Kinetics and potential risk of heavy metal release from commercial organic fertilizers in northern China

Commercial organic fertilizers (COFs) derived from animal manure often contain elevated levels of heavy metals (HMs). This study determined the contents of Cr, Cu, Zn, As, Cd, and Pb in a total of 72 COF samples manufactured in northern China, and evaluated their release characteristics with a 56-day immersion test. The average contents of Cu, Zn, Cd, and Pb in these COF samples were 51.3, 300.7, 0.6, and 47.9 mg/kg, which are much higher than their background levels in Chinese surface soils. Besides animal manure, the minerals/salts added during fertilizer shaping process, were also revealed to be a significant source of HMs in COFs. The release of HMs from COFs depended strongly on the decomposition of the fertilizer's organic matrix. The bioavailability of HMs in the COF samples generally decreased in the order of As > Cu > Cd > Zn > Pb ≥ Cr. While the risk level of HMs in the COFs generally ranges from no to medium based on their bioavailability, the actual risk would be much higher as their organic matrices would breakdown completely in the amended soils over time. The cumulative fraction of As released from the granular COFs reached up to 66.3 % by the end of the immersion test. It was estimated that continuous application of the COFs would bring excessive accumulation of Cd in agricultural soils within approximately 25.9 years. These findings indicate the significant need for controlling the contents of HMs, particularly As and Cd, in COFs to prevent pollution of agricultural soils by their continuous application.