*2.3. Experimental Design*

A 1 ha area of a 7 ha field was divided into three sections: (1) control, (2) miscanthusamended, and (3) willow-amended. Each experimental section covered an area of about 330 m2. Chopped miscanthus and willow material was added to the soil at a rate of 15 t ha−<sup>1</sup> (Figure 1). This rate is thought to be adequate to compensate for the average annual C losses observed in this type of soil (Dessureault-Rompré et al., 2020).

**Figure 1.** Experimental sections (miscanthus on the **left** and willow on the **right**) pictured before the chopped biomass was incorporated into the soil.

On 12 June 2019, the baled miscanthus was chopped and spread using a FP240 (New Holland Agriculture, New Holland, PA, USA) pull-type forage harvester equipped with a hay pickup. The bales were loosened and laid by hand in windrows on the ground to be picked up by the forage harvester and blown from the chute onto the plot. As the willow was already chipped when received, a lime spreader was used to spread it over the plot. The chopped plant material was then incorporated to a depth of 15 cm using a chisel.

## *2.4. Ionic Exchange Resin*

The use of Plant Root Simulator (PRS®) ion exchange resins is an economical and rapid way to quantify the concentrations of a range of nutrients and contaminants in soils by simulating their uptake by plant roots [29–31]. In this study, the soil nutrient supply was assessed using PRS® cationic and anionic exchange resin probes. At six locations in each experimental plot, two cationic exchange resin probes and two anionic exchange resin probes were inserted at a depth of 15 cm in the root zone of two lettuces and into two root exclusion cylinders (bulk soil) measuring about 15 cm in height and 10 cm in diameter (Figure 2). The resin probes remained in the soil for seven days, and they were then replaced with fresh ones (Table 2). The removed resin probes were cleaned with a toothbrush and rinsed with demineralized water to remove any soil particles that could continue to exchange ions with the resins. Although laborious, this cleaning procedure is essential to ensure that no soil particles are left on the resin surface, which could continue to exchange nutrients. Finally, the cleaned resin probes were stored at 4 ◦C before being sent to Western Ag Innovations' laboratories (Saskatoon, SK, Canada) for complete analysis. At Western Ag Innovations' laboratories, inorganic N (ammonium and nitrate) in the eluant was determined colorimetrically using automated flow injection analysis (Skalar San++ Analyzer, Skalar Inc., Breda, The Netherlands). The remaining nutrients (P, K, S, Ca, Mg, Fe, Mn, Cu, and Zn) were measured using inductively coupled plasma (ICP) spectrometry (Optima ICP-OES 8300, PerkinElmer Inc., Waltham, MA, USA).

**Figure 2.** Experimental design with PRS® probes installed in the root zone and in root exclusion cylinders. The probes are shown here before they were completely inserted into the soil.


**Table 2.** Burial and retrieval dates for the PRS® probes and related periods.
