The application of organic fertilizer could be accompanied by potential hazards to soil and humans caused by trace metals. A wide survey of organic fertilizers was carried out in northern China. A total of 117 organic fertilizer samples were collected to analyze the concentrations of seven trace metals. Simulation models were used to estimate the trace metal accumulation risk in soil and non-carcinogenic and carcinogenic risks to the human body. The concentrations of trace metals varied widely (Cr: 2.74–151.15; Ni: 2.94–49.35; Cu: 0.76–378.32; Zn: 0.50–1748.01; As: 1.54–23.96; Cd: 2.74–151.15; and Pb: 1.60–151.09 mg·kg−1). Chinese organic fertilizer standard limits were exceeded by0.85% for Cr, 5.98% for As, 1.71% for Cd, and 4.27% for Pb. Monte Carlo simulations showed that repeated application of organic fertilizer likely significantly increased the concentrations of Zn, Cd, and As in soil compared with the soil background levels according to the Soil Environmental Quality Standards of China. As and Cr pose high risks to human health, especially as carcinogenic risk factors with a skin exposure pathway. Reducing the content of Cr, Cu, Zn, As, and Cd in organic fertilizer would be of great significance for minimizing the damage caused by trace metals.

Nitrogen fertilizers are widely applied to increase rice yields, but excessive fertilization poses an environmental risk. It has been shown that rice-duck farming can be more efficient in terms of N use by improving rice growth. Several pathways have been proposed for how ducks may improve rice growth in paddy fields. The aim of this study was to investigate the isolated effect of rice-duck farm soil on rice, namely the N content in different plant organs and whether it differs among rice varieties. In a 116-day greenhouse pot experiment, six different Japonica rice varieties (JinU99, Jinyuan98, Jindao18, Jinyuan89, Jinhei1 and Jindao201) were grown in fertilized duck and no-duck soil after which the N content in their organs and the numbers of surviving and grain-producing plants were compared. The straw and leaf N concentrations were positively influenced by rice-duck farm soil while in the roots, this effect was recorded in only two rice varieties. Grain N content differed among varieties, but was not significantly influenced by soil type. Overall, N concentrations in straw and leaf, and roots of some rice varieties, but not in grain grown on duck soil were higher than that in those grown in soils not influenced by rice-duck farming. This study for the first time demonstrates that rice-duck farm soil alone can influence rice growth, namely an increase in the N content of certain rice plant organs.