Balancing agricultural benefits and environmental risks: Trace element contamination from prolonged straw incorporation

Straw return (SR) is an effective resource recycling practice that improves soil properties by providing essential nutrients and organic matter, while also reducing agricultural waste. However, emerging evidence indicates that SR enhances the bioavailability of metals (e.g., Cd and Hg) in soils and crops, potentially threatening food safety and sustainable agriculture. Nevertheless, this conclusion has not been tested globally. We conducted a global-scale meta-analysis to examine the effects of SR on major trace elements (TEs), including heavy metals in cropland, using 1306 paired datasets. Our findings show that SR increases the concentration of total Cd (T-Cd) (3.47 %), T-Hg (8.21 %), T-Cu (2.00 %), T-Pb (3.28 %), and T-Cr (1.10 %) in soil; T-Hg (24.78 %), T-Pb (6.50 %), T-Ni (25.35 %), and T-Fe (25.48 %) in straw; and T-Zn (7.06 %) and T-Mn (8.16 %) in grain. Further analysis reveals that SR significantly increases TEs concentration in the topsoil. SR exhibits a time-accumulative effect on TEs concentrations, with a significant increase observed for durations exceeding 5 years ( p < 0.05). TEs contamination is more severe in paddy fields than in upland and upland-paddy rotation systems. The random forest model demonstrates that high temperatures, heavy rainfall, low initial soil bulk density, low nitrogen input, and prolonged SR increase the risk of TEs pollution. Furthermore, SR promotes TEs accumulation in the soil and their migration to crop straw and grains through three mechanisms: the dissolution effect (lowered pH increasing metal ion solubility), the complexation effect (DOM complexation), and the physiological effect (enhanced root growth promoting metal adsorption and accumulation). Overall, our research highlights TEs pollution resulting from SR, elucidates its driving factors and mechanisms, and provides theoretical guidance for implementing safe SR practices. • Effect of straw return (SR) on trace elements (TEs) was assessed through a meta-analysis. • SR enhances TEs content in soil, straw, and grain. • Response of TEs to SR varies with soil depth, experimental duration, and land use type. • The underlying mechanism by which SR increases TEs pollution is revealed.