Qualifying and quantifying linuron degradation capacity with colorimetric reaction? Test case on a linuron treated agricultural soil

The IPU-degrading Sphingomonas sp. strain SRS2 was grown on R2A agar medium supplemented with 20 mg L-1of IPU (Attachment 1 & 2). The Variovorax strain SRS16 capable to degrade both linuron and diuron, was grown on R2A agar medium supplemented with 20 mg L-1of linuron (Attachment 1 & 2).
Prior to inoculation, the cells were washed two times in 0.9% NaCl and the optical density at 600 nm (OD) was adjusted to 0.1. For each herbicide 50 ml of minimal medium supplemented with 20 mg L-1of the herbicide was inoculated with 1% of cell suspension (OD600=0.1) of the appropriate strain. Only for the diuron-degradation test, the minimal medium was supplemented with 1% of yeast (Attachment 1 & 2), as suggested by Sorensen et al. (2008). Microbial degradation tests were performed in duplicate.
For each herbicide a control was included which was not inoculated. All flasks were incubated in the dark at 25°C on a rotary shaker. Everyday a sample of 1.5 mL was taken from each flasks to test for the presence of the metabolite. The analysis were performed with HPLC and the coupling-diazotization reaction (par 2.1).