Combined Application of Compost and Biogas Residues Reduces Soil and Pore Water Metal Concentration and Their Residual Toxicity to Wheat Seedlings

Purpose: Soil contamination with heavy metals is a global threat to food safety and sustainable crop production. This study aimed to assess the potential of compost (COM) and biogas residues (BGR) alone and in combination to mitigate cadmium (Cd) and lead (Pb) solubility in soil and soil pore water while evaluating their effectiveness in reducing residual metal toxicity to microbial activity and wheat (Triticum aestivum) seedlings. Methods: Metal (Cd, Pb) spiked homogenized soil was mixed with COM and/or BGR and filled in pots. Rhizon samplers were inserted in each pot to collect soil pore water at 7, 15, 30, and 60th day of exposure. A bioindicator assay was performed to assess residual toxicity of heavy metals on wheat seedlings and soil microbial activity. Bioavailable fractions of metals were calculated based on soluble and extractable Cd and Pb in soil. Results: In the COM treatment, the bioavailable fractions in terms of soluble and extractable Cd were reduced by 77% and 35%, whereas Pb by 3% and 57%, respectively, compared to the control treatment on 60th day. Application of COM, BGR, and COM + BGR significantly reduced bioavailable metal fractions in soil pore water by 53–81%, 31–38%, and 56–61%, respectively. The bioindicator assay revealed that COM + BGR promoted soil microbial activity (up to 81% respiration) and reduced residual phytotoxicity to wheat seedlings. COM application increased seed germination by 22% and the germination index by 33%, while COM + BGR increased seed germination by 30%, seedling length by 43%, and seedling biomass by 34%, compared to control. Conclusion: This study concludes that COM either alone or in combination with BGR, effectively reduces the bioavailability of Cd and Pb in contaminated soil and soil pore water, thereby mitigating residual metal toxicity to plants and microbes. Future studies should focus on optimizing application rates and assessing long-term impacts on soil health and crop productivity under field conditions.