Spatial variability of soil CO2 efflux in drip-irrigated old and young citrus orchards and its dependence on biotic and abiotic factors

- Soil respiration (Rs) was measured in adult and young orange orchards.
- The spatial distribution of soil moisture and root density are affected by both cover fraction and irrigation.
- Fine-root density explains a great part of the spatial distribution of Rs.
- The ratio Rh/Rs decreases with stand age, Rh being predominant in the young orchard and Rr in the old one.
- Stand structure dictates the spatial variability and partitioning of Rs at tree scale.

Quantification of soil respiration (Rs) is an essential step in modelling soil carbon fluxes associated with stand structure and management. This study aims to characterise the spatial variations of Rs at tree scale in adult (AO) and young (YO) drip-irrigated orange orchards, with special focus on abiotic (soil moisture, θs) and biotic (stand age and root traits) factors determining the variability of Rs. Rs, soil temperature (Ts), θs, soil organic carbon content (OC) and root traits were sampled along three transects parallel to the drip-line. Measurements were carried out in autumn, with mean Ts over the sampling area close to 18 °C. Heterotrophic respiration (Rh) was determined in undisturbed non-rooted soil, and root-derived (or rhizosphere) respiration (Rr) was estimated from the difference Rs − Rh. Root distribution and root density were closely correlated with θs. Fine-root density appeared to play a major role in the control of Rs, explaining a great part of the spatial variability of Rs. The ratio Rh/Rs, estimated by means of a simple partitioning model, appeared to decrease with stand age (~ 0.68 and ~ 0.32 in YO and AO, respectively), indicating the predominance of Rh in the young orchard and of Rr in the old one. The study highlights that stand structure plays a determinant role in driving the spatial variability of Rs and its partitioning between heterotrophic and root-derived respiration in irrigated orchards.