Benefits and Trade-Offs of Long-Term Organic Fertilization Substitution: Wheat Grain Nutrition and Heavy Metal Risks in an 11-Year Field Trial

Optimizing organic fertilizer substitution is essential for enhancing the sustainability of agriculture and achieving a balance between crop productivity, nutritional quality, and environmental safety. Here, we conducted an 11-year field experiment to evaluate the effects of substituting 50% of mineral fertilizers with pig manure (PM) or cattle manure (CM) on the nutritional quality of wheat grain, heavy metal (HM) accumulation, and associated human health risks. The yield and protein content were highest in the mineral fertilizer (MF) treatment, and grain micronutrients (Fe, Mn, Cu, Zn) were 6.7–13.8% higher under organic substitution (PM/CM) than in the MF treatment. The Ni, Pb, and As contents were 35.4–43.0% higher in the PM treatment than in the MF treatment, which stems from the higher HM content in pig manure. Health risk assessments indicated that the Hazard Index (HI) for children exceeded 1 in the PM treatment, primarily due to As, which accounted for 69.6% of the HI. All treatments remained within safe thresholds, although As and Pb posed detectable carcinogenic health risks. The higher levels of Ni and As in pig manure likewise led to a significant increase in the health risk associated with the PM treatment compared to the MF treatment. We developed a novel Grain Quality Index (GQI) that combined nutrient and HM data, which indicated that the nutritional quality of wheat grain was similar in the CM and MF treatments. The GQI was 9.1% lower in the PM treatment than in the MF treatment. These findings suggest that the substitution of mineral fertilizer with cow manure can help achieve a balance between yield, nutrition, and safety, and more stringent regulation of HMs is required for the use of pig manure. Our findings provide actionable insights with implications for sustainable wheat production policies.