Willow Root Development and Morphology Changes Under Different Irrigation and Fertilization Regimes in a Vegetation Filter

Determining the appropriate wastewater dose to supply nutrients while avoiding overirrigation is essential for achieving more balanced above- and belowground development of the plants, thereby ensuring high filtering functionality of a willow vegetation filter system. Aboveground biomass and root development of a 3-year-old willow vegetation filter in response to three increasing doses of secondary-treated urban wastewater under both fertilized and unfertilized conditions were evaluated. Fine, coarse and total root biomass, fine root length density (FRLD), specific fine root length (SFRL) and specific root area (SRA) were assessed in soil core samples collected at four different depths (0–20, 20–40, 40–60 and 60–80 cm) in all treatments. While aboveground biomass increased as wastewater amounts and fertilization increased (from 23.89 Mg ha−1 in non-irrigated condition to 47.07 Mg ha−1 in the highest wastewater dose, from 31.5 Mg ha−1 in unfertilized plots to 40.1 Mg ha−1 in the fertilized), total and fine root biomass decreased when willow was irrigated with the highest wastewater dose (i.e. total root biomass is 2.54 Mg ha−1 in non-irrigated condition—1.36 Mg ha−1 in the highest wastewater dose). The root biomass did not follow the same development pattern, and a significant decrease in fine root biomass was observed when the highest irrigation dose was applied. In these circumstances, we believe that the supply in water and nutrient is sufficient to the development of the willows which react by diminishing the amount of roots proportionally to the aboveground parts. This may lead to a higher transpiration rate and allow willows to avoid a surplus of water in the soil.