Objectives Soil zinc (Zn) fertilization is an important measure to alleviate the available Zn deficiency in soil and increase the Zn concentration in crops. However, in calcareous soils, a variety of factors, such as high soil pH, high carbonate content, low organic matter content, etc., greatly reduce the effectiveness of Zn fertilizer, and the efficiency of Zn fertilizer utilization is low. Exploring the effect and mechanism of combined application of zinc fertilizer with organic materials to enhance the effectiveness of soil Zn, providing a theoretical basis for the realization of Zn enrichment in crop grains through efficient soil application of Zn fertilizer.

Methods A field positioning experiment of Zn fertilizer combined with organic materials was conducted in 2016 at the experimental field of Northwest A & F University. The tested soil was a calcareous soil with potential zinc deficiency, classified as an Earth-cumulic Orthic Anthrosol. The following eight treatments were no Zn and no organic material control (CK), Zn fertilization alone (Zn), fulvic acid application alone (FA), wheat straw application alone (St), sheep manure application alone (M), fulvic acid application combined with Zn fertilization (Zn+FA), wheat straw application combined with Zn fertilization (Zn+St), and sheep manure application combined with Zn fertilization (Zn+M). The wheat yields were measured at the maturity stage in 2018 and 2019, and the Zn content in various parts of wheat was analyzed. Soil samples were also collected to determine their physicochemical properties as well as the content of different forms of Zn.

Results Compared with CK, the FA, St, and M treatments did not improve the total Zn and DTPA-Zn contents, and the contents of all Zn forms significantly, while the four Zn treatments significantly increased those and Zn mobility factors (M_S). Compared with Zn treatment, Zn+FA, Zn+St, and Zn+M treatments significantly increased the content of Zn loosely bound to organic matter (Lom-Zn) and the M_S value by 19.9%−50.4% and 0.2−0.7 percentage points, respectively. The DTPA-Zn content in Zn+M and Zn+St treatments were increased by 33.6%−54.3% and 24.7%−27.1%, respectively, while the Zn+FA treatment had no significant change compared to the Zn treatment. There was a significant positive correlation between Lom-Zn and dissolved organic matter (DOM) in soil, which explained the reason of the promotion of Lom-Zn content by the carbon input of organic materials in Zn+M and Zn+St treatments. Zn alone and Zn plus organic material increased wheat grain Zn concentration and accumulation, with Zn+M treatment exhibiting the highest effects, increasing Zn concentration and accumulation by 32.1% and 39.9%, respectively, compared to those of CK. The wheat grain Zn concentration (39.4 mg/kg) in Zn+M treatment nearly met the human health requirement (40−50 mg/kg). Compared with CK, the St and M treatments increased the phytic acid content in grains, thereby increased the phytic acid to Zn molar ratio (PA/Zn) by 30.3% and 22.1%, respectively. In contrast, the Zn+M treatment reduced the PA/Zn value in the grains, decreasing by 15.0% compared to the CK and by 16.8% compared to the Zn treatment. Zn concentration and accumulation in wheat grains were positively correlated with soil Lom-Zn, DTPA-Zn, and exchangeable Zn, with the Lom-Zn being the most important factor.

Conclusions The combined application of Zn fertilizer with fulvic acid, crop straw, or sheep manure could more effectively promote the conversion of Zn to the Zn loosely bound to organic matter with high availability than that of Zn fertilization alone. In particular, the combination of Zn fertilization with sheep manure could improve the deficiency of soil available Zn more significantly, and enhance the Zn concentration and bioavailability of wheat grains. Therefore, soil application of Zn fertilizer combined with manure can be an effective measure for Zn nutritional enhancement of wheat grain in calcareous soils.