UC Davis

Soil hypoxia occurs when soil oxygen is used by roots and soil organisms at a faster rate than it is replenished by diffusion through the soil. Once soil oxygen levels fall below the amount necessary to support aerobic cellular respiration, new root production is reduced, water and nutrients uptake decline and energy demand switches from active ion uptake to cellular maintenance. In the Mediterranean climate of California, soil hypoxic conditions happen typically in periods of high rainfall, but they can also occur during the growing season if a high amount of irrigation water is applied. As the diffusion of oxygen through water filled pores is approximately 10,000 times slower than through air filled pores, the concentration of soil oxygen can drop rapidly, especially if root respiration is high. To test the impact of hypoxia on root growth and activity at different phenological stages, we flooded the root systems of young (5th leaf) almond (Prunus dulcis) trees three times for 3 days during the growing season and measured root population dynamics and root/soil activity.

We found that there were no impacts of saturating the rootzone of almond trees for three consecutive days in either late spring, summer, and fall, even though the treatments succeeded in creating hypoxic conditions. We had hypothesized that hypoxia would have the greatest negative impact in the spring when most new fine root production was expected to occur. However, due to very dry conditions and resulting little root production prior flooding, we observed that most root production occurred in the two weeks after each spring and summer saturation event. We concluded that this response is likely due to drought alleviation reducing soil strength and enhancing root production, rather than a response to the short-term hypoxic conditions. Future field studies will need to incorporate higher irrigation and perhaps longer saturation treatments.