Abstract:
Objectives To investigate the effects of zinc fertilizer and its combination with organic fertilizer on soil zinc content and wheat zinc accumulation in dry-crop wheat fields under different biochar application rates in the long term.
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 zinc application (CK), zinc fertilizer alone (Zn), and zinc fertilizer combined with organic manure (Zn+M). Following the 2021 winter wheat harvest, we measured aboveground biomass and zinc concentrations in plant tissues. Soil samples were collected to analyze total zinc content, bioavailable zinc, and zinc fraction distribution.
Results After nine years of biochar application, no significant effects were observed on total zinc (Tol-Zn) or available zinc (DTPA-Zn) in the soil. Zinc application significantly increased soil Tol-Zn and DTPA-Zn levels. Compared to the control (CK), Zn and Zn+M treatments elevated Tol-Zn by 7.5% and 12.5%, respectively, and DTPA-Zn by 210.2% and 249.7%. Furthermore, Zn+M enhanced Tol-Zn and DTPA-Zn by 4.7% and 12.7% compare to Zn alone (P<0.05). Under CK (no zinc), biochar reduced exchangeable zinc (Ex-Zn) but increased loosely organic-bound (Lom-Zn) and manganese oxide-bound zinc (Mon-Zn). In Zn-treated soils, biochar raised Lom-Zn, tightly organic-bound zinc (Sbo-Zn), and carbonate-bound zinc (Carb-Zn), while reducing Mon-Zn. With Zn+M, Lom-Zn levels under C50 and C30 biochar treatments were significantly higher than C10 and C0 (P<0.05), though no differences were observed for other zinc fractions. Compared to CK, both Zn and Zn+M enhanced Lom-Zn, Carb-Zn, Sbo-Zn, and residual zinc (Res-Zn), with Zn+M demonstrating greater efficacy. The combined application of biochar and zinc significantly increased winter wheat grain biomass. Across plant tissues, biomass and zinc content followed the order: Zn+M > Zn > CK (P<0.05). Under no zinc application (CK), C30 and C50 treatments increased zinc content in wheat stems, with C50 also enhancing leaf zinc content. However, both treatments reduced zinc levels in grains. Compared to CK, Zn and Zn+M treatments increased average grain zinc content by 13.7% and 28.3%, respectively, with Zn+M demonstrating significantly greater enhancement than Zn alone. Compare to CK, stem+leaf accumulation rose by 203.0% and 257.2%, grain+shell by 40.0% and 64.8%, and total aboveground accumulation by 60.6% and 89.1%. Zn+M again outperformed Zn in enhancement effects. Correlation analysis revealed significant positive associations (P<0.01) between wheat zinc accumulation and all measured soil zinc 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 zinc accumulation in winter wheat.
Conclusion Long-term biochar application did not increase total or available zinc in rainfed wheat field soils but elevated more bioavailable Lom-Zn fractions. Zinc fertilizer application, particularly when combined with organic manure, significantly enhanced total zinc, available zinc, and organic-bound zinc fractions (loosely- and tightly-bound), thereby enhancing its potential availability. Without zinc fertilization, high biochar rates increased winter wheat grain biomass but reduced grain zinc content. When zinc fertilizer was applied, biochar further boosted grain yield without compromising zinc content at application rates of 10-30 t/hm2. The combined use of zinc fertilizer and organic manure proved significantly more effective than zinc fertilizer alone in improving soil zinc availability and grain zinc enrichment.