Fate of glyphosate and its degradation products AMPA, glycine and sarcosine in an agricultural soil: Implications for environmental risk assessment

Highlights

• ¹³C/¹⁵N-mass balances of glyphosate and degradation products fate were determined.

• Only traces of glyphosate and about 30% of AMPA were extracted from soil.

• High amounts of NERs_biogenic were determined for glyphosate, glycine and sarcosine.

• The NERs from AMPA were mainly NERs_unknown and thus potentially NERs_xenobiotic.

Abstract

Glyphosate can be biodegraded via the aminomethylphosponic acid (AMPA) and the sarcosine/glycine pathway leading to the formation of three intermediate products AMPA, sarcosine or glycine. The fate of the three intermediate compounds of glyphosate biodegradation including nature of non-extractable residues (NERs; harmless biogenic [NERs_biogenic] versus hazardous xenobiotic [NERs_xenobiotic]) in soils has not been investigated yet. This information is crucial for an assessment of environmental risks related to the speciation of glyphosate-derived NERs which may stem from glyphosate intermediates. Therefore, we incubated ¹³C- and ¹⁵N-labeled glyphosate (2-¹³C,¹⁵N-glyphosate) and its degradation product AMPA (¹³C,¹⁵N-AMPA), sarcosine (¹³C₃,¹⁵N-sarcosine) or glycine (¹³C₂,¹⁵N-glycine) in an agricultural soil separately for a period of 75 days. ¹³C₂-glycine and ¹³C₃-sarcosine mineralized rapidly compared to 2-¹³C-glyphosate and ¹³C-AMPA. The mineralization of ¹³C-AMPA was lowest among all four compounds due to its persistent nature. Only 0.5% of the initially added 2-¹³C,¹⁵N-glyphosate and still about 30% of the initially added ¹³C,¹⁵N-AMPA was extracted from soil after 75 days. The NERs formed from ¹³C,¹⁵N-AMPA were mostly NERs_xenobiotic as compared to other three compounds for which significant amounts of NERs_biogenic were determined. We noticed 2-¹³C,¹⁵N-glyphosate was biodegraded via two biodegradation pathways simultaneously; however, the sarcosine/glycine pathway with the formation of harmless NERs_biogenic presumably dominated.

Introduction

Glyphosate is one of the most widely used herbicide worldwide due to its great efficacy against a wide variety of weeds [5]. Glyphosate and its transformation product aminomethylphosphonic acid (AMPA) were most frequently found pesticides in agricultural European Union soils [34]. Such a widespread occurrence of both glyphosate and AMPA in soils triggers public concern about the safety of glyphosate use for the environment and humans [35]. Glyphosate can be biodegraded via two well-documented pathways: the AMPA and the sarcosine pathway [10], [35], [40]. The two pathways of glyphosate biodegradation result in formation of different intermediate products that have different environmental fate and implications for environmental risk assessment [24], [40]. For instance, the AMPA pathway produces persistent AMPA and glyoxylate which may further form amino acid glycine [40]; see Fig. 1). In contrast, the sarcosine pathway yields sarcosine which is readily oxidized to glycine [31], [37]. However, Li et al. [22] suggested that the C-N bond of glyphosate also can be cleaved directly to glycine bypassing sarcosine formation.

Isotope mass balance of the glyphosate fate comprising mineralization, extractable parent compound & its degradation products and non-extractable residues (NERs) was well documented in various soils [28] and in planted filters [19]. In contrast, the isotope mass balance studies of the fate of AMPA, glycine or sarcosine in soils are still lacking. Previous studies reported only half-life dissipation (DT₅₀) of AMPA (151–173 days; [4], [15] in soils, as well as of glycine (0.89 day; [36] and sarcosine (0.99 day; [36] in the soil-water system.

The NERs that can be only determined using isotope tracers are often a ‘black box’ in the mass balance study of the chemical fate in soils due to their unknown identity [33]. The NERs are remaining residues of an isotope labeled parent chemical or its degradation product(s) in soils that cannot be extracted using aquatic or organic solvents [23]. The parent chemical or its degradation product(s) can be strongly sorbed to soils as hazardous xenobiotic NERs (NERs_xenobiotic) with a remobilization potential and delaying the environmental risk [23], [33]. However, a chemical also can undergo microbial degradation accompanied with the formation of CO₂ and microbial biomass [33], [21]. After the death of microorganisms, biomass compounds and in particular proteins are stabilized in soil matrix as harmless biogenic NERs (NERs_biogenic) [29], [33]. The NERs_biogenic can be a result of assimilation of inorganic C and N (CO₂ or NH₄⁺) or monomeric molecules (e.g. amino acids) from a biodegraded compound into microbial biomass [39]. When the NERs_biogenic constitute a major portion of the NERs, the environmental risks associated with the NERs_xenobiotic formation will be overestimated [29]. The lack of information about the NER speciation is thus a ‘bottleneck’ in fate studies of chemicals since it impedes an assessment of environmental risks related to the NERs_xenobiotic [33], [21].

The intermediates of glyphosate, AMPA, glycine or sarcosine may determine the NER speciation resulting from the glyphosate degradation (Fig. 1). We hypothesize that an enhanced transformation of glyphosate to AMPA in the AMPA pathway will result in an increased formation of hazardous NERs_xenobiotic. The AMPA (DT₅₀: 151–173 days) is more resistant to biodegradation than glyphosate (DT₅₀: 7–60 days) [4], [15], [37]; and it is thus expected to be mainly sorbed to soils as NERs_xenobiotic with release potential to waters [39], [4], [6]. In contrast, enhanced degradation of glyphosate via the sarcosine/glycine pathway accompanied with the glycine formation may yield NERs_biogenic. Both glycine and sarcosine are biomolecules, which are readily transformed to CO₂ and microbial biomass [12], [22]. The glycine may be either assimilated as a monomeric ‘building block’ into microbial biomass and then into the NERs_biogenic or mineralized to CO₂ or NH₄⁺ which are then integrated into the biomass (see Fig. 1 and S1).

Formation of three degradation products of glyphosate and their proportions: AMPA, glycine and sarcosine can therefore determine environmental risks associated with the NERs_xenobiotic formation during the glyphosate degradation in soil. To date, mass balance of the fate of the three glyphosate degradation products and in particular the formation of NERs_biogenic has not been reported. This information may help to predict more accurately the NER speciation (hazardous NERs_xenobiotic versus harmless NERs_biogenic) of glyphosate in soil and which may stem from glyphosate intermediates. Therefore, the objectives of this study were (i) to elucidate the fate of glyphosate & its three degradation products: AMPA, glycine and sarcosine in soil microcosm experiments, and (ii) to determine the NERs_biogenic formation from these compounds using stable isotope double-labeling approach (¹³C + ¹⁵N). The ¹³C- and ¹⁵N-mass balance of the fate of 2-¹³C,¹⁵N-glyphosate, ¹³C,¹⁵N-AMPA, ¹³C₂,¹⁵N-glycine and ¹³C₃,¹⁵N-sarcosine was determined and comprised of mineralization (CO₂), extractable residues (ERs) of parent compound & its degradation products and NERs. The NERs_biogenic were based on the quantification of ¹³C- or ¹⁵N-amino acids (¹³C- or ¹⁵N-AAs) hydrolyzed from soil proteins.