In China, heavy metal pollution of farmland has become more and more concerned. Government and policymakers pay close attention to the control and remediation of heavy metal polluted soil. The standard-exceeding rates of heavy metals in surveyed soil on a national scale were 7.0% for Cd, 2.1% for Cu, 0.9% for Zn, and 4.8% for Ni. Heavy metals mainly accumulate in surface soil and there is potential danger to migrate and transport via surface runoff during rainfall events, and then polluting surface water and crops, and thereby posing substantial risks to human and animal health.

The concentrations of heavy metals in surface runoff were affected by rainfall. Moreover, erosion of topsoil can be a significant form of pollutant loss. Vegetation cover and mulches have been proven to protect the soil surface against the erosive forces of rain and runoff. Various organic mulches had been applied for soil and water conservation purposes. However, few studies had concentrated on the migration and transportation of heavy metals in runoff of agricultural soils in response to both natural and simulated rainfall.

Researchers from the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) conducted a simulated rainfall experiment  to determine the effect of different rates of rice straw mulching on the concentrations, transportation forms and cumulative amounts of Cd, Cu, Zn, and Ni transported via surface runoff.

The simulated rainfall was 90 mm hr^-1 with a uniformity of 85%. Soil box used in the study was 200*50*20 cm (L*W*H) with a slope gradient of 10°. Rice straw was used at the level of 0, 200 and 500 g m^-2. The ICP-OES was used to determine the concentrations of Cd, Cu, Zn, and Ni. The enrichment ratio (ER) was used to reflect the transportation and enrichment of heavy metals in sediment particles. All the data were processed  and showed as the means of the determined. The difference in the determined indices between treatments was tested by the repeated measures ANOVA within general linear model (GLM) following the Greenhouse-Geisser correction using the SPSS 19.0 statistical software.

Scientists found that rice straw mulching delayed runoff generation by 4 min for 200 g m^-2 treatment, and by 25 min for 500 g m^-2. Straw mulching, particularly at a high rate, may significantly reduce sediment and runoff in the soil. Applying rice straw of 200g m^-2 significantly reduced the cumulative runoff volume and cumulative sediment by 31% and 93%. Rice straw mulching at 500g m^-2 significantly reduced the cumulative runoff volume and cumulative sediment by 50% and 97%.

Also, scientists found that tested heavy metals Cd, Zn and Ni in straw mulching treatments enriched in sediments in runoff. For the rice straw mulching of 200g m^-2 and 500g m^-2 treatments, the ER of Cd, Zn and Ni varied from 1.03 to 1.51 and 1.21 to 1.86, respectively. It was larger than the critical value of 1.0. Moreover, straw mulching can effectively reduce the pollution from heavy metals with sediments in runoff. Particulate bound heavy metals in sediments decreased by 88.1-88.9% for the200g m^-2 treatment and 94.5-97.1% for the 500g m^-2 treatment, respectively.

Furthermore, the proportion of particulate bound heavy metals to total heavy metals in rice straw mulching treatments decreased significantly. Rice straw mulching could be beneficial to control heavy metals pollution from sediments in runoff.

This research was supported by funding from the National Key Technologies R&D Program of China (2015BAD05B02), the National Natural Science Foundation of China (41371318, 41101300), and Natural Science Foundation of Hunan Province (13JJ4113).

The study entitled "Effect of rice straw mulching on migration and transportation of Cd, Cu, Zn, and Ni in surface runoff under simulated rainfall" has been published in the Journal of Soils and Sediments, details could be found at https://link.springer.com/article/10.1007/s11368-016-1391-8.