Experimental study on the movement of heavy metal Zn in paddy soil under different irrigation quota of reclaimed water

To reveal the mechanism of heavy metal Zn migration in the irrigated paddy field reclaimed water, this study investigated irrigation quota of 75, 150, 225 and 300 m(3)/hm(2) for three consecutive years. The results showed that with the same irrigation quota, firstly the content of Zn, its variation and the rate of change in soil increased, and hereafter decreased with the increase of soil depth, and finally become stable of reclaimed water. Study results identified that when the irrigation quota was 75, 150, 225 and 300 m(3)/hm(2), the average content of Zn in the soil reached the maximum with the value of 9.60, 12.10, 16.75 and 18.50 mg/kg respectively at the depth of 30 cm. The average content of Zn in soil found maximum values of 13.51, 16.01, 19.02 and 20.98 mg/kg, respectively on the 120th day of cultivation. This study also identified that the content of Zn, its variation and the rate of change increased with the increase of irrigation quota at the same soil depth. Additionally, when the soil depth or plant growth time was the same, the content of Zn, its variation and the rate of change increased with the increase of irrigation quota. However, at the soil depth of 30 cm, the content of Zn in the irrigation quota of 75, 150 and 225 m(3)/hm(2) decreased by 48.11%, 34.59% and 9.46%. The fertility time of 120 days also decreased by 35.71%, 23.81% and 9.52% respectively compared to an irrigation quota of 300 m(3)/hm(2). All the findings are explored by a nonlinear regression under different situations and timing. The mean value of the standard error between the statistical and measured value is found insignificant. However, the correlation coefficient is found greater than 0.9400 and statistically significant. Thus, the findings by nonlinear regression reflected the migration law of soil Zn duly with soil depth and plant growth time in the rice field. This study provided theoretical support for the comprehensive treatment and ecological restoration of heavy metals to the farmland soil in China.