Transformation and binding of 13C and 14C-labelled atrazine in relation to straw decomposition in soil

As a source of organic matter, crop residues affect the behaviour of pesticides in agricultural soils. The fate of [U‐ring‐¹³C] and [U‐ring‐¹⁴C] atrazine (6‐chloro‐N‐ethyl‐N‐isopropyl‐1,3,5‐triazine‐2,4‐diamine) was investigated during laboratory incubation under controlled conditions in a loamy soil amended with wheat straw at two different states of decomposition: no preliminary decomposition or 6 months’ preliminary decomposition. After 3 months, non‐extractable, so‐called ‘bound’, ¹³C‐atrazine residues were recovered in three particle‐size fractions (> 200, 50–200 and < 50 μm), and investigated with solid‐state ¹³C‐NMR spectroscopy. Parallel incubations with [U‐ring‐¹⁴C] atrazine were carried out to quantify the bound residues as well as the extractable and mineralized fractions. The effect of straw residues on atrazine behaviour depended on whether they had been previously decomposed or not. When straw was decomposed for 6 months prior to incubation, atrazine mineralization was enhanced to 50% of the initial ¹⁴C in contrast to 15% of the initial ¹⁴C in soil alone and soil amended with fresh straw. In parallel, atrazine bound residues were formed in greater amount representing up to 20% of the initial ¹⁴C. CP/MAS ¹³C‐NMR on soil size fractions of soil–straw mixtures after incubation with ¹³C‐atrazine showed that bound residues contained mostly triazinic C, corresponding to atrazine or primary metabolites. Non‐humified organic materials recovered in size fractions > 200 and 50–200 μm contained significant amounts of bound residues, especially when straw was added to the soil. CP/MAS ¹³C‐NMR analysis of humic acids obtained from < 50‐μm fractions was difficult due to overlapping of the native carboxyl ¹³C signal with the ¹³C‐atrazine signal.