How does irrigation with treated wastewater affect soil oxygen dynamics in avocado (Persea americana) orchards planted in clayey soils and implications to plant Na homeostasis

Treated wastewater (TWW) is a promising source of irrigation-water worldwide. However, it is now evident that irrigation with TWW can have negative implications on orchards, specifically avocado and citrus orchards planted in clayey soils. Increasing evidence is pointing toward lack of oxygen as having a role in the damage. In order to understand the soil dynamics leading to the lack of oxygen in TWW irrigated orchards we performed continuous measurements of soil gaseous-oxygen and soil water content in the root-zone (35 cm depth) of ‘Hass’ avocado trees irrigated with either TWW or fresh water (FW) together with root growth and mineral content analysis. Marked differences between treatments appeared in the soil water dynamics – the changes in volumetric water content during the day and night driven by evapotranspiration and drainage respectively were both three-fold smaller in the TWW plots as compared to the FW plots. The retarded drying in TWW irrigated plots led to longer periods with soil moisture values that largely prevented oxygen penetration to the root-zone, leading to longer periods with low oxygen compared with FW-irrigated plots. A negative correlation was observed between root growth and the period with oxygen deprivation, so that trees exposed to longer periods of hypoxia had smaller roots mass. In addition, roots in TWW irrigated trees had three times larger Na concentration, which also negatively correlated with their growth. These findings support the idea that there is a negative feedback process in TWW irrigated avocado by which hypoxia leads to low Na selectivity by the roots and to deteriorated root growth, this is turn constraints the plant water uptake so that wetter and more hypoxic conditions are instigated. Agro-technical solutions devised based on this insight are currently being tested and will be presented.