**4. Discussion**

#### *4.1. Nitrogen*

The use of plant-based, high-C/N-ratio amendments has been proposed as a conservation strategy in cultivated peatland (Dessureault-Rompré et al., 2020). In the present study, PRS® probes were used to evaluate the impact of incorporating miscanthus straw and willow chip amendments in the field on nutrient availability in the rhizosphere of a lettuce crop. The field was fertilized as usual by the grower. Miscanthus and willow amendments decreased fresh lettuce yield by 35% and 14%, respectively. Although not statistically significant, such a yield decrease is necessarily relevant for growers as it could have substantial economic consequences.

Straw mulches from different plant sources have been studied and used in vegetable crop production for years, with both positive and negative effects on crop yield [36–38]. In the present study, however, the miscanthus straw and willow chips were not used as a mulch but were incorporated into the soil as an organic amendment. Few studies have investigated the effect of straw incorporation on vegetable crop yields. More numerous studies have focused on the effect of straw incorporation on wheat or wheat-rice rotation [39–41] and corn yields [42–44], with most reporting a neutral to positive crop response. In the present study, however, the quality of the straw was different, and straw was added at a much higher rate.

The present study highlighted the significant impact of miscanthus straw and willow chip incorporation on the field soil nutrient supply rate. Nitrogen was by far the most affected by the soil amendments, with a 75% reduction in the N supply rate. In a modeling study on the effect of 8 t ha−<sup>1</sup> of wheat straw incorporation on nitrogen dynamics, Garnier et al. [45] observed a 13% decrease in the net amount of nitrogen mineralized 13 months after the straw was incorporated. In an incubation experiment, both wheat straw and spruce sawdust added at a rate of 4.5 t C ha−<sup>1</sup> [46] were found to cause nitrogen immobilization, reaching a maximum level of 42 kg N ha−<sup>1</sup> and then decreasing to 8–15 kg N ha−<sup>1</sup> after a few weeks. In a microcosm study, rice straw incorporated at a rate of 10 t ha−<sup>1</sup> was found to immobilize 16–39% of applied N over the course of a 160 days incubation period [47]. In the present study, in the bulk soil, the difference between amended treatment and the nonamended soil did not attenuate over time (42 days) 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 (Figure 4). In a 56 days incubation study, Bourdon et al. [11] observed that the addition of 11 t ha−<sup>1</sup> of miscanthus straw and willow chips to a moderately decomposed histosol reduced mineral N from a KCl extract by 60% and 54%, respectively, overall, with the effect of these two amendments increasing over the course of the incubation period. In a 182 days incubation experiment, Marmier [20] observed that following miscanthus straw and willow chip amendments at a rate of 15 t ha−1, N immobilization remained stable, increasing or decreasing over the course of the incubation period, depending on the type of histosol and amendment. Finally, the plant N uptake index in the control was found to increase until week 3 and then decrease until the end of lettuce growth. The plant N uptake index was much lower in the miscanthus treatment than in the control treatment and decreased from the beginning to the end of lettuce growth. The index for the willow treatment was initially negative and increased slightly until week 3. These results showed that miscanthus and willow amendments, by their negative impact on the supply rate of N, decrease the capacity of the lettuce crop to absorb N, which is consistent with the impact observed on lettuce yield. Interestingly, soil extract at the end of the growing season showed no difference in mineral N concentrations between the control and amended soil. However, a significant decrease in soluble organic N was observed. Dynamic changes in the fast-cycling mineral N pool might be better captured using PRS® probes than soil extract; however, future research should explore the time evolution between mineral and organic N pools under field-amended cultivated peatland.
