Long-term assessment of the environmental fate of heavy metals in agricultural soil after cessation of organic waste treatments.

The current study examined the anthropogenic accumulation and natural decrease in metal concentrations in agricultural soils following organic waste application. Three common organic wastes, including municipal sewage sludge, alcohol fermentation processing sludge, and pig manure compost (PMC), were applied annually to an agricultural soil under field conditions over 7 years (1994-2000) at a rate of 12.5, 25, and 50 ton ha(-1) year(-1) and the soil accumulation of three metals of concern (Cu, Pb, and Zn) was monitored. Subsequently, organic waste amendments ceased and the experimental plots were managed using conventional fertilization for another 10 years (2001-2010) and the natural decrease in metal concentrations monitored. Although Cu and Zn concentrations in all experimental plots did not exceed the relevant guideline values (150 mg kg(-1) for Cu and 300 mg kg(-1) for Zn), significant increases in metal concentrations were observed from cumulative application of organic wastes over 7 years. For instance, PMC treatment resulted in an increase in Cu and Zn from 9.8 and 72 mg kg(-1) to 108.2 and 214.3 mg kg(-1), respectively. In addition, the natural decrease in Cu and Zn was not significant as soils amended with PMC showed only a 16 and 19 % decline in Cu and Zn concentrations, respectively, even 10 years after amendment ceased. This research suggested that more attention must be paid during production of organic waste-based amendments and at the application stage.

Hazardous effects of eight years of application of four organic waste materials on earthworm numbers and biomass in field lysimeters.

The hazardous effects on the earthworm numbers and biomass of eight consecutive yearly applications of three levels (12.5, 25, and 50 tons of dry matter/ha/y) of four different organic sludges (municipal sewage, industrial sewage, alcohol fermentation processing, and leather processing) to earthworm-free soils were examined using field lysimeters. Results were compared with those of pig manure compost (PMC)-treated soil. Of five species (Amynthas agrestis, A. sangyeoli, A. hupeiensis, Drawida koreana, and D. japonica) from 390 adult specimens collected 4 and 8 years after treatment, the number of species appeared to be lower in sludge-treated soils than in PMC-treated soils. Earthworm populations and biomass were correlated negatively with the pollution index [∑(heavy-metal concentration in soil/tolerable level)/number of heavy metal] 4 and 8 years after treatment. These results suggest that the long-term application of these sludges, particularly from industrial sewage and leather processing, might affect adversely the establishment of Megascolecid and Moniligastrid earthworms in field conditions.

Modeling uptake kinetics of cadmium by field-grown lettuce.

Cadmium uptake by field grown Romaine lettuce treated with P-fertilizers of different Cd levels was investigated over an entire growing season. Results indicated that the rate of Cd uptake at a given time of the season can be satisfactorily described by the Michaelis-Menten kinetics, that is, plant uptake increases as the Cd concentration in soil solution increases, and it gradually approaches a saturation level. However, the rate constant of the Michaelis-Menten kinetics changes over the growing season. Under a given soil Cd level, the cadmium content in plant tissue decreases exponentially with time. To account for the dynamic nature of Cd uptake, a kinetic model integrating the time factor was developed to simulate Cd plant uptake over the growing season: C Plant=C Solution.PUF max.exp[-b.t], where C Plant and C Solution refer to the Cd content in plant tissue and soil solution, respectively, PUF max and b are kinetic constants.