil, rich of nutrients, on clayey base materialII Sorghum Bontioli - agriculturally used since 15 years, subtropical ferruginous soil, partly leached, on sandy base materialIII Sorghum Dano, IV Cotton Dano, V Peanut Dano - agriculturally used since several decades, subtropical ferruginous soil, leached, on sandy base materialIDry seasonIRainy seasonII III IV VMaterials and methodsFor determination of the soil-atmosphere exchange of N2O, CH4 and CO2 the static chamber technique was used. Two major field campaigns were carried out during the vegetation periods in 2005 and 2006. Air samples (15 ml) were taken every 10 min and analysed afterwards by a gas chromatography system (see Butterbach-Bahl et al. 1997).Additionally a fertiliser experiment on the Dano sorghum site was set up using an automated chamber system (Breuer et al. 2000). The sorghum field was divided into three square plots with a side length of 10 m each. The amount of added inorganic fertiliser (NPK) ought to represent realistic scenarios of farmers who have access and financial support for (i) high (1000 kg per ha) and (ii) ordinary (375 kg per ha) fertiliser applications.ReferencesBreuer L, Papen H, Butterbach-Bahl K (2000): N2O emission from tropical forest soils of Australia. J Geophys Res 105:26353-26367Butterbach-Bahl K, Gasche R, Breuer L, Papen H (1997): Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions. Nutr CyclAgroecosys 48:79-90Results and discussion (1) –Impact of soil moisture and soil temperature on trace gas exchangeWhile the savannah soil showed consistently higher water contents resulting in higher respiration rates, differences among the sorghum, cotton and peanut fields were marginal (Fig.2). Average N2O fluxes were very low (3-10 μg N2O-N m-2 h-1), whereas the highest flux rates in each year were measured in June which coincided with the onset of the rainy season. While on the agriculturally used fields mean CH4 uptake rates of 3-8 μg CH4-C m-2 h-1 were observed, average CH4 emissions of 80 μg CH4-C m-2 h-1 were measured in the nature park during the rainy season in 2005 (Fig.2). CH4 uptake was observed at high soil temperature and low water content levels, while highest emissions occurred at soil moisture values above 20 % (Fig.3). Fig.4 and 5 show the time course of N2O and NO fluxes after wetting. Both, N2O and NO fluxes were highest at medium water contents between 15-20 %. Soil dry up resulted in a longer lasting decline of fluxes, while constantly high water contents led to relatively low N2O and NO emissions.Results and discussion (2) -Effect of N fertiliser application on N2O exchangeBefore adding inorganic fertiliser on the sorghum field, fluxes ranged from low release rates (1-10 μg N2O-N m-2 h-1) to even lower uptake rates (1-8 μg N2O-N m-2h-1) on all three untreated fields (Fig.6). After the application of fertiliser, there was no abrupt increase of fluxes, but a constant rise on the two treated plots to values up to 30 μg N2O-N m-2 h-1 (plot B) and 120 μg N2O-N m-2 h-1 (plot A). ConclusionOn the agricultural fields, the flux rates and directions of N2O and CH4 indicate very low input and turnover of N. In the near-natural savannah park, however, CH4 emissions are among the highest ever observed in a ‘dryland’ ecosystem. This CH4 source strength is normally a characteristic trait of a wetland and is even amplified by enormous CH4 release of insular termite nests (data not shown). Highest CH4 fluxes in the park could be explained by the assumption that the input of organic material into the soil decreases rapidly after conversion to cropland, leading to significantly decreased C and N availability and turnover.Fig.6: Effect of N-fertiliser application on N2O exchange on a sorghum site in Dano.Fig.1: Seasonal courses of N2O, CH4 and CO2 exchange, precipitation, VWC and air temperature on a near-natural savannah site and a sorghum site in Bontioli.Fig.2: Mean N2O, CH4 and CO2 flux rates of all investigated land use types of field campaigns 2005 and 2006.Fig.3: Impact of soil moisture and soil temperature on CH4 exchange in the nature park Bontioli.Fig.2Fig.3Fig.4: Time courses of N2O fluxes after wetting. Left charts show a singular wetting event with a subsequent dry up; right charts show permanent wetting events with constant water content.Fig.5: Time courses of NO fluxes after wetting. Left charts show a singular wetting event with a subsequent dry up; right charts show permanent wetting events with constant water content.(2)Center for Development Research (ZEF)Department of Ecology and Resource ManagementUniversity of Bonn, Walter-Flex Strasse 353113 Bonn, Germany(3)German Aero Space Center (DLR)German Remote Sensing Data Center (DFD)OberpfaffenhofenD-82234 Wessling, Germany

Soil-atmosphere exchange of N₂O, NO, CH₄ and CO₂ in natural savannah and savannah converted to agricultural land in Burkina Faso (W.Africa)

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Soil-atmosphere exchange of N₂O, NO, CH₄ and CO₂ in natural savannah and savannah converted to agricultural land in Burkina Faso (W.Africa)

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