Seasonal CO2 fluxes and energy balance in a kiwifruit orchard.

This study reports the seasonal assessment of energy balance components and carbon dioxides fluxes over a drip irrigated kiwifruit orchard, with a Leaf Area Index (LAI) of 3 at full canopy development. The most remarkable amount of energy partitioning throughout the whole season was due to latent heat flux that had values approximating to those of sensible heat flux (about 200 W m-2 s-1 at midday) only in May, when leafy biomass was still developing. From June to September, latent heat flux kept constantly at about 50% of net radiation (Rn), and did not appear to be influenced by the air temperature, very high in its maximum values during the three months of June, July, August. Canopy carbon assimilation reached maximum values of about ¡V14 ƒÝmol m ¡V2 s ¡V1 during May and June, and values decreased in July and August respectively to ¡V12 and ¡V10 ƒÝmol m ¡V2 s ¡V1 , turning to ¡V13 ƒÝmol m ¡V2 s ¡V1 again in September. Going on with the season, a marked decrease of the canopy photosynthesis was observed. A diurnal decline in photosynthesis similar to that recorded from single leaf measurements is reported and an amount between 60% and 75% of the total carbon is fixed by the orchard in the morning between 7 and 12.30. The seasonal timing and the amplitude of the net system carbon flux as well as the ones of the components fluxes corresponding to assimilatory and respiratory processes has been quantified. The Net Ecosytem Exchange (NEE) of the kiwifruit system was at around 3 t of carbon during the seven months leafy period (useful to produce 12.5 t total biomass ha-1 , more than 50% partitioned to perennial structures,), when 8.5 t of carbon were lost in ecosystem respiration. Such fluxes are the sum of contribution to both kiwifruit plants and cover crop, present during all the season. Daily and seasonal canopy water use efficiency (WUE) is also reported. Midday averaged monthly values indicate that WUE is 0.0027 and 0.0029 in May and June respectively, and decreases to 0.0022 in July up to a minimum of 0.0018 in August. WUE values increase after that time reaching higher values in September (0.0033) and October (0.0046). In each month, the diurnal trend shows constant higher water use efficiencies in the morning hours, evidently matching the photosynthetic daily patterns.