Prolonged cultivation reduces imidacloprid retention in citrus soil

Increasing crop age in citrus groves may compromise the soil’s ability to retain the insecticide imidacloprid (imidacloprid, CAS 38261-41-3, IRAC 4A), according to a study conducted by scientists at Jiangxi Agricultural University, who analyzed soils that had been under continuous cultivation for 10, 30 and 50 years in red soil regions in southern China. The researchers used advanced spectroscopic techniques to understand the interactions between the insecticide and dissolved organic matter (DOM) in the soil.

Over the years, the soil in these orchards has accumulated more organic matter. The content has increased by almost 58% over the course of five decades. This accumulation is a reflection of the decomposition of plant residues and the action of microorganisms stimulated by fertilization.

However, the older the orchard, the lower the affinity between the soil and imidacloprid. This is due to the increasing complexity of the organic molecules, which become larger and more aromatic, making it difficult to adsorb the pesticide.

The research found that imidacloprid interacts preferentially with fractions of dissolved organic matter similar to fulvic acids. These compounds have a high capacity for electronic exchange and carboxylic groups that favor binding with the insecticide. As soils age, these fractions decrease proportionally. Humic acids, more abundant in old orchards, demonstrated lower binding efficiency.

Another important factor is soil pH. According to scientists, with the constant application of nitrogen fertilizers, the soil became more acidic over time. The pH dropped from 4,83 in younger orchards to 4,08 in 50-year-old orchards. This acidification contributes to the chemical transformation of organic matter, making it less likely to form complexes with the insecticide.

Even with more dissolved organic matter present, older soils retained less imidacloprid. This phenomenon, described by the authors as a “quantity–quality paradox,” shows that greater availability of organic matter does not translate into greater adsorption capacity. The adsorption coefficient normalized by organic carbon (Koc) decreased as the soil aged.

Fluorescent analyses also revealed structural changes in organic matter. In older soils, new fractions associated with microbial byproducts emerged. These fractions behaved differently during binding with the pesticide.

Furthermore, tests showed that the most reactive functional groups, such as hydroxyls and amines, become less available after decades of cultivation, reducing the formation of stable complexes.

The order of interaction between dissolved organic matter and the insecticide was also observed. First, imidacloprid binds to fulvic fractions. Then, to humic fractions. Finally, to protein fractions. This sequence reinforces the idea that soils richer in humic fractions — typical of old orchards — have lower retention efficiency.

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