Straw return enhances the risks of metals in soil?

Interactions between organic matter (OM) and metals in soils are important natural mechanisms that can mitigate metal bioaccumulation in terrestrial environments. A primary source of OM in soils is straw return, accounting for more than 65% of OM input. Straw-OM has long been believed to reduce metal bioaccumulation, e.g., by immobilizing metals in soils. However, there is growing evidence that straw return could possibly enhance bioavailability and thus risks (i.e., food safety) of some metals in crops, including Cd, Hg, and As. Poor understanding of straw return-induced increases in metal bioavailability would add uncertainty in assessing or mitigating risks of metals in contaminated farming soils. Here, 863 pieces of literature (2000–2019) that reported the effects of straw return on metal bioavailability and bioaccumulation were reviewed. Mechanisms responsible for the increased metal mobility and bioavailability under straw return are summarized, including the effects of dissolution, complexation, and methylation. Effects of straw return on the physiology and the absorption of metals in plants is also discussed (i.e., physiological effect). These mechanisms are then used to explain the observed increases in the mobility, bioavailability, and bioaccumulation of Cd, Hg, and As under straw amendment. Information summarized in this study highlights the importance to re-consider the current straw return policy, particularly in metal-contaminated farmlands. Image 1 • Straw return impacts metal mobility by changing soil pH, Eh, and POM/DOM levels. • Straw return increases metal bioavailability by forming soluble metal-DOM complexes. • Straw-derived DOM enhances microbial activities and thus Hg/As methylation. • Straw return can affect plant physiology and thus metal bioaccumulation. [ABSTRACT FROM AUTHOR]