Monitoring soil greenhouse gas (GHG) emissions in a Sahelian agrosilvo-pastoral parkland

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 (CO₂, H₂O, CH₄, N₂O) and ammonia (NH₃) emissions were measured in a Sahelian agro-silvo-pastoral parkland dominated by Faidherbia albida 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 CO₂ fluxes were compared to the Net Ecosystem Exchange of CO₂ (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 Faidherbia albida area of influence. Indeed, for a given day, soil CO₂ respiration is significantly higher under trees (shade) than far from trees (full sun) due to trees ‘island effect’ (p<0.0001).

Soil CO₂ respiration was very low at the end of the dry season, with an average of about 0.6 µmol CO₂ m^-2 s^-1. During the wet season, the maximum soil respiration at night was about 5 µmol CO₂ m^-2 s^-1 and  the  maximum  net  CO₂  uptake  during  the  day  was  around -6 µmol CO₂ m^-2 s^-1. Only negligible fluxes of CH₄, N₂O and NH₃ were recorded for all seasons. The low N₂O 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 N₂O and CH₄. The CO₂ 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 CO₂ respiration magnitude of the chambers with groundnut plant were much lower (1.26 µmol CO₂ m^-2 s^-1) than the ecosystem respiration as seen from the EC tower (3.74 µmol CO₂ m^-2 s^-1), and the difference was even worse for diurnal net CO₂ uptake (by a factor of 7).