Sand fixation by willows: change of physical and chemical parameters in an indoor pot experiment

Due to fast growth and resilience, different species of willow (Salix sp.) have been historically used for sand fixation and reclamation in various regions of Europe, Asia and North America. Planting willow cuttings was proclaimed as an important step of afforestation on sandy soils, as a part of combatting desertification in semi-arid and arid regions, and for slope stability improvement. Willow planting is also applied as a bioenergy crop, and as a bioremediation measure for soils contaminated with low concentration of organic pollutants or heavy metals. These useful properties of willows are delivered not only by the plants themselves, but also by their symbionts: root-associated bacteria, ectomycorrhizal and arbuscular mycorrhizal fungi. Willow cuttings can potentially be used for creating a resilient plant cover on river embankments and coastal infrastructure which provides a living element for coastal protection.

Our objective was to estimate the changes in physical and chemical parameters of a clean heat-treated silica sand during the growth of willow cuttings. A series of controlled indoor pot experiments were performed where willow cuttings were planted in a uniform 0.65 mm silica sand for up to 150 days. Samples were systematically analysed from different pots that were disassembled on day 30, 60, 90, and 150 of the experiment. For each sampling time, we determined the shoot and root architecture and dry biomass as a proxy of overall plant health condition. We measured total and water-extractable organic carbon (TOC and WEOC), pH, DNA concentration at 3 depths in root-affected areas, in each case sampling from 4 pots. Permeability and direct shear tests were performed on pots containing the plant. Furthermore, the pullout strength required for removing willows from the sand and the tensile strength of individual roots were measured.

In agreement with expectations and similar findings for other plant types, we observed a clear trend of TOC, WEOC and DNA concentrations’ increase with time, despite the variability of willow biomass in replicated pots. Dry biomass of shoots and roots also increased. pH remained in range 7.5-8.5. Pullout strength was obviously affected by plant age and health condition: it was higher in case of better-established willow cuttings with higher root and shoot dry biomass. However, root tensile strength was comparable for roots sampled at different times; presumably, due to the constant growth of roots and presence of relatively young roots in all pots. Permeability values were constant within the same order of magnitude; there was no clear trend of its change with time. Despite localized root formation close to the pot walls, sand aggregation around roots was observed in pots sampled at day 150 for roots close to the stem in the middle of the pot. Conclusively, our findings show that the growth of willow cuttings leads to sand fixation by direct root reinforcement and aggregation in a time frame of 5 months.

This work is part of the project SOil Is Alive (SoIA) granted by the Carlsberg Foundation as part of the consolidator excellence grant Semper Ardens: Accelerate.