Instantaneous Measurement of Radiation and Water Use Efficiencies of a Maize Crop

Field-scale measurement of CO₂ flux between vegetation and the atmosphere is a direct way to quantify short-term performance of an agricultural crop through the growing season. This study measured CO₂ and H₂O exchange in a maize (Zea mays L.) field during a growing season, in order to (i) estimate crop net photosynthesis (Pₙ) and canopy nighttime respiration and (ii) calculate crop radiation and water use efficiencies. Maize net photosynthesis was calculated for 850 h (day) and canopy respiration for 175 h (night) from measurements of CO₂ fluxes above the crop and estimates of CO₂ fluxes at the soil surface. Under high radiation, Pₙ reached 2 mg m⁻² s⁻¹ at 30 d after emergence, remained between 2 and 2.5 mg m⁻² s⁻¹ for the next 30 d, and then slowly decreased until first frost. Two negative exponential equations were proposed to describe the relationship between Pₙ and intercepted photosynthetically active radiation (IPAR) during the growing season: one from planting to maximum leaf area index (LAIₘₐₓ) and one from LAIₘₐₓ to the first fall frost. These results confirmed that, in absence of water stress and with adequate fertilization, simple models based on IPAR could account for about 90% of the variation in Pₙ. The higher efficiency of diffuse than of direct beam radiation was documented. Instantaneous radiation use efficiency (IRUE) was shown to decrease by 66% from cloudy to clear sky conditions, so it is desirable to incorporate IRUE for estimation of short-term (hourly) P. Canopy respiration rates at night ranged from 0.1 to 0.2 mg m⁻² s⁻¹ for LAI between 1 and 3, or about 10% of daily photosynthesis. The relationship between Pₙ and water vapor flux (Fq,ₐ) was nonlinear, with slope decreasing with increasing Fq,ₐ. Water use efficiency at LAIₘₐₓ was 17 mg g⁻¹ for Fq.ₐ = 0.05 g m⁻² s⁻¹ and about 10 mg g⁻¹ for Fq,ₐ = 0.20 g m⁻² s⁻¹. Direct evaporation of water from the soil surface (at low LAI) or from wet plant parts resulted in considerable noise in the Pₙ - Fq,ₐ relationship. The normalization of Fq,ₐ by the vapor pressure deficit (VPD) linearized the relationship. The slope of the Pₙ - Fq,ₐ/VPD curve was larger for cloudy than for clear sky conditions, probably as a result of the larger RUE under diffuse radiation. CLBRR Contribution no. 95-60.