Changes of zinc forms in the decomposition of Astragalus sinicus under waterlogged incubation conditions

A 90-day indoor cultivation experiment investigated how the decomposition of Chinese milk vetch (Astragalus sinicus) under waterlogged incubation conditions in quartz sand affects the medium’s zinc forms and their availability. Five treatments, including a control (no A. sinicus added) and four A. sinicus treatments at 1.50 (S₁), 2.25 (S₂), 3.00 (S₃), and 3.75 (S₄) t·ha⁻¹ were set up. Sand samples were regularly collected to determine the pH value and contents of various zinc forms and plant-available zinc (DTPA-Zn). We found that the DTPA-Zn, exchangeable (Ex-Zn), carbonate-bound (Carb-Zn), manganese oxide-bound (OxMn-Zn), and tightly bound organic (Sbo-Zn) zinc contents increased, whereas the loosely bound organic zinc (Wbo-Zn) content decreased with the decomposition of A. sinicus under waterlogged conditions. Zinc occurred mainly as Sbo-Zn and Ex-Zn. Residual zinc (Min-Zn) was not detected. Adding A. sinicus promoted the transformation of Wbo-Zn into Sbo-Zn, Carb-Zn, and OxMn-Zn over time. Increasing the amount of A. sinicus favored the conversion to OxMn-Zn over Carb-Zn. The average Zn conversion rate was lowest in S₂ (30.9%) and highest in S₄ (35.9%). Adding A. sinicus reduced the Zn migration factor (S₃ had the lowest; 43.70%) and increased the Zn distribution index (S₂ had the highest; 37.17%) by the end of the experiment. This indicates the transformation of Zn into less available forms. The Ex-Zn content peaked after one month of decomposition. After that, further decomposition promoted the conversion of highly available zinc forms into less available ones. After three months of decomposition, the Sbo-Zn content was the highest. Zinc availability and transformation varied with the amount of A. sinicus added; applying 2.25 t·ha⁻¹ resulted in the greatest decline in Zn availability by the end of the experiment.