NPK-Enriched Date Palm Biochar Improves Soil Carbon–Nitrogen Status and Barley Yield Under Arid Conditions

Climate change and soil degradation threaten agricultural sustainability in arid oases, where water and nutrient limitations constrain crop production. In Tunisia, date palm residues are abundant but frequently burned despite their potential as soil amendme. This study assessed the effects of date palm waste biochar (B; 10 t ha−1), mineral fertilizers (NPK), and their combination as enriched biochar (BNPK) on soil fertility, including total organic carbon (TOC) and total nitrogen (TN), as well as barley (Hordeum vulgare L.) yield over two consecutive cropping seasons (2023–2024) using a randomized complete block design with three replications. During 2024, B increased TOC to 0.5% (control: 0.18%), while NPK enhanced TN to 0.037% in 2023; however, in 2024, nitrogen levels returned to values comparable to the control condition (0.017%). BNPK combined these beneficial improvements, maintained them in 2024, and resulted in a C/N ratio of 16.7 (control: 9.6), reflecting the most favorable balance between soil carbon accumulation and nitrogen retention. Grain yield increased by 21% (B), 80% (NPK), and 79% (BNPK) relative to the control (3.12 t ha−1), while BNPK reduced soluble sugars in grains (fructose 100%), glucose 86% (control: 0.09, 0.014) and increased grain nitrogen content to 1.80% (control: 0.74). Principal component analysis revealed a clear separation among treatments, with BNPK strongly associated with improved soil fertility, grain yield, and grain quality. These results demonstrate that integrating biochar with nutrient management enhances soil fertility and supports sustainable agriculture in arid oasis agroecosystems.