Effects of water-saving irrigation on the residues and risk of polycyclic aromatic hydrocarbon in paddy field

Highlights: • Water-saving effectively reduced the leaching quantity of PAHs into groundwater. • Soil risk of PAHs was reduced up to 55%–73% by water-saving vs. flooding irrigation. • Rice carcinogenic risk of PAHs was reduced up to 30–45% by water-saving. • Total Toxic Equivalency Concentration of PAHs in rice grain was reduced about 50%. • This is related to the change of paddy field environment due to moisture control. The effects of different water-saving modes on PAHs residue and risk, field environment conditions and enzyme activities in paddy field were investigated in a field experiment plot in Laoyaba, Nanjing, China. Results showed that (1) water-saving treatment affected greatly the ΣPAHs in water and soil. The order of ΣPAHs residue in surface water and groundwater in farmland is as follows: dry fields < water-saving paddy field < flooding irrigation paddy field. The ΣPAHs in water during rice tillering stage were obviously higher than that in rice booting stage and milky stage, and the percentage of high-ring PAHs gradually reduced in water. (2) The residue of ΣPAHs in soil in flooding irrigation paddy field (534.4 ± 186.7 ng/g) were more than water-saving irrigation (454.3 ± 128.1 ng/g) and dry cultivation paddy field (430.2 ± 143.4 ng/g), and the ΣPAHs in dry field gradually decreased with the increase of water furrow number in farm. (3) When compared with flooding irrigation (337.87 ng/g), water-saving (228.39 ng/g) and dry cultivation (206.62 ng/g) could obviously decrease the residue of ΣPAHs in rice tissues (35%–55%), generally the concentration of ΣPAHs in leaf > root > stem > rice grain. (4) Water-saving irrigation evidently decreased soil ecological risk (up to 55%–73%) and rice carcinogenic risk (up to 30%–45%) caused by PAHs compared with flooding irrigation. Water-saving irrigation could also reduce the Total Toxic Equivalency Concentration of PAHs in rice grain up to 50% relative to flooding irrigation. (5) The significant negative correlations were observed between the residual PAHs and the activities of laccase and dioxygenase (p < 0.019), and the physical and chemical indexes (temperature, redox potential and dissolved oxygen of field, p < 0.041). The changes of field environment conditions and enzyme activities induced by moisture control may be the main key factors affecting PAHs residue in water, soil and rice.