Influx and efflux of arsenic in cotton fields irrigated with arsenic-contaminated groundwater

Abstract

Samples of rhizosphere soil, groundwater used for drip irrigation, and mature cotton plants were collected from farms at Kuitun, Xinjiang and analyzed for their arsenic (As) levels to account for the influx and efflux of arsenic in the field. The pH value and the soil-grain-size distribution were found to correlate with the different levels of arsenic accumulated by cotton plants and arable soil layers. The principal component analysis (PCA) indicates that the arsenic level in soil is influenced by the silt content and the arsenic level in groundwater, while the arsenic level in plant roots is affected mainly by the silt content and the arsenic level in soil. As estimated, about 64.6 ± 2.7 μg of arsenic was extracted from soil by one cotton plant, and more than 44% of it was stored in husks. About 0.8-2.6 mg As/(m2 yr) was extracted by plants from soil according to the average dry weight of the plant and the field planting density, while 2.4–28.0 mg As/(m2 yr) entered the soil based on the irrigated groundwater volume and the arsenic level in groundwater. Taking account of the cotton residues left after harvest, up to 85.8% of arsenic in the plants returned to the soil. After calculation, about 0.7–2.2 mg As/(m2 yr) returned to the soil, and thus the actual net arsenic input to the soil could reach 2.3–27.6 mg As/(m2 yr). Because of an obvious rise of the soil arsenic level in Kuitun, Xinjiang, and certain preventive measures should be taken to prevent the propagation of arsenic in the agricultural field.