Eddy covariance measurement of CO₂ concentration and turbulent flux above cowpea (<i>Vigna unguiculata</i> [L.] Walp) at an agricultural site in Ile-Ife, Nigeria

Abstract

This study investigated Net Ecosystem Exchange (NEE) over a leguminous plant, cowpea (Vigna unguiculata [L.] Walp) at an agricultural site in Ile-Ife, Nigeria, monitored during two transition seasons: dry-to-wet (March - June) and wet-to-dry (August - November) in 2015. Measurements of carbon dioxide (CO₂) concentration and turbulent flux were made by eddy-covariance (EC) technique alongside Photosynthetically Active Radiation (PAR) and net radiation. Physiological parameters (e.g., Leaf Area Index, plant height and surface albedo) were observed throughout the growth stages of the plant from emergence to senescence. High levels of CO₂ mass concentrations, 850.0 mgm^-3 - 1200.0 mgm^-3, were found during the nighttime periods which are adduced to increased soil and plant respirations. In the late afternoons, around 1500 hrs LT, pronounced minima with values of about 650.0 mgm^-3 were recorded. The drop in the observed value of CO₂ concentration during the daytime is largely due to photosynthetic activity which increases as PAR values increase. The intensity of PAR (maximum value recorded was about 1800 Wm^-2 ) is a dominant factor responsible for the depletion of CO₂ , from emergence to senescence. Based on the net production of CO₂ at the study site, it can be concluded there was an uptake of carbon by the cowpea, irrespective of the developmental stage of its growth.

Keywords: Cowpea (Vigna unguiculata [L.] Walp), developmental stages, CO₂ concentration and flux, Net Ecosystem Exchange, seasonal transitions