Nutrient dynamics and GHG emissions in Azolla and Typha based cultivation on inundated former agricultural soils

Restoration and novel creation of wetlands is crucial as they store and purify water, sequester carbon, and are biodiversity hotspots. However, wetland rehabilitation on agriculturally-used soils typically causes water quality issues, low biodiversity and high methane emissions. To tackle these challenges in a novel, cost-effective way, Azolla filiculoides, a water-fern capable of nitrogen fixation and phosphorus (P) accumulation, could be cultivated after inundation to simultaneously extract nutrients and provide a commercial product.

We cultivated A. filiculoides and a polyculture of A. filiculoides and Typha angustifolia, an emergent macrophyte, on two P-rich former agricultural mineral soils in an outdoor mesocosm experiment during two years. We measured nutrient dynamics in soil, water, and biomass, diffusive and ebullitive methane (CH₄) emissions, and nitrous oxide emissions.

Open water controls showed substantial P mobilisation to the surface water and were dominated by microalgae or emergent macrophytes. Azolla cultivation lowered surface water P concentrations, but did not negate them in the most P rich soil. Infestation with the Azolla weevil (Stenopelmus rufinasus) severely constrained Azolla growth. Thus, P extraction rates were moderate: up to 38 kg ha^-1 yr^-1 in the Azolla monoculture, and 67 kg ha^-1 yr^-1 in the polyculture with T. angustifolia. Methane emissions were substantial and ebullition-dominated in all treatments, and not affected by Azolla cultivation.

Azolla cultivation shows potential in the transition from agriculture to wet nature, while recovering P from former agricultural soils. Remaining challenges include pest control, product development, and technologies for large-scale implementation.

The online version contains supplementary material available at 10.1007/s11104-025-08032-y.