4.2.1. Amendments

For nutrients other than N, the effect of the amendments was significant only on P and S, with the willow treatment decreasing the phosphorous supply rate and both the willow and miscanthus treatments increasing the sulfur supply rate (Figures 7 and 11). Several authors have reported P immobilization with the incorporation of straw under different pedo-climatic conditions [48–50], while others have noted increased available P or P losses [51–53]. Bourdon et al. (2021) pointed out that because the P content of Quebec peat soils can be excessively high [54], P immobilization could help reduce eutrophication. Although Bourdon et al. (2021) found S immobilization to occur during a 56 days incubation

experiment with miscanthus and willow straw, the mineralization–immobilization of S has been shown to be related to the C:S ratio of the amendment [55]. In the present field study using PRS® probes, the S supply was found to increase substantially, by 38% (miscanthus) and 59% (willow). The S supply changed over time, however, increasing to a peak in the fourth sampling period and then decreasing, indicating that S was mineralized and then immobilized during lettuce growth.

#### 4.2.2. Sampling Zone

Significant differences were found in the root zone for all nutrients analyzed, except P and K. The effect of sampling zone showed decreases in N, Ca, and Mg in the root zone as compared to the bulk soil zone. The opposite effect was noted for S, Fe, Mn, Cu, and Zn. Depletion and accumulation zones have often been observed in the rhizosphere [56]. Lettuce crops have shown important quantitative differences in root exudates depending on soil type, even under well-fertilized conditions [57]. Although we did not find studies on root exudates from lettuce grown in cultivated peatland, root exudation and activities are known to be intimately related to plant nutrition [58] and, therefore, might have contributed to the micro-nutrient mobilization observed in this study.
