Soil-Atmosphere Exchange of N2O and NO in Near-Natural Savanna and Agricultural Land in Burkina Faso (W. Africa)

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In a combined field and laboratory study in the southwest of Burkina Faso, we quantified soil-atmosphere N2O and NO exchange. N2O emissions were measured during two field campaigns throughout the growing seasons 2005 and 2006 at five different experimental sites, that is, a natural savanna site and four agricultural sites planted with sorghum (n = 2), cotton and peanut. The agricultural fields were not irrigated and not fertilized. Although N2O exchange mostly fluctuated between −2 and 8 μg N2O-N m−2 h−1, peak N2O emissions of 10-35 μg N2O-N m−2 h−1 during the second half of June 2005, and up to 150 μg N2O-N m−2 h−1 at the onset of the rainy season 2006, were observed at the native savanna site, whereas the effect of the first rain event on N2O emissions at the crop sites was low or even not detectable. Additionally, a fertilizer experiment was conducted at a sorghum field that was divided into three plots receiving different amounts of N fertilizer (plot A: 140 kg N ha−1; plot B: 52.5 kg N ha−1; plot C: control). During the first 3 weeks after fertilization, only a minor increase in N2O emissions at the two fertilized plots was detected. After 24 days, however, N2O emission rates increased exponentially at plot A up to a mean of 80 μg N2O-N m−2 h−1, whereas daily mean values at plot B reached only 19 μg N2O-N m−2 h−1, whereas N2O flux rates at plot C remained unchanged. The calculated annual N2O emission of the nature reserve site amounted to 0.52 kg N2O-N ha−1 a−1 in 2005 and to 0.67 kg N2O-N ha−1 a−1 in 2006, whereas the calculated average annual N2O release of the crop sites was only 0.19 kg N2O-N ha−1 a−1 and 0.20 kg N2O-N ha−1 a−1 in 2005 and 2006, respectively. In a laboratory study, potential N2O and NO formation under different soil moisture regimes were determined. Single wetting of dry soil to medium soil water content with subsequent drying caused the highest increase in N2O and NO emissions with maximum fluxes occurring 1 day after wetting. The stimulating effect lasted for 3-4 days. A weaker stimulation of N2O and NO fluxes was detected during daily wetting of soil to medium water content, whereas no significant stimulating effect of single or daily wetting to high soil water content (>67% WHCmax) was observed. This study demonstrates that the impact of land-use change in West African savanna on N trace gas emissions is smaller—with the caveat that there could have been potentially higher N2O and NO emissions during the initial conversion—than the effect of timing and distribution of rainfall and of the likely increase in nitrogen fertilization in the future.

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