*4.3. Retrospective on the Initial Hypotheses*

The present study was developed based on three main hypotheses. The first one stated that both amendments similarly reduced the availability of nutrient, mainly nitrogen. This is actually true for nitrogen where the overall impact of miscanthus and willow on the reduction of the N supply rate was −76% for miscanthus and −75% for willow. However, more precise information came from the investigation of this effect at the root zone scale and allowed us to put forward that the supply of N under the willow amendment seemed slower for the first half of the lettuce growth period (see Figure 6) as compared to the N supply rate observed under the miscanthus treatment. In addition, few other nutrients were impacted by the amendment (only P and S).

The second hypothesis stated that the intensity of the reduction observed decreased over time. In the present study, we distinguished this effect for the bulk soil and root zone. In the bulk soil, the difference between the amended treatment and the nonamended soil did not attenuate over time for willow and increased in the miscanthus-amended treatment. In the rhizosphere, the difference between the amended and nonamended treatments increased over time for both amendments (see Figure 4).

The third and last hypothesis stated that rhizosphere strategies compensate for the effect of amendment. At the temporal (weekly) and spatial (root zone and bulk soil) scales used in this study, we did observe that in the rhizosphere zone under willow amendment, the N supply rate was higher as compared to the N supply rate observed in the bulk soil for the first two weeks and half of the lettuce growth, which could lead to a nutrient mobilization strategy (Figure 4). However, with the data in hand, we cannot distinguish between a mobilization strategy or a slower N uptake by the lettuce root. In addition, the difference between the root zone and the bulk soil under miscanthus treatment and for the control treatment was identical (−49% in the root zone as compared to the bulk soil).
