Carbon balance of citrus plantations in Eastern Spain

• The carbon assimilation of representative citrus plantations in Eastern Spain was estimated.
• Our plantation was responsible for a net carbon fixation rate of higher than 10 Mg C ha−1 yr−1.
• Leaf assimilatory processes mainly determined the annual pattern of C sequestration.
• Under typical culture conditions, soil respiration rates accounted for low C losses.
• The results of this study are relevant to climate change mitigation.

Global warming due to the continuous rise in CO2 emissions has been documented in the last few decades. This work is a first effort to estimate the net carbon incorporation in citrus plantations cultivated under typical land use. The approach involves a biomass-based study of carbon accumulation and a complementary analysis of the associated CO2 fluxes. The total C content allocated to trees aged 2–14 years was determined through the direct and destructive harvesting of all tree organs. A stable pattern of biomass production in tree components was observed in plants 12 years old and older and was responsible for the sequestration of more than 50 kg C tree−1. Annual C fixation in fruit and new vegetative flushes accounted for up to approximately 75% of the total amount sequestered per year, whereas the contribution of the old, permanent organs (branches, trunk, and tap-coarse roots) was minor (approximately 25%). Further experiments were conducted on adult 12-year-old trees to confirm the data and determine the particular contribution of CO2 fluxes from tree organs and soil to the final values. Data revealed that leaves were responsible for a total net C fixation of 15.4 Mg C ha−1 yr−1 (higher than 55% of the total C fixed). The complementary, regular monitoring of fruit respiration rates showed that fruit respiration played only a minor role, responsible for the emission of 2.3 Mg C ha−1 yr−1. Minimum losses were also found when soil respiration rates were investigated, accounting for a total annual C loss of 2.7 Mg C ha−1 yr−1.Taken together, these results indicate that our plantation was responsible for a net C fixation of close to 10 Mg C ha−1 yr−1. Assimilatory processes in leaves accounted for the highest proportion of C allocated to the tree, while losses due to leaf and fruit respiration were of minor importance. Under typical culture conditions (drip irrigation and absence of ground cover), soil respiration rates accounted for a low level of C loss to the atmosphere. Because citrus is the second largest fruit crop cultivated in the EU, such data are very relevant to the mitigation of climate change.