Digestate biochar effects on soil Pb bioaccessibility, crop Pb concentrations, and human health risk in urban vegetable agroecosystems.

Biochar is an effective method of Potentially Toxic Element (PTE) remediation in urban agricultural systems; however, its remediation potential may be determined by the extent to which it alters soil properties. Where soil properties are minimally changed with biochar application, remediation potential may be minimally effective. A controlled experiment was used to compare the effects of 5%w/w digestate biochar in no-crop, lettuce, carrot, and garlic agrisystems with differing soil Pb concentrations, high soil pH and organic matter. Total soil and vegetable Pb and bioaccessible soil Pb were quantified using acid digestions and Unified BARGE Bioaccessibility Methodology (UBM). Risk indices (ADI, THQ, CR), were used to assess grower (soil), consumer (vegetable) and combined (soil and vegetable) Pb ingestion risks for each vegetable agrisystem, and were supported by Monte Carlo and sensitivity analyses. Digestate biochar significantly reduced soil-derived Pb ingestion risks (ADI/THQ/CR) in high Pb no-crop systems, high Pb and low Pb carrot systems, and low-Pb garlic systems, though had no effect on ingestion risk in low Pb no-crop and high Pb garlic agrisystems. Notably, biochar did not affect vegetable-derived Pb ingestion risks for any agrisystem. When soil and vegetable risk indices were combined, biochar only lowered overall Pb ingestion risks in high-Pb no-crop and low-Pb lettuce systems, with minimal or inconsistent effects elsewhere. Ultimately, biochar had limited and variable effects on reducing Pb ingestion risks in the studied vegetable agrisystems, which suggests that its remediation potential in urban agrisystems is context-dependent and may offer limited protection for growers and consumers in soils with elevated pH and organic matter.