Long-term mesocosm experiments to investigate degradation of fluorescent tracers

Abstract

The choice of uranine (UR) and sulforhodamine B (SRB) as hydrological tracers has recently been questioned since they might interact with the soil or become degraded. In this context, microbiological degradation of UR and SRB and the factors influencing it remain poorly studied. Here, we conducted a long-term mesocosm experiment where the effects of plants and hydrologic conditions on the dissipation and degradation of UR and SRB was separately investigated. Mass balances were combined with excitation emission matrix (EEM) fluorescence spectroscopy to characterize microbially-derived organic matter. Results revealed that most of SRB accumulated in the sand while UR was mainly found in the pore water. The estimated degradation of UR and SRB was greater in the treatments with plants and under unsaturated conditions. Overall, UR exhibited higher presumable degradation than SRB. Two components were identified by parallel factor analysis of the EEMs in addition to the UR-related component (U), one humic-like of high molecular weight primarily derived from terrestrial or soil organic matter sources (A + C) and a second humic-like of lower molecular weight related to recent microbial activity (M). A high positive correlation (Spearman’s rho = 0.74, p < 0.001) between M and U suggested a possible link, in which the presence of UR fostered microbial processes, thus supporting the hypothesis of biodegradation of UR, which seemed independent of the presence of plants, whereas it was not possible to point out such a correlation for SRB. These results show for the first time that plants and the alternation of oxic and anoxic conditions are favourable to increase degradation of UR and SRB and that microbiological degradation can be involved and dominant in UR dissipation. Keywords: Hydrological tracers, Wetland mesocosm, Degradation, Sorption, Excitation-emission matrix fluorescence spectroscopy