Objectives This study aimed to investigate the effects of zinc (Zn) fertilizer and its combination with organic fertilizer on soil Zn content and wheat Zn accumulation in dryland wheat fields under long-term different biochar application rates.

Methods A long-term field experiment investigating biochar application was established in 2012 with four treatment levels: 0 t/hm² (C0), 10 t/hm² (C10), 30 t/hm² (C30), and 50 t/hm² (C50). In 2020, three subplot treatments were implemented within each main plot: no Zn application (CK), Zn fertilizer alone (Zn), and Zn fertilizer combined with organic manure (Zn+M). Following the 2021 winter wheat harvest, we measured aboveground biomass and Zn concentrations in plant tissues. Soil samples were collected to analyze total Zn content, bioavailable Zn, and Zn fraction distribution.

Results After nine years of biochar application, no significant effect was observed on total Zn (Total-Zn) or available Zn (DTPA-Zn) in the soil. Zn application significantly increased soil Total-Zn and DTPA-Zn levels. Compared to the control (CK), Zn and Zn+M treatments elevated Total-Zn by 7.5% and 12.5%, respectively, and DTPA-Zn by 10.8% and 14.8%. Furthermore, Zn+M enhanced Total-Zn and DTPA-Zn by 4.6% and 12.7% compare to Zn alone (P<0.05). Under CK treatment, biochar reduced exchangeable Zn (Ex-Zn) but increased loosely organic-bound (Lom-Zn) and manganese oxide-bound Zn (MnO-Zn). In Zn-treated soils, biochar raised Lom-Zn, tightly organic-bound Zn (Sbo-Zn), and carbonate-bound Zn (Carb-Zn), while reducing MnO-Zn. With Zn+M, Lom-Zn levels under C50 and C30 biochar treatments were significantly higher than C0 (P<0.05), though no difference was observed for other Zn fractions. Compared to CK, both Zn and Zn+M enhanced Lom-Zn, Carb-Zn, Sbo-Zn, and residual Zn (Res-Zn), with Zn+M demonstrating greater efficacy. The combined application of biochar and Zn significantly increased winter wheat grain biomass. Across plant tissues, biomass and Zn content followed the order: Zn+M > Zn > CK (P<0.05). Under no Zn application (CK), C30 and C50 treatments increased Zn content in wheat stems, with C50 also enhancing leaf Zn content. However, both treatments reduced Zn levels in grains. Compared to CK, Zn and Zn+M treatments increased average grain Zn content by 13.7% and 28.3%, respectively, with Zn+M demonstrating significantly greater enhancement than Zn alone. Compared to CK, stem+leaf Zn accumulation increased by 203.0% and 257.2%, grain+shell Zn accumulation increased by 40.0% and 64.8%, and total aboveground Zn accumulation increased by 60.6% and 89.1%. Zn+M again outperformed Zn in enhancement effects. Correlation analysis revealed significant positive relation (P<0.01) between wheat Zn accumulation and all measured soil Zn fractions, with the strongest relationships observed for DTPA-Zn and Lom-Zn. Random forest analysis identified Sbo-Zn, Lom-Zn, and DTPA-Zn as the primary contributors to Zn accumulation in winter wheat.

Conclusions Long-term biochar application did not increase total or available Zn in rainfed wheat field soils but elevated more bioavailable Lom-Zn fractions. Zn fertilizer application, particularly when combined with organic manure, significantly enhanced total Zn, available Zn, and organic-bound Zn fractions (loosely- and tightly-bound), thereby enhancing its potential availability. Without Zn fertilization, high biochar rates increased winter wheat grain biomass but reduced grain Zn content. When Zn fertilizer was applied, biochar further boosted grain biomass without compromising Zn content at application rates of 10−30 t/hm². The combined use of Zn fertilizer and organic manure proved significantly more effective than Zn fertilizer alone in improving soil Zn availability and grain Zn enrichment.