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Article

Multi-Year Pseudo-Persistence, Mobility, and Degradation of Glyphosate and Its Degradation Product (AMPA) in a Gleysol in Quebec (Canada)

by
Stéphane Petit
1,*,
Marc Lucotte
1 and
Gilles Tremblay
2
1
Département des Sciences de la Terre, UQAM-Geotop, C.P. 8888 succ. Centre-ville, Montréal, QC H3C 3P8, Canada
2
Ministère de l’Agriculture, des Pêcheries et de l’Alimentation du Québec (MAPAQ), Direction Régionale de la Montérégie-Est, 1355 Rue Daniel-Johnson O bureau 3300, Saint-Hyacinthe, QC J2S 8W7, Canada
*
Author to whom correspondence should be addressed.
Agriculture 2025, 15(1), 110; https://doi.org/10.3390/agriculture15010110
Submission received: 11 November 2024 / Revised: 31 December 2024 / Accepted: 3 January 2025 / Published: 6 January 2025
(This article belongs to the Special Issue Effects of Tillage Management on Agricultural Soil Characteristics)
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Abstract

:
This study evaluates the pseudo-persistence of glyphosate over three growing seasons in agricultural soils (gleysol) in Québec, Canada. The experiment was carried out in long established plots following a corn–soybean–wheat rotation cycle with various combinations of N-fertilization (mineral N-fertilization, organic N-fertilization, without fertilization) and tillage techniques (conventional tillage and no-till). The periods between glyphosate applications were 250, 326, and 398 days. Soil sampling was carried out at 0–20 cm and 20–40 cm just before each new application of herbicide. Glyphosate was not detected in any sample. Its main degradation product, aminomethylphosphonic acid (AMPA), was found and quantified in approximately 50% of the samples. The detection frequency of AMPA was higher for conventional tillage compared to no-till. Levels ranged between 0.09 and 0.46 μg.g−1. The molar balance per hectare over the first 40 cm showed that the amount of glyphosate present in the form of AMPA in the soils sometimes exceeds the amount of glyphosate applied during the previous season (10.54 or 5.27 mol glyphosate.ha−1). The cumulative effect of glyphosate applications on AMPA levels over the 3 years, however, has not been demonstrated. The effect of conventional tillage on the persistence of AMPA is significant in 2 out of 3 years. The persistence of AMPA was higher for combinations of conventional tillage/mineral N-fertilization and conventional tillage/without fertilization practices. We suggest that conventional tillage can modify parameters related to soil structure or to the structural or functional composition of the bacterial community, which could impact the degradation and leaching of glyphosate and AMPA.

1. Introduction

Non-selective foliar glyphosate-based herbicides (GBHs) have been the most widely used herbicides family in the world since the mid-1990s [1]. This success is primarily due to the effectiveness of the active molecule (N-phosphonomethylglycine) and then to the marketing of cotton, corn, and soybean cultivars of the Roundup Ready© (RR) type, which have been genetically modified to tolerate glyphosate [2,3]. This major innovation facilitates the logistics of weed control and promotes the practice of no till practices, recommended in particular for the conservation of organic matter in agricultural soils [4]. In recent years, the use of GBHs has also been supported by increasing application rates per hectare due to increasing cases of weed resistance to the active ingredient, glyphosate [5].
RR cultivars have been authorized since 1999 in Canada [6]. In Quebec, the quantity of GBHs purchased by the agricultural sector increased by a factor of five over a few years to reach nearly 2100 tons of active ingredient in 2022 [7], which represents between 1 and 3% of what is used in Argentina, Brazil, or the USA [8,9]. Thus, about 35,000 tons of glyphosate were sprayed in agricultural soils in Quebec between 1999 and 2023 [6,7]. These soils represent a surface area of around 800,000 ha, essentially located in the St. Lawrence lowlands. They are cultivated with corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) mostly of RR type [10].
Glyphosate is considered a probable human carcinogen by the World Health Organization (WHO) [11], differentiating itself from Canadian, American, and European agencies in particular [12,13]. Monitoring glyphosate concentrations in the aquatic environment is a major environmental issue for preserving drinking water resources [1] and aquatic ecosystems [14,15,16], particularly in agricultural areas close to urban centers. The Government of Quebec has therefore set up a network of well and river sampling stations sampled regularly during the summer season since 2005 [17]. Between 2015 and 2017, glyphosate was detected above the limit of detection (LOD) of 0.04 μg.L−1 in 82.8% of river samples [18]. In the same samples, 78.1% of the analyzes of aminomethylphosphonic acid (AMPA), the main metabolite of glyphosate, have concentrations above the LOD of 0.2 μg.L−1 [18]. The detection frequencies of glyphosate and AMPA in surface waters of the agricultural region have been increasing since 2005, although they are not or only slightly (less than 5% of samples) detected in groundwaters [18,19,20].
The glyphosate and AMPA concentrations measured in Quebec rivers [21], as well as elsewhere in Canada [22] and across the world [16,23,24], are linked to the interaction of leaching, degradation, and adsorption–desorption of these two molecules in soils. These processes are related to soil pH, presence of metal oxides [25,26], phosphate content [27], water content [28], microbial diversity, and quality of organic matter [29,30]. These parameters are themselves impacted at different time scales by interannual pedoclimatic variations [31,32] and farming practices [33,34]. The half-life in soils (DT 50soil) is the main integrating indicator of these processes [35]. In Quebec, the half-life value used for calculating the glyphosate Environmental Risk Index (IRE) is 13.7 days in a controlled environment. Being a degradation product, there is no DT 50soil value assigned for AMPA [36]. Values reported in the literature range from 1 to 197 days for glyphosate and from 23 to 958 days for AMPA [37,38,39], thus showing the possibility that some AMPA persists over a year in the soil from the last GBH application to the next in the following spring, leading to a risk of the long-term accumulation of this compound. Results in a study in Quebec confirm this supposition [14]. The risk of the pseudo-persistence of glyphosate and AMPA in agricultural soils (defined by the constant addition of new molecules that replenish the molecules that are being removed) is important for the management of long-term water resources [40], for the preservation of certain biological functions of the soil [41], and to ensure good crop growth, including RR crops [42].
In this study, we set out to evaluate to what extent glyphosate can accumulate in the long term in an agricultural soil subjected to combinations of cultural practices in terms of crop rotations, fertilization techniques, and tillage practices. The hypotheses tested are that (i) some glyphosate persists between GBH applications from one crop season to the next, (ii) some glyphosate accumulates in soils year after year, and (iii) tillage or plowing has a negative effect on the long-term pseudo-persistence or accumulation of glyphosate in agricultural soils. The field experiment took place during 3 years of corn–soybean–wheat crop rotation with tillage and fertilization techniques compatible with the current agricultural practices in Quebec. The GBH application rates were not imposed. The choice of GBH application dates and rates sprayed was left to the field experts, according to favorable climatic conditions for spraying and weed pressure, to reflect the reality of the use of herbicides by farmers. The soil glyphosate and AMPA levels were measured before each new GBH application in the 0–20 cm and 20–40 cm horizons. The molar balance per hectare is compared to the quantity of glyphosate applied to the previous GBH treatment and tested according to the combinations of agricultural practices. The risk of long-term accumulation is assessed by the evolution of the levels during the three cropping seasons and the estimation of the mobility of glyphosate around the 20–40 cm horizon.

2. Materials and Methods

2.1. Experimental Site and Agricultural Managements

This study was carried out at the Grain Research Center (CEROM, 45°58′ N, 73°24′ W) in the St Lawrence Lowlands (Quebec, Canada). The climate is humid continental (Dfb) according to the Köppen–Geiger classification [43], with annual rainfall of about 1000 mm and an average annual temperature of 5.3 °C [44]. The soil is a clayey gleysol (Saint Urbain series) whose Ap horizon is composed of approximately 70% of silty–clayey particles on a thick bed of fine clays [45]. The water pH is 7.09 ± 0.2 and topography < 1%.
The field experiment was set up in 2008. Before its implementation, the soil was subjected to successive crops of corn–soybean–corn–corn–soybean between 2003 and 2007 treated with GBHs. General plowing before the experiment was carried out at a 20 cm depth in November 2007. The split-plot experimental design includes 2 forms of tillages, plowing (L) and direct seeding (D), and 3 treatments: mineral fertilization (M), organic fertilization (O), and without fertilization (A). The plots were treated with a 3-year crop cycle of corn–soya–wheat (rotation R1). Our study, therefore, covered 6 agricultural practices (L R1 M, L R1 O, L R1 A, D R1 M, D R1 O, D R1 A) replicated 3 times, i.e., 18 plots monitored for 3 years (2015, 2016, 2017). Residues from previous crops were left behind after harvest. The plots were divided into 3 blocks of repetitions divided into 2 sub-blocks where the variants of the main factor are assigned at random. The sub-blocks were divided into 3 plots themselves arranged randomly. The plots were 6 m wide and 20 m long (see Supplementary Materials).
For plots with tillage, plowing was carried out at a depth of 20 cm in the fall, supplemented by a passage of a harrow or disks (<5 cm) before sowing as soon as the weather conditions allowed it (from end of May to early June). Mineral fertilization consisted of 170 kg.ha−1 of nitrogen for the years with maize crops (2008, 2011, and 2014) and 50 kg.ha−1 for wheat crops (2010, 2013). There was no fertilizer input for soybean crops (2009, 2012, and the sampling year 2015). The fertilization procedure was recurrent since 2008. Organic fertilization consisted of pig manure or chicken manure applied in 2009, 2010, 2012, and 2016 at a rate following the regulatory recommendations (45 m3.ha−1). The site is naturally sufficiently rich in potassium and assimilable phosphate (P Melhich III > 100 kg/ha) so that supplements are not necessary according to the recommendations of the Reference Center for Agriculture and Agri-food of Quebec [46]. Applications of GBHs (Glyphosate Factor 540© potassium salt with 540 g.L−1 of active ingredient) were carried out on corn and RR soybean crops by airboom on days when the weather conditions permitted. The GBH application rates were 3.33 L.ha−1 (1.79 kg active ingredient.ha−1) in 2014 and 1.67 L.ha−1 (0.67 kg active ingredient.ha−1) in 2015 and 2016. The application and sampling dates are specified in Table 1. In 2014 and 2015, GBH was applied in late spring. In 2016, GBH was exceptionally applied at the end of the growing season, after the wheat harvest due to weed pressure.

2.2. Soil Sampling

Sampling was carried out in an inter-row located in the middle of the plot to avoid edge effects. Soil samples were taken with a manual auger 7 cm in diameter at 0–20 cm and 20–40 cm depth. The sampling campaigns took place on 18 June 2015, 10 May 2016, and 4 May 2017 (Table 1), i.e., 398 days, 326 days, and 250 days, respectively, after the previous GBH application. This study, therefore, covers 36 samples per year (6 plots sampled at 2 depths in 3 replicates) over 3 years, i.e., 108 in total. The soils were frozen at −18 °C on the day of sampling until being freeze-dried prior to the analysis.

2.3. Carbon Analyzes

Total organic carbon (Corg) was determined with a Carlo Erba NA-1500 elemental analyzer (Thermo Fisher Scientific, Waltham, MA, USA) after fumigation with hydrochloric acid vapor to limit traces of carbonates. The reproducibility of the measurements is ±0.1 mg of Corg.g−1 of dry soil, as described by [47].

2.4. Analysis of Glyphosate and AMPA Contents in Soils

The method used for the analyzes of glyphosate and AMPA was adapted from [48]. About 5 g of freeze-dried, finely ground, and homogenized soil was subsampled and extracted with 40 mL of an alkaline solution of NH4OH (0.25 M) buffered with KH2PO4 at (0.1 M). The samples were placed for 30 min on a rotary mixer set at 200 revolutions.min−1 and centrifuged for 15 min at 3500 revolutions.min−1. The extracts were filtered (Nylon filter ≤ 0.22 μm), whereas 20 μL was retrieved and dried under N2 flow. A mixture of 0.5 mL of trifluoroethanol and 1 mL of trifluoroacetic anhydride was used for derivatization on a hot plate at 100 °C for 3 h before being dried under N2 flow. The samples were diluted in 1 mL of isopropyl acetate and injected (0.5 μL) into a CP 3800 gas chromatograph (Varian) coupled with an electron capture detector (GC-ECD), as described by [49]. The injector and the detector were maintained at 280 °C and 300 °C, respectively. The gas used was hydrogen at a flow of 1.4 mL.min−1. The oven temperature was programmed at an initial temperature of 70 °C maintained for 1 min, to be brought to 6 °C min−1 up to 100 °C maintained for 6 min, then 130 °C at 2 °C min−1, and finally at 250 °C at 60 °C min−1 maintained for 8 min. To take into account the matrix effect observed during the protocol validation tests, the working standards were prepared in an extract of uncultivated forest soil taken in the immediate vicinity of the experimental site (500 m). The limit of detection (LOD) and limit of quantification (LOQ) for AMPA were 0.03 μg.g−1 and 0.09 μg.g−1, respectively, and 0.015 μg.g−1 and 0.045 μg.g−1 for glyphosate, respectively. The extraction yields were 71.74% ± 7.78 (n = 12) for AMPA and 90.10% ± 4.41 (n = 12) for glyphosate. As adapted from [50], the five point calibration curves showed good linearity for both glyphosate (r2 = 0.99; p < 0.0001) and AMPA (r2 = 0.97; p < 0.0001). Each sample batch included a standard curve made of five standards (0, 0.2, 0.4, 0.6, 0.8 mg·kg−1 for AMPA and 0, 0.1, 0.2, 0.3, 0.4 mg·kg−1 for glyphosate) ran in the extract of the uncultivated forest soil.

2.5. Data Processing

A molar balance per hectare of glyphosate and AMPA was carried out to compare the measured levels expressed in μg.g−1 of dry soil with the historical glyphosate quantities applied in the plots. The quantities are expressed in moles of glyphosate equivalent for the 0–20 cm horizon (Glyeq.ha−1)0–20, for the 20–40 cm horizon (Glyeq.ha−1)20–40, and for the sum of the two horizons (Glyeq.ha−1)tot = (Glyeq.ha−1)0–20 + (Glyeq.ha−1)20–40. The molar quantities of glyphosate and AMPA in a given soil horizon (Glyeq.ha−1) were calculated for each sample according to the following formula:
(Glyeq.ha−1)h = ([Gly] × MGly + [AMPA] × MAMPA) × Vh × ρh
  • ith
  • [Gly], [AMPA] = weight of glyphosate and AMPA per unit weight of dry soil;
  • MGly, MAMPA = molar mass of glyphosate and AMPA;
  • Vh = volume of soil from the “h” horizon sampled over 1 ha;
  • ρh = apparent soil density of the sampled horizon. ρh is calculated from the total organic carbon measured in the sample according to the formula used by Kämpf et al. [51]:
ρh = 100/((OM/0.244) + (100 − OM)/1.64))
With OM representing the fraction of organic matter and calculated from the organic carbon content (Corg) following the formula OM = 1.72 × Corg [51]. The average apparent density calculated with this method was 1.37 ± 0.04 for the 0–20 cm horizon and 1.45 ± 0.06 for the 20–40 cm horizon.
The relative proportion of moles of glyphosate equivalent per hectare in the 20–40 cm horizon compared to the 0–20 cm horizon was calculated for each sample by the following formula:
IP = 100 ∗ (Glyeq.ha−1) 20–40/(Glyeq.ha−1) 20–40 + (Glyeq.ha−1) 0–20
With IP denoting the depth index. If the quantity of glyphosate equivalent per hectare in the first 40 cm of soil is equally distributed between the 0–20 cm horizon and the 20–40 cm horizon, the index will be equal to 50. If 0 < IP < 50, the amount of glyphosate equivalent per hectare is higher in the 0–20 cm horizon than in the 20–40 cm one. If 50 < IP < 100, the quantity of glyphosate equivalent per hectare is higher in the 20–40 cm horizon than in the 0–20 cm one.

2.6. Statistical Analyzes

The effects of agricultural practices on the means of (Glyeq.ha−1) 0–20, (Glyeq.ha−1) 20–40, Glyeq.ha−1 tot, and Ip were tested by Student’s test (α < 0.05) for each year. The effects of fertilization and tillage on these same values were tested with an analysis of variance (ANOVA), with Fischer’s test at the threshold α = 0.05 by a logarithm (log) function to meet the normality conditions. For measurements below the LOD, the values corresponding to half of these were used, i.e., 0.008 μg.g−1 and 0.015 μg.g−1 for glyphosate and AMPA, respectively. For measurements below the LOQ, the values of 0.023 μg.g−1 and 0.045 μg.g−1 were used for glyphosate and AMPA, respectively.

3. Results

3.1. AMPA and Glyphosate Levels in Soils

Table 2 presents the AMPA levels measured for all samples. No glyphosate level above the LOD was observed. In 2015, 10 out of 18 measurements in the 0–20 cm horizon had AMPA levels above the LOQ. The levels ranged from 0.09 to 0.46 μg.g−1. Ten measurements out of eighteen presented AMPA levels above the LOQ in the 20–40 cm horizon. The levels ranged from 0.09 to 0.32 μg.g−1. In 2016, 8 out of 18 measurements showed levels above the LOQ for the 0–20 cm horizon. The AMPA levels were between 0.09 and 0.33 μg.g−1 for the 20–40 cm horizon; 8 samples out of 18 presented levels ranging from 0.09 to 0.32 μg.g−1. In 2017, 12 out of 18 measurements were above the LOQ in the surface horizon and 3 out of 18 were above the LOQ in the 20–40 cm horizon. The AMPA levels ranged from 0.10 to 0.30 μg.g−1 in the 0–20 cm horizon and from 0.15 to 0.26 μg.g−1 in the 20–40 cm horizon. Of the 51 measurements where the AMPA levels were higher than the LOD, 28 were located in plots with plowing.

3.2. Annual Molar Balance and the Effect of Agricultural Practices

The average molar balance per hectare for the year 2015 (Table 3) in the 0–20 cm horizon is 3.61 ± 3.40 mol of glyphosate equivalent for all the samples. This balance is significantly lower for D R1 A practice with 0.5 ± 0.02 mol of Gly eq.ha−1. The molar balance of the L R1 A practice is significantly higher with 6.20 ± 1.94 mol of Gly eq.ha−1. Other values range between 1.74 ± 2.12 mol of Gly eq.ha−1 for the L R1 O practice and 5.19 ± 5.64 mol of Gly eq.ha−1 for the D R1 M practice. These results show a great variability between the measurements. The analysis of variance (Table 4) shows an effect of tillage and fertilization. The values in the 20–40 cm horizon of D R1 M plots are between 1.20 ± 1.14 mol of Gly eq.ha−1 and 4.45 ± 2.88 mol of Gly eq.ha−1 in D R1 O plots. The comparison of the means by Student’s test and the analysis of variance does not indicate any significant effects of the combinations of practices or of one practice in particular (Table 3 and Table 4). The molar balance over the first 40 cm (0–40 cm) is 6.4 ± 4.51 mol of Gly eq.ha−1 for all samples. The assessment of the L R1 A practice is significantly higher with 9.56 ± 1.84 mol of Gly eq.ha−1. Values for the D R1 A practice are significantly lower with 2.12 ± 1.93 mol of Gly eq.ha−1. The analysis of variance (Table 4) does not indicate any effect of a particular tillage or fertilization practice on the average molar balance. The Ip calculated for all plots is 47.5 ± 27.5. Table 5 shows that the Ip is higher in plots D R1 A (Ip = 63.3 ± 21.5) and L R1 M (Ip = 59.1% ± 31.3). The D R1 M practice has the lowest Ip (Ip = 27.3% ± 20.2) without any significant differences.
The molar balance per hectare for the year 2016 in the 0–20 cm horizon is 2.35 ± 2.51 mol of Gly eq.ha−1 on average. Values range from 0.51 ± 0.02 mol Gly eq.ha−1 in D R1 A plots to 4.71 ± 3.94 mol of Gly eq.ha−1 in L R1 M plots (Figure 1). Table 3 and Table 4 indicate that there are no significant differences between the combinations of agricultural practices. Values in the 20–40 cm horizon have an average of 2.70 ± 2.31 mol of Gly eq.ha−1. The lowest values are measured in D R1 A plots and D R1 M plots with 0.53 ± 0.04 mol of Gly eq.ha−1 and 0.53 ± 0.02 mol of Gly eq.ha−1, respectively. The values for other plots range from 4.35 ± 3.35 mol of Gly eq.ha−1 in L R1 M plots to 4.71 ± 3.79 mol of Gly eq.ha−1 in D R1 O plots. None of the differences are statistically significant (Table 3 and Table 4). The molar balance over the first 40 cm of D R1 A plots is 1.03 ± 0.07 mol of Gly eq.ha−1 and 1.82 ± 1.34 mol of Gly eq.ha−1 for D R1 M plots. The molar balance of the other combinations of agricultural practices is greater than the amount of glyphosate applied 326 days previously (5.27 mol of glyphosate.ha−1). Values range from 6.55 ± 2.42 mol Gly eq.ha−1 in D R1 O plots to 9.06 ± 2.76 mol of Gly eq.ha−1 in L R1 M plots. Plowing has a significant effect on the 0–40 cm molar balance (Table 4). Table 5 shows that the Ip indicator is higher in D R1 O plots (Ip = 63.6 ± 44.3) and L R1 O plots (Ip = 63.8% ± 23.9). D R1 M plots have the lowest Ip (Ip = 39.7% ± 20.7). There are no significant differences in Ip between practices.
In 2017, the glyphosate equivalent molar balance is 3.45 ± 2.21 mol Gly eq.ha−1 on average in the 0–20 cm horizon (Figure 1). Means range from 2.13 ± 1.4 mol of Gly eq.ha−1 for the L R1 O practices to 5.42 ± 0.84 mol of Gly eq.ha−1 for the D R1 M practices (Figure 1). Table 3 and Table 4 indicate that there are no significant differences between the combinations of agricultural practices. The lowest values for the 20–40 cm horizon are measured in D R1 O and L R1 M plots with 0.53 ± 0.01 mol of Gly eq.ha−1 and 0.77 ± 0.42 mol of Gly eq.ha−1, respectively. The maximum is 2.61 ± 3.59 mol of Gly eq.ha−1 in L R1 A plots. The molar balance over the first 40 cm is 4.92 ± 3.04 mol of Gly eq.ha−1. This value is lower than those observed in 2015 and 2016. The values range from 3.31 ± 3.81 mol of Gly eq.ha−1 in the D R1 O plots to 7.11 ± 2.12 mol of Gly eq.ha−1 in the D R1 M plots. The 0–40 cm molar balance of the D R1 M, L R1 M, and L R1 A plots is greater than the quantity of glyphosate applied 250 days previously (5.27 mol of glyphosate.ha−1). There is no significant effect of combinations of agricultural practices (Table 3 and Table 4). The Ip indicators are 32.7 ± 21.2 for all plots. This value is lower than those observed in 2015 (47.5 ± 24.5) and in 2016 (53.2 ± 27.5). The values range from 14.02 ± 5.7 in L R1 M plots to 47.54 ± 13.2 in L R1 O plots. All Ip means are <50 (Table 5).

4. Discussion

4.1. AMPA Persistence in Soil from One Growing Season to the Next

The AMPA levels are higher than the LOD in more than 50% of the samples (Table 2). The maximum level of 0.46 μg.g−1 measured in the D R1 M plots is higher than those reported in another study in agricultural soils in the United States (0.34 μg.g−1) [23]. In European soils cultivated for cereal production, the values are of the same order of magnitude as those of our study (maximum of 0.5 μg.g−1) [52]. In a study in Argentina, the levels measured are up to 2.26 μg.g−1 [9]. These values come from large-scale random sampling campaigns. The dates and rates of GBH applications are not known, and these studies do not report levels nearly one year after the last GBH application, and the present study includes one year of dissipation of glyphosate and AMPA. Other studies with field experiments with GBH application rates similar to those of our study show comparable AMPA values in soils 613 days after the last GBH application [27]. In our study, the detection of AMPA levels 250 days, 326 days, and 398 days after the last GBH application makes it possible to validate the first hypothesis of this study, i.e., that part of glyphosate persists in the form of AMPA over a winter season.
AMPA in soils can be eroded along with particles by wind towards rivers, as shown in a study in Argentina [53]. AMPA can also be leached under dissolved or particulate form into water systems. Plots plowed in the fall (L R1 A and L R1 M), where AMPA is mostly detected, are potentially the most prone to surface and sub-surface leaching [33]. Our study suggests that this process could take place all year round, especially during heavy rains in the fall and spring and during snow melt in spring. This result complements a review [54], which indicates that the risk of AMPA leaching into rivers is mainly linked to rainfall events shortly after GBH application. Our results show the need to obtain data on the presence of AMPA and glyphosate in Quebec rivers throughout the year to complete studies carried out in summer, a season corresponding to GBH application [10]. As well, information on the dissolved, colloidal, or particulate form of AMPA (or glyphosate), and measurements in the sediments of Southern Quebec rivers, is necessary to better understand the processes related to the transfer of these compounds to aquatic environments.
The in-depth transfer of AMPA to groundwater has been documented [23]. The Ip index (Table 5) calculated for 2017 shows that the AMPA contents of the 20–40 cm horizon represent 32.7% ± 21.2 of the total AMPA for all plots. This result is lower than that of 2016 and 2015. These results suggest a certain AMPA mobility at depth (greater in 2015 and 2016 than in 2017) and a risk for groundwater contamination. The local geological context is important for the process of the infiltration of AMPA towards groundwater [25]. However, [20] detected little AMPA in the sampled groundwaters. The gleysols of the region where our experimental field was installed lie over a thick bed of impermeable clays [44] located less than 1 m deep, which can limit transfers to groundwater.
The agronomic impact linked to the presence of AMPA in cultivated soils, particularly at the time of sowing and the start of plant growth, is not yet well known. The authors in [55] showed that the chelating properties of AMPA, although weaker than those of glyphosate, can affect the transition metal nutrition of plants. Unlike glyphosate, the adsorption of AMPA by plant roots has not been demonstrated [56].

4.2. Pseudo-Persistence of AMPA in Agricultural Soils

In some plots, the molar balance calculated per hectare represents more than 100% of the quantity of glyphosate equivalent applied during the previous GBH treatment (in 2016: D R1 O, L R1 A, L R1 M, L R1 O plots; in 2017: D R1 M, L R1 A, L R1 M plots; Figure 1). However, our results do not show a progressive accumulation of AMPA in soil year after year, which invalidates our second hypothesis. However, our analyzes show a cumulative effect in the medium term (period of 2–3 years), during which the rate of the dissipation of glyphosate is lower than the rate of application [52]. These results highlight the importance of taking AMPA into account when calculating environmental risk indicators associated with GBH usage [16]. The half-life in soils (DT 50soil) is one of the main indicators used to estimate the risk of the persistence of a compound in the environment, as is the case in Quebec [35]. However, models describing the kinetics of the degradation of AMPA show that it does not follow a first-order differential equation but rather a differential equation with two compartments [34]. The value of DT 50soil can minimize the time required for the degradation of the second half of the quantified AMPA [57,58]. This issue may be the cause of the heterogeneity of DT 50soil values in the literature, which range from 23 to 958 days for AMPA [38,39], with a median value of 141 days [54]. The use of the DT 90soil indicator (the time required for 90% of an initial quantity of AMPA to degrade) seems more relevant in environmental risk assessments. One study shows, for example, that the DT 50soil for AMPA is 54.7 days and the DT 90soil is 182 days in a soil cultivated with soybeans compared to 71 days (DT 50soil) and 236 days (DT 90soil) in a soil cultivated with corn [37].

4.3. Pseudo-Persistence of AMPA Favored by Tillage Practices

AMPA is mainly detected in plowed plots (65% of plots sampled). The effect of plowing on the molar balance per hectare of glyphosate equivalent is significant in 2016 and 2015 with the combined effect of the type of fertilization (Table 3 and Table 4). These results validate the third hypothesis of this study, that cultural practices impact the persistence of glyphosate in agricultural soils, as shown by [59]. Similar results have already been reported in other types of soils [34,57,60,61,62,63]. Some of these studies are based on soil samples retrieved in long-term experimental settings with controlled environments. However, to our knowledge, there are no other studies presenting these results in a long-term experimental design in an open field environment using GBH application rates consistent with farmers’ practices.
The variable AMPA contents observed in plots that received identical GBH application rates may be linked to variations in the intensity of the degradation process (1) of glyphosate into AMPA and (2) of AMPA itself. AMPA is produced by breaking the C-N bond of glyphosate [64]. Breaking the C-P bond of glyphosate or AMPA allows for the production of orthophosphates and organic compounds like sarcosine. In soils, these reactions are essentially of biotic origin through the action of bacterial or fungal enzymes [65]. Ref. [66] showed, for example, that actinobacteria (Gram + bacteria) can degrade glyphosate directly into sarcosine via C-P lyase without producing AMPA. The abiotic degradation of glyphosate to AMPA followed by the degradation of AMPA by synthetic birnessite has been shown [67,68], whereas actinobacteria can produce biogenic birnessite [69]. More generally, this phylum is recognized for its potential in the bioremediation operations of soils polluted by heavy metals or pesticides [70]. However, the relative abundance of these bacteria in soils can be linked to agricultural practices. In Quebec, ref. [71] have shown the link between the quantity of labile organic matter and the presence of actinobacteria, and ref. [72] showed the variations in the abundance of actinobacteria during the degradation of crop residues integrated into the soil. Further research on the impact of agricultural practices on the degradation of glyphosate and AMPA in relation to the intensity of microbial activity or the structural or functional composition of bacterial or fungal communities is therefore needed [73]. These explanatory hypotheses also apply to AMPA produced by RR and non-RR plants following applications of GBHs. The AMPA produced can be exuded by the roots or accumulate in the aerial and underground storage organs of plants [49,56].
The various AMPA contents in plowed plots may be related to variations in the intensity of the leaching process. This is closely related to the adsorption/desorption of glyphosate or AMPA by the phosphonate group of the two molecules to metal oxides or to the surface of clays [25]. Plowing modifies the structure of the soil, in particular by breaking up the macro-aggregates [74] and by promoting the infiltration of water [75]. The increase in the contact surface between water and the inorganic soil fraction can accelerate the weathering of phyllosilicates and modify the cation exchange capacity, the anion exchange capacity, and the specific exchange surface, as reviewed in [76]. These same authors report the potential of certain clays such as illite to adsorb compounds with a positively charged amine group, such as glyphosate and AMPA at the soil pH range of our study. The lower average Ip indicator in 2017 vs. 2015 and 2016 (Table 5) may indicate that the infiltration of glyphosate and AMPA is greater in years when GBH is applied in spring compared to an application in autumn. These differences may be related to the structuring of the soil by the roots of soybeans and maize favoring preferential interstitial flows [28,75]. The importance of the action of organic acids exuded from the roots for the desorption of organic or inorganic matter from the surface of clays has been shown [77]. Finally, these differences can be explained by soil bioturbation by roots or earthworms during the summer [41].

5. Conclusions

The pseudo-persistence of glyphosate in the form of AMPA between GBH applications during annual crop cycles was observed. These results were produced thanks to 3 years of monitoring in a long-term experimental device and under open field conditions with GBH applications resembling common agricultural practices. The molar balance of glyphosate equivalent per hectare is greater than the quantity of herbicide applied during the previous herbicide treatment in certain plots. The year-to-year accumulation of glyphosate or AMPA in agricultural soils subjected to recurrent GBH treatments has not been demonstrated, but these observations suggest that the process of the transfer of AMPA from soils to aquatic environments could have taken place at other times of the year than summer only, the season usually most studied in environmental monitoring. The effect of tillage by plowing on the pseudo-persistence of AMPA is significant in some years. Plowing enhances the pseudo-persistence of glyphosate in the form of AMPA. These results lay the foundations for taking into account, on the one hand, agricultural practices for the estimation of soil vulnerability in relation to the risk of the pseudo-persistence of AMPA and, on the other hand, the agronomic importance of the spring awakening of the biocenosis of agricultural soils subjected to AMPA residues, which may be equivalent to the molar

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture15010110/s1, split plot experimental design.

Author Contributions

Conceptualization, S.P., M.L. and G.T.; methodology, S.P., G.T. and M.L.; validation, S.P.; formal analysis, S.P.; resources, M.L. and G.T.; data curation, S.P.; writing—original draft, S.P.; writing—review and editing, S.P., M.L. and G.T.; visualization, S.P.; supervision, M.L. and G.T.; project administration, M.L.; funding acquisition, M.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Strategic Partnership Program by the Natural Sciences and Engineering Research Council of Canada (NSERC). The grant number is STPGP 506291-17.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials unavailable due to privacy.

Acknowledgments

Laboratory of M.L. This paper is a part of the Ph.D. Thesis of M. Stephane PETIT, presented at UQAM (University of Quebec at Montreal), Canada.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 15 00110 g001aAgriculture 15 00110 g001b
Figure 1. Molar balance per hectare for the years 2015 (a), 2016 (b), and 2017 (c) in moles of glyphosate equivalent calculated from the apparent densities calculated for each soil horizon. Dotted lines represent the amount of glyphosate per hectare applied 398 days before the 2015 sampling, 326 days before the 2016 sampling, and 250 days before the 2017 sampling. Error bars represent the standard error.
Figure 1. Molar balance per hectare for the years 2015 (a), 2016 (b), and 2017 (c) in moles of glyphosate equivalent calculated from the apparent densities calculated for each soil horizon. Dotted lines represent the amount of glyphosate per hectare applied 398 days before the 2015 sampling, 326 days before the 2016 sampling, and 250 days before the 2017 sampling. Error bars represent the standard error.
Agriculture 15 00110 g001aAgriculture 15 00110 g001b
Table 1. Total precipitation and average temperatures 15 days before and 15 days after GBH applications in 2014, 2015, 2016, and 2017. Total precipitation (rain or snow) between the date of GBH application and sampling the next year.
Table 1. Total precipitation and average temperatures 15 days before and 15 days after GBH applications in 2014, 2015, 2016, and 2017. Total precipitation (rain or snow) between the date of GBH application and sampling the next year.
YearCropSampling
AAAA/MM/JJ		GBH
Application AAAA/MM/JJ		Total Precipitation (mm)/Average Temperature (°C) 15 Days After GBH ApplicationTotal Precipitation (mm)/Average Temperature (°C) 15 Days Before GBH ApplicationNumber of Days Between the Last GBH Application and Sampling/Total Rainfall (mm) During This Period
2014Cornnot applicable12 May 2014 a70/16.427/10.9
2015Soy18 June 201519 June 2015 b55.4/1889/17.8398 days/1166 mm
2016Wheat10 May 201627 August 2016 b43/18.4155 */19.3326 days/973 mm
2017Corn4 May 201717 May 2017 a48/17.572/18.4250 days/865 mm
Note: The data come from station no. 7027361 located in Saint-Hyacinthe (45°34′ N; 72°55′ W) 15 km away from the sampling site. (*): Precipitation is mainly due to two events of more than 60 mm each on 13 and 16 August 2016. a: GBH application rate 3.33 L.ha−1 (1.79 kg of active ingredient.ha−1). b: GBH application rate 1.67 L.ha−1 (0.67 kg of active ingredient.ha−1).
Table 2. Aminomethylphosphonic acid ([AMPA]) content measured at depths 0–20 cm and 20–40 cm depending on the agricultural practices, and year of sampling.
Table 2. Aminomethylphosphonic acid ([AMPA]) content measured at depths 0–20 cm and 20–40 cm depending on the agricultural practices, and year of sampling.
Treatments[AMPA]
μg.g−1		BlocSampling Year
2015
398 Days a.a		2016
326 Days a.a		2017
250 Days a.a
D R1 A[AMPA] 0–201<LOQ<LOQ<LOQ
2<LOQ<LOQ0.25
3<LOQ<LOQ<LOQ
[AMPA] 20–401<LOQ<LOQ<LOQ
20.13<LOQ<LOQ
3<LOQ<LOQ<LOQ
D R1 M[AMPA] 0–2010.14<LOQ0.18
20.460.110.25
3<LOQ<LOQ0.21
[AMPA] 20–401<LOQ<LOQ<LOQ
20.09<LOQ<LOQ
3<LOQ<LOQ0.15
D R1 O[AMPA] 0–2010.350.15<LOQ
20.22<LOQ0.30
3<LOQ<LOQ<LOQ
[AMPA] 20–4010.15<LOQ<LOQ
20.320.31<LOQ
3<LOQ0.21<LOQ
L R1 A[AMPA] 0–2010.150.120.16
20.290.09<LOQ
30.290.120.10
[AMPA] 20–4010.130.14<LOQ
20.120.34<LOQ
30.12<LOQ0.26
L R1 M[AMPA] 0–2010.090.330.22
2<LOQ<LOQ0.10
30.240.220.28
[AMPA] 20–4010.28<LOQ<LOQ
20.190.23<LOQ
3<LOQ0.24<LOQ
L R1 O[AMPA] 0–201<LOQ<LOQ0.10
2<LOQ<LOQ<LOQ
30.170.310.13
[AMPA] 20–4010.13<LOQ<LOQ
2<LOQ0.20<LOQ
3<LOQ0.290.18
Note: <LOQ: <at the limit of quantification of 0.09 μg.g−1 of dry soil. a.a: Number of days after the last application of GBHs.
Table 3. Comparison of the average molar balances per hectare of glyphosate equivalent using Student’s test (α < 0.05) (n = 108). For each column, different letters mean that the means show significant differences between them. (a) represents the group with the highest average.
Table 3. Comparison of the average molar balances per hectare of glyphosate equivalent using Student’s test (α < 0.05) (n = 108). For each column, different letters mean that the means show significant differences between them. (a) represents the group with the highest average.
201520162017
0–20 cm20–40 cm0–40 cm0–20 cm20–40 cm0–40 cm0–20 cm20–40 cm0–40 cm
D R1 Ababaacaaa
D R1 Mabaabaabcaaa
D R1 Oabaabaaaaaa
L R1 Aaaaaaaaaa
L R1 Mabaabaaaaaa
L R1 Oabaabaaabaaa
Table 4. Analysis of variance (ANOVA) with Fischer’s test (F) at threshold α = 0.05 on the effect of combinations of agricultural practices on the molar balance per hectare of glyphosate equivalent. In bold: the effects are significant at p < 0.05. dof: degree of freedom. F: fertilization. T: tillage.
Table 4. Analysis of variance (ANOVA) with Fischer’s test (F) at threshold α = 0.05 on the effect of combinations of agricultural practices on the molar balance per hectare of glyphosate equivalent. In bold: the effects are significant at p < 0.05. dof: degree of freedom. F: fertilization. T: tillage.
F (α = 0.05)
201520162017
Origin of Variationdof0–20 cm20–40 cm0–40 cm0–20 cm20–40 cm0–40 cm0–20 cm20–40 cm0–40 cm
F20.32370.03490.11310.11451.02200.77332.64940.13010.3865
T10.79421.25770.20393.50783.47537.43500.14360.41470.8228
T × F24.28902.58173.69041.15110.97191.37470.13540.87190.7308
Total17
Table 5. Means and standard deviations of the depth index (Ip) for the 6 agricultural practices in triplicate from 2015, 2016, and 2017 (n = 108). For each column, the means followed by the same letter in parentheses have no significant differences between them with Student’s test (α < 0.05). The letter (a) represents the group with the highest average.
Table 5. Means and standard deviations of the depth index (Ip) for the 6 agricultural practices in triplicate from 2015, 2016, and 2017 (n = 108). For each column, the means followed by the same letter in parentheses have no significant differences between them with Student’s test (α < 0.05). The letter (a) represents the group with the highest average.
IP 2015IP 2016IP 2017
D R1 A63.3 ± 21.6 (a)51.5 ± 1.1 (a)39.9 ± 18.9 (a)
D R1 M27.3 ± 20.23 (a)39.7 ± 20.7 (a)20.4 ± 19.2 (a)
D R1 O48.4 ± 15.4 (a)63.6 ± 44.3 (a)36.2 ± 25.8 (a)
L R1 A36.9 ± 10.1 (a)49.4 ± 32.8 (a)37.9 ± 31.5 (a)
L R1 M59.1 ± 32.3 (a)51.2 ± 43.4 (a)14.1 ± 5.7 (a)
L R1 O49.9 ± 38.7 (a)63.8 ± 23.9 (a)47.5 ± 13.2 (a)
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Petit, S.; Lucotte, M.; Tremblay, G. Multi-Year Pseudo-Persistence, Mobility, and Degradation of Glyphosate and Its Degradation Product (AMPA) in a Gleysol in Quebec (Canada). Agriculture 2025, 15, 110. https://doi.org/10.3390/agriculture15010110

AMA Style

Petit S, Lucotte M, Tremblay G. Multi-Year Pseudo-Persistence, Mobility, and Degradation of Glyphosate and Its Degradation Product (AMPA) in a Gleysol in Quebec (Canada). Agriculture. 2025; 15(1):110. https://doi.org/10.3390/agriculture15010110

Chicago/Turabian Style

Petit, Stéphane, Marc Lucotte, and Gilles Tremblay. 2025. "Multi-Year Pseudo-Persistence, Mobility, and Degradation of Glyphosate and Its Degradation Product (AMPA) in a Gleysol in Quebec (Canada)" Agriculture 15, no. 1: 110. https://doi.org/10.3390/agriculture15010110

APA Style

Petit, S., Lucotte, M., & Tremblay, G. (2025). Multi-Year Pseudo-Persistence, Mobility, and Degradation of Glyphosate and Its Degradation Product (AMPA) in a Gleysol in Quebec (Canada). Agriculture, 15(1), 110. https://doi.org/10.3390/agriculture15010110

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