In-situ atrazine biodegradation dynamics in wheat (Triticum) crops under variable hydrologic regime
Introduction
The herbicide atrazine (ATZ) is a broadleaf triazine weed suppressor introduced in agriculture in 1958 (IPSC, 1990). Between 2012 and 2013, 46,350 tonnes of triazines were applied worldwide (FAO, 2013), while other herbicides such as glyphosate may now be more widely used (Benbrook, 2016). Despite soils with a long history of ATZ applications show short ATZ half-life near the surface (Shaner et al., 2007), ATZ can accumulate and persist in aquifers (Thurman et al., 1994), as observed in European countries that have stopped using ATZ before 2004 but still detect ATZ concentrations above 0.1 μg/l in aquifers (ISPRA, 2016, Vonberg et al., 2014, EC, 2004). ATZ degrades to hydroxylated and chlorinated metabolites eventually leading to acetone, acetaldehyde, NH3, HCl, and CO2 via a number of hydrolytic, oxidative, and oxidative deamination catabolic reactions (Boundy-Mills et al., 1997, Kumar and Singh, 2016, Mandelbaum et al., 1995, Martinez et al., 2001, Solomon et al., 2013), with half-life shorter than 4 days (Katz et al., 2000, Radosevich et al., 1995). Bacterial strains that degrade ATZ rapidly, allowing for short half-life, have been found to have a high specific biomass affinity (la Cecilia and Maggi, 2016); hence, low-efficiency ATZ soil biodegraders, together with disadvantageous environmental conditions that reduce ATZ bioavailability and microbial activity (Krutz et al., 2008, Singh and Cameotra, 2013), may slow down ATZ biodegradation and lead to longer ATZ half-life (Alvey and Crowley, 1997, Eisler, 1989). As ATZ residence time increases, also the chance for long-range transport away from the contamination source point may increase, and phytotoxicity and endocrine system disrupting potential to amphibians and humans may extend reach in space and time (Fan et al., 2007, Hayes et al., 2002). ATZ reaching aquifers is unlikely to degrade (Widmer et al., 1993), while it may be only slowly diluted over decadal time scales.
Numerical models in use to predict contaminants concentrations in soil would ideally integrate all interplaying physical, chemical, and biological processes that govern contaminants transport, including boundary conditions and biogeochemical reactions. Only few reactive transport models are so comprehensive (e.g., HYDRUS, (Yu and Zheng, 2010); MODFLOW-RT3D, (Johnson and Truex, 2006); TOUGHREACT, (Xu et al., 2011); and BRTSim, (Maggi, 2015)), whereas the greatest number does not account for biological processes in a mechanistic framework, and simplistic models deal with transport processes only. In addition, chemical and biological reactions in soil are complex and sometimes unknown; therefore, modeling hypotheses have to be used to describe those processes and understand how their superposition affects contaminants dynamics in soil under different hydrometeorological forcing. While no model can capture all these processes in their entirety, there are some essential aspects that influence contaminants dynamics in the environment and should therefore be integrated in models. Soil water dynamics, strongly driven by boundary conditions, transport contaminants far from their source points, equilibrium and kinetic chemical reactions may either favor contaminants persistence or their degradation, while biological processes may reduce the contaminants concentration.
The aim of this paper is to propose an ATZ biodegradation reaction network coupled with the nitrogen cycle (Maggi et al., 2008) in soil, and test it in a comprehensive, general-purpose numerical solver, which satisfies most of the above mentioned requirements for applications to contaminant hydrology. For this purpose, laboratory experiments were retrieved from the literature to estimate the kinetic parameters of biologically-mediated hydrolytic, oxidative, and deaminative ATZ biodegradation reactions. Equilibrium chemical reactions including complexation, adsorption, and gas volatilization, and kinetic biochemical reactions were implemented in a multicomponent and multiphase bioreactive transport computational solver, BRTSim-v2.1 (based on Maggi (2015)), and were used to predict the dynamics of ATZ and its byproducts over the soil profile from the vadoze zone to the aquifer for various scenarios of ATZ applications over a 100-year-long period.
Section snippets
ATZ biodecomposition pathways
In soil, three microbial functional groups can degrade ATZ to acetone, acetaldehyde, NH3, HCl, and CO2 via a number of pathways (Fig. 1); three of these pathways break ATZ down to cyanuric acid (CYA) with different enzymes, while three other pathways biodegrade CYA, ethylamine (ETA), and isopropylamine (IPA) produced during biodegradation of ATZ metabolites (Fig. 1).
ATZ biodegradation rate and Specific biomass affinity
In the reference simulation (2 kg/ha/year), aerobic ATZ hydrolysis (P1R1a) was the main mechanism for ATZ biodegradation (79.2%); anaerobic hydrolysis (P1R1b) biodegraded 18.2% of applied ATZ, while oxidative reactions P2R1 and P3R1 contributed nearly 0.11 and 0.15%, respectively (Table 2). HOATZ, which does not contain Cl, was the main byproduct of ATZ hydrolysis (P1R1a and P1R1b), and organochlorides mass was reduced to a 2.6% of the total applied mass. Krutz et al. (2010) assumed the
Conclusion
An advanced bioreactive transport solver was used to assess a comprehensive ATZ biodegradation network to its end products under various hydrological conditions in a wheat crop. 97.4% of applied ATZ was biodegraded to its non phytotoxic metabolite HOATZ by BATZhyd, while only 0.2% was biodegraded by BATZoxi. BATZhyd had a higher specific biomass affinity than BATZoxi, thus providing evidence that this parameter can be used to select the optimal pollutant-degrading functional group in
Acknowledgments
This research was partially funded by the Civil Engineering Research and Development Scheme 2015 (CERDS) of The University of Sydney. We thank two anonymous reviewers for their feedbacks on this work. BRTSim and the input files for the atrazine biogeochemical cycle can be downloaded at https://www.dropbox.com/sh/wrfspx9f1dvuspr/AAD5iA9PsteX3ygAJxQDxAy9a?dl=0, or alternatively by contacting the authors.
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