Physicochemical Properties of Biochar Produced from Grapevine-Pruning Residues of 12 Cultivars

The valorization of grapevine pruning residues through pyrolysis provides a sustainable approach to agricultural waste management, producing biochar with agricultural use potential and carbon sink functionality. This study investigated pruning residues from 12 grapevine cultivars to evaluate the cultivar effects on biochar properties. Samples were collected along the Croatian coast from Istria to Dalmatia and included six indigenous cultivars (Malvazija istarska, Pošip, Maraština, Teran, Plavina, and Plavac mali) and six introduced cultivars (Chardonnay, Pinot blanc, Sauvignon blanc, Merlot, Cabernet Sauvignon, and Syrah). For each cultivar, residues were collected from three distinct vineyards with three replicates per vineyard. Pyrolysis was conducted in a muffle furnace at 400 °C. The pruning residues showed acidic pH (4.79–5.45), moderate electrical conductivity (1694–2390 µS cm−1), and ash contents of 2.65–3.49% among all cultivars. Significant differences were observed among cultivars in residue carbon content and ash fraction, which were reflected in the resulting biochar. Biochar yield ranged from 32% to 35%, while pH values were alkaline, ranging from 10.20 to 11.13. Total carbon increased from 43.77 to 45.36% in grapevine-pruning residues to 65.88–71.57% in biochar. FT-IR spectra revealed cultivar-dependent variation in aromatic C=C intensification, while SEM analysis indicated differences in pore abundance and surface area (1.63–4.13 m2 g−1) between cultivars. These results demonstrate that carbon-dense cultivars produced biochars with greater structural stability, indicating enhanced resistance to decomposition. Spectroscopic and microscopic analyses consistently showed increased aromatic condensation, reduced aliphatic functionality, and greater porosity following pyrolysis. These cultivar-dependent differences highlight pruning residues as a chemically heterogeneous but predictable feedstock, with biochar properties primarily governed by the intrinsic characteristics of the source material.