Biomass production and control of nutrient leaching of willows using different planting methods with special emphasis on an appraisal of the electrical impedance for roots

In many countries there is an increasing interest in willows to be used in biomass production and environmental projects. Vegetative reproduction can be achieved through vertically or horizontally planted cuttings. Conventionally, willow plantations are established by inserting cuttings vertically into the soil. There is, however, a lack of information about the biomass production and its effect on the reduction of nutrient leaching of plantations established through horizontally planted cuttings. A greenhouse experiment with potted cuttings and a field trial lasting three years were carried out to investigate whether horizontally planted Salix schwerinii cuttings have a positive effect on stem yield, root distribution and nutrient leaching in comparison with vertically planted cuttings with different planting densities. The height of the shoots of horizontally planted cuttings was significantly smaller than that of vertically planted cuttings in the pot experiment, but only during the first two weeks after planting. Thereafter, no significant effect of planting orientation on the stem biomass was observed in the two experiments that were conducted. In both experiments the total stem biomass increased with the planting density. It was also found that the fine root biomass and the specific root length were not affected by the planting orientation or density, while the fine root surface area and the absorbing root surface area (ARSA) were affected only by the planting density. The planting orientation did not affect the nutrient concentrations in the soil leachate, apart from SO4-S and PO4-P in the pot experiment. The horizontally planted cuttings were slightly more effective for reducing the SO4-S leaching and the vertically planted cuttings were slightly more effective for reducing PO4-P leaching. Lower PO4-P leaching was observed only in the context of higher planting density. The ARSA in the pot experiment was assessed by using the earth impedance method. The applicability of this method was further evaluated in a hydroponic study of willow cuttings where root and stem were measured independently. The results showed that electrical resistance had a good correlation with the contact area of the roots with the solution. However, the resistance depended strongly on the contact area of the stem of cuttings with the solution, which caused a bias in the evaluation of root surface area. A similar experimental set-up in hydroponics but combining an approach using of electrical impedance spectroscopy was employed to study the relationship between the electrical parameters and root morphology of willow cuttings. A good fit was obtained between the impedance spectra data and the corresponding proposed lumped models. The model parameters were correlated with the contact area of roots and/or stem in the hydroponic solution. In conclusion, the horizontal planting method can be used as an alternative method in connection with short-rotation coppice willow used for bioenergy and nutrient leaching. Electrical impedance spectroscopy is a promising new non-destructive method in root research, but further more laboratory and field studies are undoubtedly needed.