LAUSR

Abstract Soil respiration (Rs) represents the largest CO2 efflux to the atmosphere of terrestrial ecosystems. The regular alternation of wet and dry soil strips prevailing in irrigated orchards is a singular case-study among ecosystems as it may help to understand the impact of the spatial variability of abiotic factors on soil respiration in orange orchards having contrasting age and structural heterogeneity. We characterised Rs in adult (AO) and young (YO) drip-irrigated orange orchards with special focus on the main factors driving Rs site-by-site and across-sites. Tree structure (leaf area index, LAI) was measured fortnightly during two years, whereas Rs, soil temperature (Ts) and moisture (θs) were measured during the same period in dry and wet longitudinal transects parallel to the tree rows. The data sets collected in the field were used to carry out a modelling exercise aimed to predict Rs from only abiotic factors (here Ts and θs) or from a combination of abiotic and biotic (LAI) factors. The modelling results indicated that the seasonal changes of Rs were temperature-driven in predominantly wet transects and moisture-driven in predominantly dry ones; and that the emerging seasonal pattern of spatially-weighted mean soil respiration (Rs,mean) was mainly explained by abiotic factors. An abiotic model assuming a constant value of the reference respiration (Rb,10 = Rs at Ts = 10 °C) was found suitable to describe the temporal variations in Rs,mean within an orchard, but was not able to reproduce the differences in Rs between orchards (R2 = 0.20). The model fitness was strongly improved (R2 = 0.80) when Rb,10 was assumed an increasing function of LAI, suggesting that LAI might be a suitable biotic variable for predicting the magnitude of soil respiration in citrus orchards.