Fluorescent tracers to evaluate pesticide dissipation and transformation in agricultural soils.

This study evaluates the mobility and dissipation of two organic fluorescent tracers (uranine, UR and sulforhodamine-B, SRB) in soil from an agricultural field. Two plot experiments were conducted for 2.5months in 2012 and 2016 to compare the behavior of reactive fluorescent tracers (UR and SRB) to the chloroacetanilide herbicide S-metolachlor (S-MET) and bromide (BR), used as a traditional conservative tracer. SRB in top soil closely mimicked the gradual recession of S-MET, while BR overrated both top soil mobility and slow leaching of S-MET in the soil column. In contrast, UR quickly receded in the soil and was entirely dissipated at the end of the study periods. Instead, a strong fluorescent signal that was stable against acidification, and non-traceable in background samples, gradually developed at an excitation wavelength of 510nm in samples from the uppermost soil layer starting 40 (2012) and 22 (2016) days after tracer application. We hypothesize that (bio-)chemical transformation of UR accelerated tracer loss with concomitant formation of the specific transformation product TP510. By LC-MS/MS analysis we propose a probable molecular structure of TP510 and sulfonation as one likely transformation process. Overall, we anticipate our results to be a starting point to use fluorescent tracers in longer term (>2months) agricultural soil studies as a proxy for S-MET and possibly also other organic pesticides, as they are non-conservative in unsaturated soil and may follow similar dissipation and transformation patterns. At the same time their analysis is less costly and they pose smaller environmental risks.

Correcting Laser-Based Water Stable Isotope Readings Biased by Carrier Gas Changes.

Recently, laser-based water stable isotope spectrometers have become popular as they enable previously impossible approaches of environmental observations. Consequently, they have been subjected to increasingly heterogeneous atmospheric conditions. However, there is still a severe lack of data on the impact of nonstandardized gas matrices on analyzer performances. Against this background, we investigated the influence of changing proportions of N2, O2, and CO2 in the carrier gas on the isotope measurements of a typical laser-based water stable isotope analyzer (Picarro L2120-i). We combined environmentally relevant mixtures of N2, O2, and CO2 with referenced, flash-evaporated water and found that isotope readings of the same water were altered by up to +14.57‰ for δ(18)O and -35.9‰ for δ(2)H. All tested relationships between carrier gas changes and respective isotope readings were strongly linearly correlated (R(2) > 0.99). Furthermore, an analyzer-measured variable allowed for reliable postcorrection of the biased isotope readings, which we additionally tested on field data. Our findings are of importance for environmental data obtained by analyzers based on the same technology. They are relevant for assays where inconsistent gas matrices or a mismatch in this regard between unknown and reference analyses cannot be excluded, which is in particular common when investigating the soil-vegetation-atmosphere continuum.

A method for in situ monitoring of the isotope composition of tree xylem water using laser spectroscopy.

Field studies analyzing the stable isotope composition of xylem water are providing important information on ecosystem water relations. However, the capacity of stable isotopes to characterize the functioning of plants in their environment has not been fully explored because of methodological constraints on the extent and resolution at which samples could be collected and analysed. Here, we introduce an in situ method offering the potential to continuously monitor the stable isotope composition of tree xylem water via its vapour phase using a commercial laser-based isotope analyser and compact microporous probes installed into the xylem. Our technique enables efficient high-frequency measurement with intervals of only a few minutes per sample while eliminating the need for costly and cumbersome destructive collection of plant material and laboratory-based processing. We present field observations of xylem water hydrogen and oxygen isotope compositions obtained over several days including a labelled irrigation event and compare them against results from concurrent destructive sampling with cryogenic distillation and mass spectrometric analysis. The data demonstrate that temporal changes as well as spatial patterns of integration in xylem water isotope composition can be resolved through direct measurement. The new technique can therefore present a valuable tool to study the hydraulic architecture and water utilization of trees.

Dissipation of hydrological tracers and the herbicide S-metolachlor in batch and continuous-flow wetlands.

Pesticide dissipation in wetland systems with regard to hydrological conditions and operational modes is poorly known. Here, we investigated in artificial wetlands the impact of batch versus continuous-flow modes on the dissipation of the chiral herbicide S-metolachlor (S-MET) and hydrological tracers (bromide, uranine and sulforhodamine B). The wetlands received water contaminated with the commercial formulation Mercantor Gold(®) (960 g L(-1) of S-MET, 87% of the S-enantiomer). The tracer mass budget revealed that plant uptake, sorption, photo- and presumably biodegradation were prominent under batch mode (i.e. characterized by alternating oxic-anoxic conditions), in agreement with large dissipation of S-MET (90%) under batch mode. Degradation was the main dissipation pathway of S-MET in the wetlands. The degradate metolachlor oxanilic acid (MOXA) mainly formed under batch mode, whereas metolachlor ethanesulfonic acid (MESA) prevailed under continuous-flow mode, suggesting distinct degradation pathways in each wetland. R-enantiomer was preferentially degraded under batch mode, which indicated enantioselective biodegradation. The release of MESA and MOXA by the wetlands as well as the potential persistence of S-MET compared to R-MET under both oxic and anoxic conditions may be relevant for groundwater and ecotoxicological risk assessment. This study shows the effect of batch versus continuous modes on pollutant dissipation in wetlands, and that alternate biogeochemical conditions under batch mode enhance S-MET biodegradation.