Soil erosion-related transport of neonicotinoids in new citrus orchards

Abstract

The widespread use of neonicotinoids has increased the risk of non-point source pollution. Previous studies mainly focused on neonicotinoid transport by runoff on gentle slopes and ignored the role of sediment. Neonicotinoid migration due to erosion could be underestimated because a considerable amount of sediment migration is caused by erosion on steep slopes. Here, we focused on the transport of two typical neonicotinoids, imidacloprid (IMI) and clothianidin (CLO), via runoff and sediment in new citrus orchards on steep slopes. The total runoff from the bare land (BL), biological crust (BC) and Kummerowia striata (Thunb.) Schindl. (KS) plots was 77.73, 66.22 and 46.94 mm, respectively, and the corresponding total sediment was 162.47, 54.00 and 3.51 kg ha-1, respectively. The on-site pollution was mainly due to IMI and CLO retention in topsoil (0-2 cm). A linear relation between the loss of IMI and CLO and the loss of runoff and sediment (p < 0.05) indicated that the migration of IMI and CLO with runoff and sediment was the main cause of off-site pollution. Compared with the BL plot, the risks of on-site and off-site pollution were reduced significantly in the BC and KS plots: BC reduced on-site IMI and CLO by 33.50% and 19.28% and off-site IMI and CLO by 57.47% and 52.78%, respectively; KS reduced on-site IMI and CLO by 44.97% and 46.42% and off-site IMI and CLO by 75.32% and 57.42%, respectively. This finding reveals that the ability of vegetation to decrease IMI and CLO transport was mainly associated with a reduction of runoff and sediment loss. Therefore, the introduction of biological crust and fast-growing plants is recommended in new citrus orchards because it is a cost-effective method of mitigating soil erosion and non-point source pollution.