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 studied extensively in semi-arid systems such as olive groves. In this study, we show a full year of continuous measurements of Rs with six automatic chambers in a fertigated olive grove with bare soil in the Mediterranean accompanied by modeled ecosystem respiration (Reco) estimated by decomposing net ecosystem exchange (NEE) measured 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 (RsAlley), and the gradient of Rs between both locations was measured in several manual campaigns in addition to angular changes about the olive trees. The results indicate that Rs Under-Tree was 3 times higher than RsAlley in the annual computations. Higher Rs was found on the southern 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 onto the ecosystem scale, whose annual balance was 1.6-2.3 times higher than the Reco estimated using EC-derived models. Rs Under-Tree represented 39 % of the Rs of the olive grove. We found values of Q10<1 in the vicinity of the olive tree in the warm period. Outbursts of CO2 emissions associated with precipitation events were detected, especially in the alley, during dry periods and after extended periods without rain, but they were not accurately detected by EC-derived respiration models. We point out an interaction between several effects that vary in time, that are different under the canopy than in the alleys, and that the accepted models for estimating Q10 and Reco do not consider. These results show high spatial and temporal heterogeneity in soil respiration and the factors involved, which must be considered in future works in semi-arid agroecosystems.