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

Abstract

Abstract. Soil respiration (R_s) 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 R_s with six automatic chambers in a fertirrigated olive grove with bare soil in the Mediterranean accompanied by ecosystem respiration (R_eco) obtained using the eddy covariance (EC) technique. To study spatial variability, the automatic chambers were distributed equally under the canopy (R_{s Under-Tree}) and in the center of the alley (R_{s Alley}), and the gradient of R_s 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 R_{s Under-Tree} was three times larger than R_{s Alley} in the annual computations. Higher R_s was found on the south face, and an exponential decay of R_s was observed until the alley's center was reached. These spatial changes were used to weigh and project R_s to the ecosystem scale, whose annual balance was 1.6–2.3 higher than R_eco estimated using EC-derived models. The daytime R_eco model performs better the greater the influence of R_{s Under-Tree} and the night-time R_eco model and R_s covaried more the higher the fraction of R_{s Alley}. We found values of Q₁₀ < 1 in the vicinity of the olive tree and R_{s Under-Tree} represented 39 % of the R_s 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 R_eco 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.