Understanding the role of pig manure, rice straw and calcium carbonate on the cadmium pollution in soil-wheat system

To explore the specific effects of pig manure and rice straw on cadmium-contaminated soils and the effect of different mass fractions of calcium carbonate on cadmium efficacy, a pot experiment was conducted with different rates of pig manure (RS), rice straw (PM) and calcium carbonate (CC). The results showed that pig manure addition increased the soil HA/FA ratio, thereby reducing the available Cd. The addition of calcium carbonate increased the soil pH and reduced the available Cd, The addition of pig manure and calcium carbonate significantly reduced the Cd content of wheat grains by 51.96% and 45.95%, respectively, at 3% and 5% application rates. Rice straw application reduced the soil pH, improved the soil Cd availability, and increased the accumulation of Cd in wheat grains. Compared with that in the control, the grain biomass increased significantly with 3% pig manure and 3% rice straw application, and the grain biomass increased significantly, by 58.86% and 39.11% and 93.3% and 75.08%, respectively, in 2020 and 2021. With the application of calcium carbonate, the grain biomass of wheat first decreased and then increased. Additionally, the grain biomass significantly increased by 26.36% and 71.22%, respectively, at the 5% calcium carbonate application rate. Therefore, high application rates of pig manure (3% and 5%) have potential application in the safe production of wheat in Cd-contaminated soils.

Methods
The wheat samples harvested in October were separated into roots, stems, grains. The plant tissues were subsequently washed with deionized water 2-3 times. The plants were then incubated at 105℃ for 30 min, then at 70°C until a constant weight was reached, after which the dry weight was measured. The dry samples of each part of the plant were ground and sieved. Soil samples were collected after plant harvest, then sieved through 0.85 mm and 0.15 mm polyethylene mesh after air drying.

The soil Cd was extracted by the DTPA [TEL (0.005 mol/L DTPA-0.100 mol/L triethanolamine)-0.010 mol/L CaCl2]  leaching method, The plant samples were digested with HNO3-HClO4, the soil samples were digested with HNO3-HClO4-HF, the digested samples were subjected to inductive-coupled-plasma-emission spectroscopy (Agilent 5110 ICP‒OES) . Other physicochemical properties of the soil were determined with reference to the Methods of Agricultural Chemical Analysis of Soil: The pH values of the soil samples (1:2.5 soil-to-water ratio, w/v) were measured by potentiometry using a pH meter (PHS-3E); the content of soil organic carbon was determined using the potassium-dichromate-digestion method; the total nitrogen of the soil was determined using the semimicro-Kjeldahl method; the Olsen's P was measured with the sodium-bicarbonate method; the available potassium was measured with the flame-photometry method. Soil active calcium carbonate was determined by potassium permanganate titration. Soil humic acids and fulvic acids were first extracted by acid precipitation , then determined by the Churin method.