Conservative tracer bromide inhibits pesticide mineralisation in soil lable at ScienceDirect Environmental Pollution xxx (2016) 1e8 Contents lists avai Environmental Pollution journal homepage www els[.]
Environmental Pollution xxx (2016) 1e8 Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol Conservative tracer bromide inhibits pesticide mineralisation in soil* Tina B Bech a, *, Annette E Rosenbom a, Sebastian R Sørensen c, Carsten S Jacobsen b a Geological Survey of Denmark and Greenland, Department of Geochemistry, Copenhagen, Denmark Aarhus University, Department of Environmental Science, Roskilde, Denmark c Novozymes, Copenhagen, Denmark b a r t i c l e i n f o a b s t r a c t Article history: Received 11 October 2016 Received in revised form December 2016 Accepted 11 December 2016 Available online xxx Bromide is a conservative tracer that is often applied with non-conservative solutes such as pesticides to estimate their retardation in the soil It has been applied in concentrations of up to 250 g Br L1, levels at which the growth of single-celled organisms can be inhibited Bromide applications may therefore affect the biodegradation of non-conservative solutes in soil The present study investigated the effect of potassium bromide (KBr) on the mineralisation of three pesticides e glyphosate, MCPA and metribuzin e in four agricultural A-horizon soils KBr was added to soil microcosms at concentrations of 0, 0.5, 2.5 and g Br L1 in the soil solution The study concluded that KBr had a negative effect on pesticide mineralisation The inhibitory effect varied depending on the KBr concentration, the type of pesticide and the type of soil Furthermore, 16 S amplicon sequencing revealed that the KBr treatment generally reduced the abundance of bacteroidetes and proteobacteria on both an RNA and DNA level Therefore, in order to reduce the effect of KBr on the soil bacterial community and consequently also on xenobiotic degradation, it is recommended that KBr be applied in a concentration that does not exceed 0.5 g Br L1 in the soil water © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Introduction The conservative tracer bromide has been used for decades to study leaching patterns in a representative range of soils, highlighting distinct differences in the hydrology of the soils and thus the leaching potential for pesticides and other contaminants The advantages of bromide are: i) it is present in low background concentrations in groundwater and soil, ii) it rarely sorbs to soil particles and iii) it is not biological degradable (Davis et al., 1980) When applying bromide, it is important that the volume used exceeds the background concentration of bromide in the receiving waters (Davis et al., 1980) Hence there may be a tendency to use higher concentrations to facilitate sufficient concentrations in receiving waters when applying a conservative tracer such as bromide A literature search revealed concentrations as high as 250 g Br L1 in irrigation water (Boesten and van der Pas, 2000) In a technical commentary, Korom and Seaman (2012) highlight that * This paper has been recommended for acceptance by Klaus Kummerer * Corresponding author Geological Survey of Denmark and Greenland, GEUS, Øster Voldgade 10, DK 1350 Copenhagen K, Denmark E-mail address: tib@geus.dk (T.B Bech) it is extremely common to read studies in which bromide is labelled “conservative” or “non-reactive” and for it to be assumed that this is the case In the commentary the main focus is on transport and how bromide may not be conservative due to anion adsorption when pH < PZC (point of zero charge) Paradoxically, at lower pH the fraction of bromide sorbed decreases with increasing aqueous bromide concentrations, making bromide appear more conservative at higher concentrations (Korom, 2000) Less is known about the biological effect of bromide in the natural environment than about its conservative behaviour In a review, (Flury and Papritz, 1993) report that concentrations ranging from 0.11 to 4.6 g Br L1 have an effect on the growth of singlecelled aquatic organisms and that a groundwater quality criterion of mg Br L1 should not be exceeded The concentration of bromide in soil water varies both spatially and temporally and is influenced by soil type, irrigation and application concentration/method A high initial dose followed by a break before precipitation/irrigation would result in the highest concentration in the upper soil In a field scale study by (Vryzas et al., 2012), 220 kg KBr ha1 was applied followed by 40 mm irrigation every 8e10 days Bromide concentrations reached 238 mg L1 in the soil water in the upper 25 cm, with a decreasing http://dx.doi.org/10.1016/j.envpol.2016.12.016 0269-7491/© 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Please cite this article in press as: Bech, T.B., et al., Conservative tracer bromide inhibits pesticide mineralisation in soil, Environmental Pollution (2016), http://dx.doi.org/10.1016/j.envpol.2016.12.016 T.B Bech et al / Environmental Pollution xxx (2016) 1e8 trend over time and with soil depth Boesten and van der Pas (2000) found a bromide concentration of 200 mg Br kg soil1 in the upper cm after one day Assuming a soil water content of 20%, this would result in a bromide concentration in the soil water of 1000 mg Br L1 Nielsen et al (2011a) found that flow characteristics influenced the bromide concentration in the soil following 50 mm irrigation containing 0.14 g Br L1 Bromide concentrations were up to 62 mg Br g1 soil near fractures/biopores, with an average concentration of 40 mg Br g1 in the surface soil Commonly the bromide concentration decreases with time and depth Concurrently, degradation of pesticides mainly occurs in the upper soil profile (Moorman and Harper, 1989; Sørensen et al., 2006) The disappearance of pesticides and residues in soil environments is a complex function of several processes, including sorption to soil matrix, dispersion, dilution, and abiotic and biotic degradation The ability of different soils to degrade pesticides largely depends on soil physical and biological properties, with sorption characteristics and the capacity for microbial degradation being two key factors (Moorman and Harper, 1989; Alexander, 2000; Jensen et al., 2004) Even though soil fungi and algae may play a role, the complete mineralisation of pesticides in soil is most often associated with bacterial metabolism (Hussain et al., 2015) Decreased pesticide degradation has previously been related to several interacting factors, such as reduced bioavailability due to sorption, low availability of organic or inorganic nutrients, reduction in the size and activity of the microbial population or the lack of a competent degradative microbial population (Moorman and Harper, 1989; Jensen et al., 2004; Radosevich et al., 1996) The discovered link between KBr and reduced herbicide degradation has to our best knowledge hitherto not been reported in the literature Therefore, the objective of the present study was to investigate how bromide in concentrations ranging from to g Br L1 influenced the mineralisation of glyphosate, MCPA and metribuzin in four agricultural soils Three herbicides with contrasting sorption and degradation properties were chosen to cover a span of physical and chemical characteristics MCPA is generally easily biodegradable and has minor sorption (Sørensen et al., 2006), metribuzin is expected to be slowly degraded and has weak sorption (Olsen et al., 2005) and glyphosate was selected due to its known strong sorption and medium degradability (Gimsing et al., 2004) 2.2 Herbicides Analytical grade N-phosphonomethyl-glycine (glyphosate; 98% purity), 4-chloro-2-methylphenoxyacetic acid (MCPA; 99.5% purity) and 4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)one (metribuzin; 99% purity) were purchased from Dr Ehrenstorfer GmbH (Augsburg, Germany) [Ring-U-14C] MCPA (1389 MBq/mmol; 100% radiochemical purity) and [P-methylene-14C] Glyphosate (871.1 MBq/mmol (23.56 mCi/mmol); >99% radiochemical purity) were purchased from Izotop (Institute of Isotopes Co., Ltd., Hungary) [Ring-6-14C] Metribuzin (1100 MBq/mmol; > 95% radiochemical purity) was purchased from Internationale Isotope (Munich, Germany) 2.3 Mineralisation Mineralisation of 14C-labelled glyphosate, MCPA and metribuzin to CO2 was studied in microcosms containing 2.5 g soil Each microcosm was spiked to an initial concentration of mg pesticide g1 soil by adding 125 mL 20 mg L1 pesticide solution with a radioactivity of approximately 10,000 DPM 125 mL KBr stock solutions were mixed into the soil, resulting in a soil water concentration of 0, 0.5, 2.5 or g Br L1 (soil water content ~ 20% after spiking) A small test tube with mL 0.5 M NaOH was placed in the flask to capture any 14C-CO2 formed by mineralisation The flask was sealed and placed in the dark at 10 C The NaOH trap was replaced on days 5, 9, 14, 19, 23, 30 and 36 for glyphosate, MCPA and metribuzin The 14C-CO2 content was determined using a Wallac 1409 liquid scintillation counter (LSC) after being mixed with 10 mL Optiphase HiSafe scintillation cocktail (Wallac, Finland) Radioactivity was converted to the percentage mineralisation of the pesticide in the microcosms 14 2.3.1 Modelling The mineralisation of glyphosate and MCPA was described applying a first-order model: h i P ¼ S0 eðktÞ where P is product formation (CO2) (% of the initial amount of substrate (S0)), and S0 is the initial amount of substrate (in terms of the maximum percentage of the initial carbon that can evolve as CO2 added at day 0) and k is the mineralisation rate constant The metribuzin mineralisation was described applying the zero-order model: Material and methods 2.1 Soil P ẳ S0 ỵ k,t The four agricultural soils represent typical soil types found in Denmark: sand, loamy sand, sandy loam and loam The soil characteristics are given in Table A-horizon soil was collected as small subsamples within a few square metres and mixed thoroughly All soils were sieved (2 mm) after sampling and stored at 10 C where P is product formation (CO2) (% of the initial amount of substrate (S0)), S0 is the initial amount of substrate (in terms of the maximum percentage of the initial carbon that can evolve as CO2 added at day 0) and k is the mineralisation rate constant The glyphosate mineralisation followed a first-order model due to the decreased mineralisation rate during the experiment This model was also chosen for MCPA even though some curves were sigmoidal indicating bacterial growth Thereby, it was possible to statistically compare the mineralisation rate constant (k) between the different KBr concentrations Zero-order model was chosen for metribuzin due to the very low mineralisation Table Physical properties of the soils Soil SOMa[%] Clayb[%] Siltb[%] Sandb[%] Gravelb[%] pHCaCl2 Sandy loam Loam Loamy sand Sand 3.35 3.57 3.93 3.83 4.54 7.91 2.76 1.18 22.50 32.02 14.84 6.58 70.80 58.25 82.24 91.32 2.16 1.82 0.15 0.93 5.95 5.71 5.49 5.81 a b Loss on ignition Clay < mm, silt 2e20 mm, sand 20e2000 mm, gravel > 2000 mm 2.4 Sorption experiments Air-dried soil samples were sieved to