Effects of straw returning on photochemical process and imidacloprid degradation in paddy water through a field experiment

Abstract

Straw returning is an important strategy for effectively using straw resources to improve soil fertility and quality, but the effects of this process on reactive intermediates (RIs) formation in paddy water are not fully explored. In this study, we conducted a field experiment to explore the effects of straw returning on RIs formation including triplet chromophoric dissolved organic matter (3CDOM*), hydroxyl radicals (•OH), and singlet oxygen (1O2) in paddy water. It was found that straw returning considerably increased the steady-state concentration of RIs, and the application of rice straw with 25% and 75% organic replacement ratio increased RIs concentration by 2.52 and 2.25 times, respectively. The optical spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) analysis and structural equation model showed that straw returning produces abundant humified and aromatic DOM containing fulvic acid, lignin and protein-like substances, which could induce more RIs under irradiation. In addition, straw returning significantly accelerated imidacloprid (IMD) degradation, yielding degradation products that were highly toxic to aquatic animals (i.e., fish and daphnia) but less toxic to aquatic plants (i.e., green algae). This study would offer an innovative viewpoint on the effects of straw returning on the photochemistry of paddy water and the naturally occurring abiotic degradation pathway of organic contaminants.