Control of heavy metals exported by runoff and fertilization in a watershed

Objectives Heavy metals eroding from agricultural watershed enter rivers and lakes and cause water pollution. This poses a great threat to the safety and health of human and livestock drinking water. Many studies suggested that fertilizer application significantly increased the heavy metal accumulation in soil, but few studies determined the linkage of fertilization to heavy metal pollution in rivers and lakes in agricultural watershed. It is crucial to understand the influence of fertilization on heavy metal pollution inflow load for the prevention and control of agriculture-induced water pollution. Thus, the objectives of this study was to measure the temporal and spatial variations in export load of typical heavy metals eroding from an agricultural watershed, and to clarify these changes in response to runoff, fertilization and crop coverage.

Methods The Nala watershed, located in the water source area of Kelan Reservoir in Guangxi Province of southern China, was selected for this study. This watershed is a representative area for intensive sugarcane plantation on slopping land in sub-tropics. It includes three sub-watersheds, one in downstream (S₁), and the other two in upstream (S₂ and S₃). Land use types and crop coverage during sugarcane growth stage were obtained by using unmanned aerial vehicle (UAV). The runoff and sediment were monitored at the outlet of each sub-watersheds through the automatic monitoring and sampling systems. The concentration of typical heavy metals (Cr, Ni, Cu, Zn, As, Cd) in the sediment were measured in the laboratory, and then the export load of heavy metal eroding from sub-watershed was calculated. the relationship between heavy metal export load and precipitation, runoff, fertilization and crop coverage was determined by using Pearson correlation analysis.

Results The heavy metal export load decreased in the following order: Cr (5.16 kg/hm²) > Ni (3.0 kg/hm²) > Zn (2.98 kg/hm²) > Cu (1.89 kg/hm²) > As (1.46 kg/hm²) > Cd (0.01 kg/hm²). The heavy metal export load from the S₁ in the downstream was 1.68 and 1.44 times of that from S₂ and S₃ in the upstream, respectively. The highest soil heavy metal export occurred at the stages of tillering and grand growth, accounted for 74.32% of total export in the whole sugarcane growth period. Soil heavy metal export changed with different sugarcane growth stage in the following order: tillering (5.64 kg/hm²) > grand growth (5.13 kg/hm²) >establishment (3.53 kg/hm²)>ripening (0.19 kg/hm²). Precipitation, runoff, fertilization and crop coverage were significantly correlated with heavy metal export load.

Conclusions Cr, Ni and Zn were the uppermost heavy metal pollutants of sediment entering the river in sugarcane growing area of Guangxi, accounting for 78% of the total inflow load of 6 typical heavy metals. Heavy metal export in sediment from the watershed mainly occurred at the stages of tillering and grand growth of sugarcane, and the export in downstream watershed was significantly higher than that of upstream. The coupling of runoff and fertilization could control the heavy metal pollution export load. These results indicated that in the intensive sugarcane growing area, regulating fertilization time, reducing fertilizer spraying, and increasing crop coverage should be encouraged to reduce soil heavy metal accumulation and eroding loss.