Energy Crop Cultivation of Miscanthus × Giganteus on Biochar‐Amended Marginal Soils with Conventional and Waste‐Derived Fertilization: Biomass Allocation and Nutrient Dynamics Toward Closing Nutrient Loops

ABSTRACT Sustainable production of lignocellulosic biomass on marginal lands requires strategies that enhance yield while minimizing nutrient losses and supporting circular resource use. In this study, a two‐year (2023–2024) greenhouse meso‐scale experiment was conducted using Miscanthus × giganteus cultivated on nutrient‐poor soil amended with wood‐ or straw‐derived biochar (450°C) and fertilized with either mineral fertilizer (NPK) or spent coffee grounds (Cof). Biomass production and allocation, nutrient mobility in leachates, elemental composition of plant biomass and resulting phyto‐ashes, root anatomy, and plant physiological status were evaluated. Total biomass ranged from 250 to 491 g d.w. m −2 in the first growing season and reached 1495 g d.w. m −2 after two growing seasons. Biomass allocation was dominated by belowground fractions (58%–89% of total biomass), particularly in Cof‐amended plots. The highest biomass production was observed in the plot amended with straw biochar and Cof, indicating potential complementarity between biochar amendment and organic fertilization under marginal soil conditions. Some biochar‐amended plots (wood biochar + Cof) showed lower and more stable event‐based nitrogen loads in leachates, consistent with reduced nitrogen mobility. Cof‐amended plots exhibited higher chlorophyll‐related indices, while the highest TKN concentration, 9726 mg kg −1 d.w., was recorded after the second growing season, indicating differences in nitrogen availability and allocation among amendment scenarios. Root anatomical organization also varied among plots. Endpoint Fv/fm values ranged from 0.72 to 0.77 and were below the range commonly reported for non‐stressed plants, while Fm/Fo and Y(II) indicated that PSII photochemical activity was maintained. Here, we demonstrate that biochar combined with organic residues (Cof) modifies biomass partitioning and nutrient dynamics in Miscanthus × giganteus cultivated on marginal soil. In addition, the resulting phyto‐ashes were characterized by elevated K and P concentrations and low levels of potentially toxic elements, supporting their potential relevance for circular nutrient management.