Dynamics in imidacloprid sorption related to changes of soil organic matter content and quality along a 20-year cultivation chronosequence of citrus orchards

The on-going and extensive use of neonicotinoids occur in orchards. However, it is still unknown whether and how orchard management affects soil properties, especially the contents and structure of soil organic matter during orchard development, and their further influences on neonicotinoid persistence. Here, surface soil sam-ples were collected from the citrus orchards with different cultivation ages (1, 10, 14, and 20 years), and their physicochemical properties were determined. Changes in the chemical structure of soil organic matter (SOM) were furtherly examined using solid-state CP/TOSS C-13 NMR. Then, the sorption isotherms of imidacloprid in these soils were investigated. The sorption coefficient (K-d) of imidacloprid at C-e of 0.05 mg/L in the orchard soils increased by 19.4- 23.3%, along a 20-year chronosequence of cultivation, which should be mainly ascribed to the increase of SOM. However, the organic carbon-normalized sorption coefficient (Koc, sorption per unit mass of OM) of imidacloprid declined with increasing cultivation ages. Moreover, the polar and aliphatic domains of SOM had a significantly positive relation to the Koc of imidacloprid, suggesting its key role in governing imi-dacloprid sorption. The results highlighted that reasonable management measures could be adopted to control the occurrence and fate of neonicotinoids in soils, mainly by affecting the content and quality of SOM.