# Willows Used for Phytoremediation Increased Organic Contaminant Concentrations in Soil Surface
## 4 Discussion
### Cutting Trees Did Not Remove the Roots  
Since willows are known for their great potential to reduce deep percolation and leaching of contaminants in ET cover applications [13], we would have expected to find stronger evidence of decreasing contamination under the Cut treatment over time. However, as the experimental site was an open system without any physical barriers between plots, the complete removal of the aerial parts of the willows in the Cut plots may not have totally stopped the impact on soil water dynamics. Due to the complexity associated with the excavation of complete root systems, few studies have investigated the root length of willows under SRIC conditions. [PERSON] et al. [42] were able to observe the belowground plant growth of willows and poplars and reported lateral root spread of 5-11 m from the stem of _Salix matsudana_, after only nine months of growth (270 days) in New Zealand. It is therefore possible to assume that each individual seven-year-old root system in the present plantation could already have extended beyond the limit of its respective experimental unit. Consequently, the willows growing outside the Cut plots could still have accessed that soil and impacted the water and chemicals mass flows, although most likely to a lesser degree. Additionally, an opening in the willow field might at the same time have allowed for increased evaporation by reducing shading, thereby influencing surface soil hydrology. Furthermore, it has been found that the roots and stumps of dead or cut trees can be kept alive through root grafts with living residual trees [43]. When tree root systems spread laterally and intermingle, connections between mature individuals may occur by grafting [44]. Natural root grafting is a fairly common phenomenon occurring among many trees including willows [45] and which has been observed under SRIC conditions in a past study conducted at a location near the study site [46]. Willows outside the Cut plots may thus have used the former root system of the cut willows directly, to absorb water and nutrients from the soil in the Cut plots. Overall, lateral root spreading, and root grafting may have maintained a sufficiently high transpiration rate in the Cut plots, thus limiting the expected decreases of contaminant concentrations over time. To prevent this, a much greater area could have been used for Cut plots. Also, a fine and deep cut could have been made around the Cut plots to inhibit living root activity in the Cut plots; however, this would not have stopped new roots from accessing it. Apart from the abovementioned phenomenon related to water dynamics, many contaminants may have remained strongly bound to the mature root systems lying in the Cut plots, thus preventing their decrease over time. It is well known that _Salix_ spp. can immobilize various TEs and organic compounds in/on their roots through absorption/adsorption [4; 11; 47; 48; 49; 50]. It would have been interesting to investigate this question in the present study, but the focus was on monitoring the soil concentrations only. The presence of fine roots in the soil samples could also explain why no significant decreases of contaminants were found under the Cut treatment over time. However, a plausible decay of at least some of the roots following cutting may also have begun to release some otherwise strongly root-sorbed contaminants into the soil. Six months after willow harvest, [PERSON] [51], reported significant increases in soil solution Pb concentrations, and interpreted that finding as an effect of root degradation and Pb release. Such increases in soil contaminant solubility could then lead to their leaching and unwanted loss in the environment [52]. Since tendencies towards contaminant decreases have been observed for some PAHs under the Cut plots, it is possible that such releases could have begun slowly, and that significant reduction of contamination levels would have begun to occur and been observed if the monitoring had lasted a few more years.