Monitoring soil greenhouse gas (GHG) emissions in a Sahelian agrosilvo-pastoral parkland
Authors: Ba Seydina Mohamad , Roupsard Olivier , Chapuis-Lardy Lydie , Bouvery Frédéric , Diongue Djim Mouhamadou Lamine , Agbohessou Yélognissè Frédi , Guérin Frédéric , Tagesson Håkan Torbern , Sambou Bienvenu , Serça Dominique 
Published: March 2022

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&lt;p&gt;Estimates of greenhouse gas (GHG) emissions from soil are essential to understand possible climate change mitigation from ecosystems. There is currently very limited and reliable information on GHG emission factors for most land-use types of Sahelian Africa. GHG (CO&lt;sub&gt;2&lt;/sub&gt;, H&lt;sub&gt;2&lt;/sub&gt;O, CH&lt;sub&gt;4&lt;/sub&gt;, N&lt;sub&gt;2&lt;/sub&gt;O) and ammonia (NH&lt;sub&gt;3&lt;/sub&gt;) emissions were measured in a Sahelian agro-silvo-pastoral parkland dominated by &lt;em&gt;Faidherbia albida&lt;/em&gt; trees (Niakhar, Senegal) using 8 automatic chambers coupled to a Picarro G2508 gas analyser. The measurements were carried out in 2021 covering the late dry season (bare soil), the full rainy season (with groundnut plants in the chambers) and the beginning of the next dry period (senescent vegetation and bare soil). The chamber-based CO&lt;sub&gt;2&lt;/sub&gt; fluxes were compared to the Net Ecosystem Exchange of CO&lt;sub&gt;2&lt;/sub&gt; (NEE) as measured by a 4.5m-eddy covariance tower (below tree crowns) installed over the same agro-silvo-pastoral field. To avoid small scale heterogeneity, we compared here EC fluxes with chamber measured fluxes far from the &lt;em&gt;Faidherbia albida&lt;/em&gt; area of influence. Indeed, for a given day, soil CO&lt;sub&gt;2&lt;/sub&gt; respiration is significantly higher under trees (shade) than far from trees (full sun) due to trees &amp;#8216;island effect&amp;#8217; (p&lt;0.0001).&lt;/p&gt;
&lt;p&gt;Soil CO&lt;sub&gt;2&lt;/sub&gt; respiration was very low at the end of the dry season, with an average of about 0.6 &amp;#181;mol CO&lt;sub&gt;2&lt;/sub&gt; m&lt;sup&gt;-2 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;. During the wet season, the maximum soil respiration at night was about 5 &amp;#181;mol CO&lt;sub&gt;2&lt;/sub&gt; m&lt;sup&gt;-2 &lt;/sup&gt;s&lt;sup&gt;-1 &lt;/sup&gt;and &amp;#160;the &amp;#160;maximum &amp;#160;net &amp;#160;CO&lt;sub&gt;2&lt;/sub&gt; &amp;#160;uptake &amp;#160;during &amp;#160;the &amp;#160;day &amp;#160;was &amp;#160;around -6 &amp;#181;mol CO&lt;sub&gt;2&lt;/sub&gt; m&lt;sup&gt;-2 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;. Only negligible fluxes of CH&lt;sub&gt;4&lt;/sub&gt;, N&lt;sub&gt;2&lt;/sub&gt;O and NH&lt;sub&gt;3&lt;/sub&gt; were recorded for all seasons. The low N&lt;sub&gt;2&lt;/sub&gt;O fluxes could be related to low soil fertility and lack of nitrogen supply, and low soil moisture in these sandy soils does not favor soil gas production processes for both N&lt;sub&gt;2&lt;/sub&gt;O and CH&lt;sub&gt;4&lt;/sub&gt;. The CO&lt;sub&gt;2&lt;/sub&gt; fluxes from the automatic chambers showed similar typical semi-arid ecosystem patterns as that of the EC tower. We saw large emission peaks during the first rain events of the rainy season, positive and negative fluxes at night and day, respectively, high fluxes when the soil was wet, and decay during the next dry season. However, in average the soil CO&lt;sub&gt;2&lt;/sub&gt; respiration magnitude of the chambers with groundnut plant were much lower (1.26 &amp;#181;mol CO&lt;sub&gt;2&lt;/sub&gt; m&lt;sup&gt;-2 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;) than the ecosystem respiration as seen from the EC tower (3.74 &amp;#181;mol CO&lt;sub&gt;2&lt;/sub&gt; m&lt;sup&gt;-2 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;), and the difference was even worse for diurnal net CO&lt;sub&gt;2&lt;/sub&gt; uptake (by a factor of 7).&lt;/p&gt;
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