Agronomic biofortification of wheat is a strategy to alleviate zinc (Zn) deficiency in human nutrition. The application of green manure (GM) to soils with low Zn availability can increase the transfer of Zn from soil to wheat, however, this practice can also influence the plant availability of the toxic metal cadmium (Cd). We tested the effect of GM addition on the transfer of Zn and Cd from soil to wheat (Triticum aestivum L.).
To obtain GM, sunflower (Helianthus annus L.) was grown and labeled with 67Zn and 111Cd in hydroponics. In a pot trial, different quantities of milled sunflower were amended as GM to a soil that either was fertilized with ZnSO4 (high Zn availability) or not fertilized (low Zn availability). The soil solution was frequently analyzed during wheat growth and the wheat was harvested at flowering and full maturity. Isotope ratios (67/66Zn and 111/110Cd) of purified soil solution and wheat samples were analyzed with Q-ICP-MS.
The addition of GM to soil increased wheat root and shoot biomass as well as pH and total nitrogen (TN) in the soil solution. Furthermore, GM increased N concentrations in the wheat shoots and initially also dissolved organic carbon (DOC) in the soil solution.
The average Zn concentration in wheat grains were higher in soils with high than with low Zn availability (high Zn: 101 ± 1.9 mg kg-1, low Zn 28 ± 0.9 mg kg-1). However, the reverse pattern was found for Cd (high Zn: 0.15 ± 0.01 mg kg-1, low Zn: 0.437 ± 0.01 mg kg-1). No significant effect of GM addition on shoot and grain Zn and Cd concentration was observed due to increased biomass (dilution effect). However, the transfer of Zn and Cd into wheat shoots strongly increased with the application of GM to soils with low Zn availability (Zn: from 67 to 121 µg, Cd: from 1.51 to 2.74 µg). In these soils, isotope mass balances revealed that the increased transfer was mainly caused by an indirect effect through solubilization of these metals from the bulk soil (Zn: + 39 µg, Cd: + 1.07 µg), but also directly through the addition of the metals with GM (Zn: + 16 µg, Cd: + 0.16 µg). Our data suggest that the increased soil-plant transfer of Zn and Cd was mainly caused by the decomposition of GM that (i) released organic compounds which solubilized Zn and Cd from the bulk soil, (ii) improved N supply in the initial phase of wheat growth, which led to a better soil exploitation and enhanced root exudates.
This study showed that GM amendments to soils can increase the soil-wheat transfer of Zn but also the transfer of Cd. A combination of agronomic biofortification measures, such as GM addition and mineral Zn fertilization could contribute to a healthier balance of Zn and Cd in wheat grains.