Co-application of carboxymethyl cellulose and biochar enhances soil hydrophysical properties and water use efficiency in a drip-irrigated jujube orchard under arid conditions

In the arid sandy soils of southern Xinjiang, low clay content, weak aggregation, and limited water-holding capacity constrain water use efficiency (WUE) and jujube (Ziziphus jujuba Mill.) productivity. However, field-scale responses to the co-application of sodium carboxymethyl cellulose (CMC) and biochar remain insufficiently understood, particularly with respect to soil hydrophysical properties, root-zone water distribution, and crop water use. Based on a two-year field experiment, six treatments were evaluated: a control (CK), CMC alone at 300 kg·hm −2 (C30), biochar alone at 3.0 and 6.0 t·hm −2 (B30 and B60), and combined applications (C30B30 and C30B60). Co-application increased saturated soil water content by 15.69%–20.73% and reduced saturated hydraulic conductivity by 22.79%–29.17% relative to CK (P < 0.05). WUE increased by up to 18.75%, and fruit quality improved through lower titratable acidity and higher soluble sugar and flavonoid contents. Among the tested treatments, C30B30 provided the most favorable overall agronomic–economic outcome when WUE, IWUE, yield, fruit grading, and economic return were considered jointly, whereas C30B60 produced the largest responses for several soil physical indicators. These findings support the combined use of CMC and biochar for improving water-use performance in drip-irrigated jujube orchards on arid sandy soils.