Spatial and temporal heterogeneity of soil respiration in a bare-soil Mediterranean olive grove

Abstract. Soil respiration (Rs) is an important carbon flux in terrestrial ecosystems and knowledge about this CO2 release process and the drivers involved is a key topic in the context of global change. However, temporal, and spatial variability has not been extensively studied in semiarid systems such as olive groves. In this study, we show a full year of continuous measurements of Rs with six automatic chambers in a fertirrigated olive grove with bare soil in the Mediterranean accompanied by ecosystem respiration (Reco) obtained using the eddy covariance (EC) technique. To study spatial variability, the automatic chambers were distributed equally under the canopy (Rs Under-Tree) and in the center of the alley (Rs Alley), and the gradient of Rs between both locations was measured in several manual campaigns in addition to azimuthal changes about the center of the olive trees. The results indicate that Rs Under-Tree was three times larger than Rs Alley in the annual computations. Higher Rs was found on the south face, and an exponential decay of Rs was observed until the alley's center was reached. These spatial changes were used to weigh and project Rs to the ecosystem scale, whose annual balance was 1.6–2.3 higher than Reco estimated using EC-derived models. The daytime Reco model performs better the greater the influence of Rs Under-Tree and the night-time Reco model and Rs covaried more the higher the fraction of Rs Alley. We found values of Q10 < 1 in the vicinity of the olive tree and Rs Under-Tree represented 39 % of the Rs of the olive grove. CO2 pulses associated with precipitation events were detected, especially in the alley, during dry periods, and after extended periods without rain, but were not accurately detected by EC-derived models. We point out an interaction between several effects that vary in time and are different under the canopy than in the alleys that the accepted models to estimate Q10 and Reco do not consider. These results show a high spatial and temporal heterogeneity in soil respiration and the factors involved, which must be considered in future work in semi-arid agrosystems.