Evaluating carbon fluxes in orange orchards in relation to planting density G.LIGUORI 1 *, G.GUGLIUZZA 2 AND P.INGLESE 1 1 Dipartimento di Colture Arboree, viale delle Scienze, 90128 Palermo, Italy 2 Consiglio per la Ricerca e la Sperimentazione in Agricoltura, S.S. 113, km 245.5, 90111 Bagheria, Italy (Revised MS received 21 April 2009; First published online 2 June 2009) SUMMARY Orange (Citrus sinensis L.) is one of the main fruit crops worldwide and its evergreen orchards may have a great potential for carbon (C) sequestration, but no data are currently available. In order to understand carbon fluxes in orange orchards, an experiment was undertaken on traditional and intensive planting systems. The experiment used C. sinensis scions grafted onto Citrus aurantium (bitter orange) rootstock. One orchard contained 14-year-old trees of the cv. Tarocco Scire` (a blood orange) grown in a traditional system with 494 trees/ha. The second orchard contained 12-year-old trees of the cv. Newhall (a seedless navel orange) grown in an intensive system with 1000 trees/ha. Net primary productivity (NPP) was obtained by measuring the annual canopy growth of single orange trees and the above ground dry biomass of the ground cover ; soil respiration seasonal pattern was measured with an infrared gas analyser (EGM-4, PP System) from June 2005 to May 2006, every 2 weeks from 12 . 00 noon to 15 . 00 h for maximum respiration and from 02 . 00 to 05 . 00 h for minimum respiration ; a 24 h cycle measurement of soil respiration was made every 3 months. Carbon fixation in the fruits and in the canopy of single trees was almost twice as much (10 . 7 kg C/ tree) in the traditional than in the intensive system (5 . 5 kg C/tree) ; however, total NPP of the orchard did not change with planting density, being 5 . 3 t C/ha/year in the traditional system and 5 . 5 t C/ha/ year in the intensive one. Carbon fixation by the ground cover was higher in the traditional (1 . 1 t C/ ha/year) than in the intensive system (0 . 5 t C/ha/year). Annual soil respiration was 5 . 9 t C/ha/year in the traditional system and 4 . 2 t C/ha/year in the intensive one. The carbon balance was almost four times higher in the intensive system (1 . 8 t C/ha/year) than in the traditional one (0 . 5 t C/ha/year), due to large differences in soil respiration. INTRODUCTION The amount of carbon (C) partitioned to the canopy and root components of the trees depends on a num- ber of factors, which include genotype, tree age (Chalmers & Van Den Ende 1975; Palmer 1988), orchard density and training systems (Robinson & Lakso 1991 ; Caruso et al. 1999), scion/rootstock in- teraction in terms of vigour (Glenn & Scorza 1992; Caruso et al. 1997), fruit production (Maggs 1963) and orchard management (Pace et al. 2000; Inglese et al. 2002). Most of the increases of orchard yields realized in the last few decades can be linked to an increase of the harvest index (HI), i.e. the fraction of annual dry matter allocated to the fruits (Forshey & McKee 1970). Indeed, the efficiency of horticultural systems can be evaluated in terms of light use (Robinson & Lakso 1991) and it is commonly related to high C partitioning to the fruit fraction and a lower carbon stock of ‘ unproductive wood ’ parts (Forshey & McKee 1970), which are considered a net cost in terms of energy and nutrient allocation (Daie 1985). The strategy applied to orchard systems of most fruit tree crops has been to use high-density planta- tions with small-sized trees, obtained through the * To whom all correspondence should be addressed. Email : giorgial@unipa.it Journal of Agricultural Science (2009), 147, 637–645. f Cambridge University Press 2009 637 doi:10.1017/S002185960900882X Printed in the United Kingdom