Annual net input fluxes of heavy metals of the agro-ecosystem in the Yangtze River delta, China

- Annual fluxes of heavy metals to topsoil in the Yangtze River delta are assessed.
- Irrigation water and atmospheric deposition were the important sources.
- The leaching amount of heavy metals is larger than the uptake of them by crops.
- The average input rates of heavy metals are calculated.
- Cadmium had the shortest time from average values to the environmental limits.

Understanding the status and extent of agricultural soil contamination in industrialized developed areas is important for developing sustainable management strategies. The annual inputs of heavy metals (As, Cd, Cr, Hg, Cu, Pb and Zn) to agricultural soils in the Yangtze River delta, China in 2007-2009 have been studied. Five fluxes through the cultivated horizon were considered: 1) atmospheric depositions, 2) fertilizers, 3) irrigation water (as inputs), 4) crops, and 5) leaching water (as outputs) and corresponding samples collected and analyzed on a large regional scale. The results show that irrigation water was the main source of metals (As, Cd, Cu and Hg), contributing 60-71% of the total inputs. Atmospheric deposition was an important source of Zn and Pb, responsible for an estimated 72% and 84% of the total inputs, respectively. The input sources for heavy metals show a clear spatial heterogeneity at the individual field scale. Furthermore, the amounts of heavy metals leached were generally larger than the uptake of heavy metals by crops. The leaching water accounted for approximately 32-98% of the total heavy metal outputs. The average input rates of heavy metals were calculated based on the assumption that the annual net inputs of heavy metals to agricultural soils are accumulated in the topsoil (0-20 cm). Zn had the highest average input rate of 224.0 μg/kg/y, Hg and Cd had the lowest average input rates of 0.1 and 2.9 μg/kg/y. Among the elements concerned, Cd had the shortest predicted time (number of years) from average values to the environmental limits. This work is helpful in developing strategies to reduce heavy metal inputs into agricultural land and policies to maintain ecological balance.