**1. Introduction**

Peatlands serve a number of essential purposes, including the production of substrates and food [1]. Thanks to their high organic matter content, drained peatlands can be highly productive and suitable for agriculture, grasslands, and forestry. However, the drainage required to make organic soils suitable for agricultural activities creates conditions conducive to rapid degradation and compaction [2]. Wind and water erosion, subsidence, and soil organic matter mineralization leading to CO2 losses are the main causes of the degraded soil conditions observed in cultivated peatlands [3–5]. Solutions proposed thus far have included restoring cultivated peatlands to their natural state [6–8] or changing land use drastically, to paludiculture, for example [9,10].

More recently, the use of chopped biomass crops as a soil amendment has been proposed as a way to make peatland cultivation more sustainable [11,12]. Implemented in conjunction with optimal water table management and erosion mitigation, this practice may lead to an integrated conservation management approach that would improve the long-term sustainability of cultivated histosols [12–14].

Cultivated peatland in Canada constitutes an important part of the agricultural economy. In the plain of Montreal, in southwestern Quebec, close to 12,000 ha of land is covered in deep organic soils [15]. Although small, the area plays an essential role in the production of high-value vegetable crops, such as *Daucus carota* (carrots), *Lactuca sativa* (lettuce), and *Allium cepa* (onion), supplying fruits and vegetables to Canada and the northeastern United States [16].

**Citation:** Dessureault-Rompré, J.; Gloutney, A.; Caron, J. Nutrient Availability for *Lactuca sativa* Cultivated in an Amended Peatland: An Ionic Exchange Study. *Nitrogen* **2022**, *3*, 26–42. https://doi.org/ 10.3390/nitrogen3010002

Academic Editor: Marouane Baslam

Received: 11 December 2021 Accepted: 10 January 2022 Published: 14 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In 2016, a large-scale project was set-up in collaboration with local growers to develop conservation strategies adapted to their soils. As Bourdon et al. (2021) explained [11]: "In addition to controlling wind and, to a lesser extent, water erosion, a combination of other strategies is currently being investigated to extend the lifespan of these highly fertile lands including the application of plant biomass with a high carbon (C)/nitrogen (N) ratio (woodchip and grass straw)." Such strategies could potentially compensate for soil losses based on recent C modeling [12,17] and restore physical properties [18].

Miscanthus (*Miscanthus x giganteus*) straw and willow (*Salix miyabeana*) chips have been found to be good candidates for these conservation strategies [12,18,19], despite some anticipated side-effects such as N and P immobilization [11,20]. Miscanthus and short-rotation willow can be grown as biomass crops in degraded land [21,22] with little fertilization and practically no weed control [23,24], making them excellent choices for on-farm soil amendment production. Unlike manure and sewage sludge, for example, these plant-based amendments comply with food safety norms regarding vegetable cultivation and are not a source of pollution for rivers and bodies of water near the amended sites [25,26].

Approximately 14–20% of peatlands are used globally for agriculture, and when drained and cultivated, they represent some of the world's most productive agricultural soils [27]. Hence, the need for the development of sustainable management practice is urgent for these precious soils. The proposed conservation strategy is new and original as it aims to maintain long-term vegetable production on these productive lands while ensuring a massive return of organic matter annually, through biomass crop production on-site, or a biomass supply locally grown on more degraded surfaces.

The general objective of this field study was to assess the impacts of incorporating plant-based amendments, specifically *Miscanthus x giganteus* straw and *Salix miyabeana* chips, on nutrient availability for the roots of *Lactuca sativa* crops cultivated on peatland. We hypothesized that (1) both amendments similarly reduced the availability of nutrients, mainly nitrogen; that (2) the intensity of the reduction observed decreased over time; and that (3) rhizosphere strategies compensate for the effect of amendment.

### **2. Materials and Methods**
