To investigate whether soil clay content, cultivar and seasonal variation have any effect on soil CO₂ emission rates and leaf CO₂ assimilation rates in a drip-irrigated commercial Citrus sinensis orchard.

The study was carried out in the field as a randomised complete block design in a 2 × 2 factorial consisting of two soil types and two citrus cultivars on a drip-irrigated commercial Citrus sinensis orchards with 2-week interval measurements of soil CO₂ emission and leaf gas exchanges for a year.

Soil clay content did not influence plant CO₂ assimilation rates and soil CO₂ emission rates in irrigated citrus. However, seasonal variation significantly influenced both processes. Soil CO₂ emission rates were highest in summer and were more than double the rates observed in winter while leaf CO₂ assimilation rates were highest in autumn and four times higher than the winter season rates. Mean seasonal soil CO₂ emission rates were strongly influenced by mean minimum seasonal temperatures while leaf CO₂ assimilation rates only showed a relatively weak relationship with mean maximum seasonal temperatures.

Soil clay content did not influence soil CO₂ emission and assimilation rates in drip irrigated citrus suggesting a non-significant effect of clay content for soils subjected to similar management practices. Citrus CO₂ assimilation rate peaks in the autumn while soil CO₂ emission rates peak in summer. A snapshot analysis of CO₂ sequestration rates suggests that irrigated citrus orchards are net sinks of CO₂ in summer, autumn and winter season.