# Willows Used for Phytoremediation Increased Organic Contaminant Concentrations in Soil Surface
## 4 Discussion
### Results Interpretation and Implications for Field Trials  
The challenges associated with monitoring changes in soil contamination over time are not new [15]. The results presented here demonstrate how counterintuitive results gathering can be in a field trial. However, field studies are essential for the development of phytoremediation, among other reasons, because not every aspect of an open field system can be tested under controlled conditions. For instance, the pumping effect that a high-density willow crop under SRIC management has on soil hydrology is a major field characteristic that cannot be taken into account in typical greenhouse experiments.  
The numerous factors interacting in field studies are a source of variance in field-gathered data that can prevent researchers from attaining practical objectives related to environmental cleaning [15]. It can also be very challenging to identify the mechanisms responsible for the observations made. Based on the findings here, important recommendations to mitigate the effects of spatial heterogeneity at field sites are: to combine subsamples into a composite samples in each experimental plot [53], to collect samples as close as possible to the same sampling point over time, and to be flexible when establishing statistical significance thresholds [15].  
To our knowledge, very few studies have reported increases in contaminant concentrations under phytoremediation, as was the case here. Since phytoremediation studies usually hypothesize soil purification as a benefit, it is possible that previous findings suggesting no effect, or even opposite effects, have just been considered as failures and regarded as unsuitable for publishing or incomplete. Studies that fail to confirm a hypothesis are generally underrepresented in the literature compared to studies that succeed in doing so, producing so-called publication bias [54]. Nevertheless, rather than remain unpublished, studies with such negative results should be available to the scientific community to help interpret other types of findings [55].  
Apparent increases of concentrations over time do not exclude the possibility that active degradation [15] and extraction are occurring in the rhizosphere [30]. In this context, the remediation effect of willows (i.e., lowering soil contamination levels), most likely by rhizdegradation, may have been masked by continuous transfer of a mobile fraction of the contaminants present near the plantation, followed by their accumulation close to willow roots. The experience acquired during this study led us to the conclusion that, on this particular site, sampling the soil close to the trees, as per usual practice, might not yield an accurate estimate of the phytoremediation process in progress. [PERSON] and [PERSON] [31] also pointed out that a reduction in pollutant concentration in soil is unlikely to be evident in samples from surface soil that is extensively penetrated by roots, due to the possible movement of chemicals towards them. Following the previous experimental phase, which took place on the same experimental site, our study group observed no significant effect of willow on any TE variation in soil, despite evidence that some TEs were substantially eliminated from the ground by plant uptake [17].  
Finally, this experiment could be considered a case of pollutant containment, which has been proven effective elsewhere [56]. If SRIC of willows leads to such increases of pollutant concentrations under trees, it is probably also because some decreases are occurring elsewhere, which is relevant and desirable in a context of low investment risk management strategy. Containing contaminants in the root zone, even if carried out in a non-perennial time frame, still implies that these compounds do not migrate into the environment, which includes the St. Lawrence River in our case.