Experimental and numerical assessment of carbon sequestration in horticultural crops inside Mediterranean naturally ventilated greenhouses

The main objective of this work was to evaluate the CO₂ sequestration of three greenhouse horticultural crops. Experimental measurements were developed in an Almería-type greenhouse with a tomato crop in 2018 and in a multispan greenhouse with tomato and cucumber crops grown in 2021. Three experimental methods and a numerical one were used to estimate carbon sequestration. First, the maximum carbon sequestration capacity of crops was determined by measuring the net photosynthesis at plants leaves with a portable analyzer along two crop cycles. Second, the whole-canopy instantaneous carbon absorption by crop photosynthesis was estimated using the greenhouse as an open-chamber system, measuring air velocity and CO₂ concentration in all greenhouse openings. Third, the whole-canopy CO₂ absorption and the spatial distribution of the CO₂ were simulated using Computational Fluid Dynamics (CFD) in a naturally ventilated greenhouse. The 3D CFD model allowed to simulate plant photosynthesis from the computed CO₂ concentration distribution. Simulations were validated through measurements carried out in the Almería-type greenhouse with the tomato crop inside. Finally, carbon plant content was calculated from the annual dry weight of fruits, leaves, stems and roots of the three crops analyzed. Maximum photosynthesis rates was around 3.2 g CO₂ m^‑2 leaves h^‑1 for the spring-summer tomato and 2.1 g CO₂ m^‑2 leaves h^‑1 for the autumn-winter cucumber. Whole-canopy carbon sequestration in the multispan greenhouse ranged between 0.1 kg CO₂ ha^‑1 h^‑1, with low wind speed and roof windows closed in winter, and 40.8 kg CO₂ ha^‑1 h^‑1 in the greenhouse fully opened with strong wind in summer. CO₂ sequestration in plant biomass was 0.78 kg m^‑2 year^‑1 for the cucumber crop in autumn-winter and 1.21 and 3.98 kg m^‑2 year^‑1 for tomato crops in spring-summer and long cycles, respectively.