Rainfall regimes control C-exchange of Mediterranean olive orchard

We investigated the relationship between Net Ecosystem Exchange (NEE), energy partitioning and the main climate variables (rainfall, temperature and solar radiation) in a typical Mediterranean rainfed olive orchard. Eddy covariance measurements covered three contrasting and extreme years, spanning over 90% of the long-term rainfall variability. Across those years, the olive orchard resulted overall a net carbon sink (3.6 Mg ha(-1) year(-1)) comparable to several Italian forestry systems. Annual and seasonal NEE was found to be mostly driven by rainfall regimes and their seasonal variability. More specifically, higher spring-like rainfall increased both spring and summer NEE. On the contrary, monthly scale NEE was poorly correlated with rainfall (r = -0.286; p-value = 0.091), whilst good correlations were observed with air temperature (r = 0.473; p-value = 0.003) and solar radiation (r = 0.541; p-value = 0.001). These results indicated that temperature and light played an important role in regulating NEE, but rainfall amounts and timing represent the most important drivers of carbon storage of rainfed olive orchard in dry environment as Mediterranean basin. Future expected reduction in rainfall pattern, with less rainy days and longer drought periods especially in the warm season, will decrease the carbon sequestration capacity of this agro-system, requiring the adoption of agronomic practices aimed at enhancing the soil water retention capacity.