Synergistic effects of biochar and deficit irrigation on soil properties and organic carbon fractions in arid sunflower farmlands

Deficit irrigation and biochar are important strategies for conserving freshwater resources and improving soil quality in arid, saline-alkaline irrigation districts, yet their effects on the molecular composition of soil organic carbon (SOC) remains insufficiently studied. This study quantified the effects of three biochar rates (0, 15, and 30 t ha⁻¹) and two drip irrigation regimes—full (100 % ETc) and deficit (60 % ETc)—on soil moisture (SM), bulk density (BD), electrical conductivity, pH, total nitrogen (TN), and SOC fractions in sunflower soils. Under deficit irrigation, 15 t ha⁻¹ biochar produced the greatest improvement in soil conditions—raising SM by 20.7 %–30.8 %, water storage by 9.7 %–46.4 %, TN by 5.6 %–16.1 %, and SOC by 16.0 %–59.1 %, while reducing BD by 1.2 %–14.6 %. In contrast, 30 t ha⁻¹ primarily altered SOC fractions, increasing particulate organic carbon (POC) by 1.3 %–59.2 %, causing an initial rise subsequent 11.1 %–35.9 % decline in easily oxidizable carbon (EOC), and producing a short-term increase followed by a decrease in dissolved organic carbon (DOC). Under full irrigation, SOC and POC increased with biochar rates, with 30 t ha⁻¹ achieving 21.2 %–95.1 % and 53.4 %–62.8 % higher levels, respectively, than no-biochar soils. Random forest and structural equation modeling showed that biochar rate was the main driver of POC and soil chemical properties exerted stronger controls on EOC and DOC than physical properties. Deficit irrigation combined with biochar improved soil physicochemical properties and enhanced labile carbon stability, with 15 t ha⁻¹ optimizing soil conditions and 30 t ha⁻¹ promoting labile carbon accumulation. • Deficit irrigation combined with 15 t ha⁻¹ biochar most effectively improved soil physicochemical properties. • Biochar-irrigation interaction significantly affected labile organic carbon fractions. • Biochar rate primarily drove POC dynamics, while soil chemical properties controlled EOC and DOC. • Biochar at 15 t ha⁻¹ improved soil quality, whereas 30 t ha⁻¹enhanced labile carbon accumulation.