Growth and yield, and nitrate and hazardous metal contents of the rice plant, percolating water and soil of a paddy field irrigated with the effluent of a rural sewage treatment plant

摘要

An experimental paddy field (370 m~2) was irrigated with the effluent of a rural sewage treatment plant (328 m~3/d) for 7 years, and the concentrations of hazardous metals in the rice plant, percolating water and surface soil as well as the growth and yield of rice were compared with those obtained from a neighboring paddy field (325 m~2) irrigated with the water of an irrigation canal. The total volume of the irrigated effluent was strongly affected by rainfall during a growing season, and ranged from 172 to 849 mm/y, with the mean value of 415 mm/y. The mean concentrations of total-N, NH_4-N, NO_3-N, total-P and K~+ in the effluent for 7 years were 14.4, 2.04, 9.25, 1.86 and 8.1 mg/l, respectively. Those of total-N and total-P in the percolating water of the control field were 0.49 and 0.038 mg/l, respectively, whereas those of the field irrigated with the effluent were 0.52 and 0.029 mg/l in the last 7th year. The mean supplies of total-N and total-P from the irrigated effluent were 61.8 and 7.89 kg/ha per crop, respectively, and increased the percolating losses by 2.0 and 0.22 kg/ha.y in comparison with those from the control field. The concentrations of Cd, Hg, Cu, Zn, Pb and As in the harvested rice grains, leaves and stems, surface soil (ca. 10 cm), and percolating water did not differ between the effluent-irrigated and the control field. No increasing trends were observed in the metal and nitrate concentrations of the percolating water as well as the soils for these 7 years. The mean yields of the rice grain harvested from the effluent-treated and the control field were 5,950 and 5,770 kg/ha per crop, respectively.