Mobility Of Glyphosate Research Articles

Characteristics of glyphosate adsorption with biochar from young coconut waste

Biochar is a carbon-rich by-product of biomass pyrolysis that can limit glyphosate mobility in soil using amelioration technology. Study of the basic principles of glyphosate adsorption behaviour in biochar from young coconut waste using the Kon-Tiki method at temperature (682°C), moisture content (81.27%), and yield ratio (20.87%) at the size of ≤2 mm. The absorption of glyphosate in biochar increased as glyphosate content rose, whereas pH declined. The capacity and adsorption coefficient of glyphosate by biochar from young coconut waste was 0.85 mg g-1 and 56.25 L Kg-1 at a pH of 8.03 and a concentration of 100 mg L-1 glyphosate. The adsorption isotherm of glyphosate occurs in the Freundlich and Langmuir models (Freundlich > Langmuir), wherein the Freundlich model (y = 1.8373x - 2.3971; R2 = 0.9771) with a value of n (0.54) and KF [0.004 (L mg-1)1/n], and the Langmuir model (y = - 17.066x + 251.58; R2 = 0.7239) with a value of Qm (0.06 mg g-1) and KL (0.07 L mg-1).

Glyphosate pollution of surface runoff, stream water, and drinking water resources in Southeast Brazil.

Glyphosate-based herbicides can be harmful to the environment and human health. Especially in developing countries, these herbicides are often used indiscriminately in agricultural and urban areas. Here, we optimized a simple and efficient flow injection-based spectrophotometric method to monitor environmentally relevant glyphosate concentrations in surface waters. The method was then used to assess the environmental mobility of glyphosate in Southeast Brazil by monitoring surface runoff from experimental agricultural soil plots that received glyphosate applications in 2015. Further, water samples from low-order streams were collected in five agricultural, urban, and natural areas, as well as from the 5th-order Rio das Mortes during the rainy season. Finally, 20 drinking water sources were sampled in urban, rural, and agricultural areas. Runoff from reference plots without glyphosate application showed concentrations below the method's detection limit of 0.49mg.L-1, whereas runoff from plots with standard glyphosate application had concentrations between 1.24 and 6.1mg.L-1. Similarly, concentrations in natural stream water were below the detection limit, whereas agricultural streams had concentrations of up to 3.7mg.L-1 (average: 0.97mg.L-1). In an agricultural stream monitored weekly, concentration peaks were observed after glyphosate applications by farmers, and concentrations were correlated to stream discharge. Urban streams had concentrations of up to 5.8mg.L-1 (average: 2.6mg.L-1), but samples from the catchment's major river were mostly below detection limits, illustrating the dilution of urban and agricultural runoff in high-order rivers. In the sampled drinking water resources, glyphosate pollution occurred mainly in the rainy season, with detectable concentrations between 0.5 and 8.7mg.L-1 in 80% of the sampled drinking water sources. In conclusion, our results suggest considerable environmental mobility of glyphosate in the studied Southeast Brazilian catchment. Substantial pollution, well above national and international limits, was detected in surface runoff, stream water, and drinking water resources.

Phosphonate herbicide interactions with quartz, montmorillonite, and quartz‐enriched agricultural soil

AbstractPhosphonate‐based herbicides are extensively used in agriculture. Glyphosate was long considered immobile in soils due to strong complexation with metal oxides, but detection in agricultural effluents and surface waters points to glyphosate mobilization and transport. In particular, mechanisms of glyphosate adsorption and subsequent mobility from runoff‐prone soils enriched in quartz have not been fully elucidated. Here we used theoretical and experimental approaches to gain new insights on the soil mobility of glyphosate and two other phosphonate herbicides, glufosinate and fosamine. Molecular modeling simulations of optimized adsorbate complexes of the phosphonate herbicides with quartz and montmorillonite revealed Ca‐enhanced interactions for all three herbicides at both mineral interfaces, weak binding of Na‐complexed phosphonate herbicides on quartz, and more favorable interactions for glufosinate and fosamine with Na‐montmorillonite than for glyphosate. We conducted adsorption and desorption experiments to evaluate the mobility of glyphosate on quartz‐enriched agricultural soils. Consistent with our theoretical findings, adsorption experiments conducted with CaCl2 resulted in greater than a twofold increase in glyphosate retention compared with experiments with NaCl. The desorption experiments revealed release of about 30–90% of the initially bound glyphosate into solution, whereby Ca in the desorption solution promoted glyphosate mobility compared with Na. Furthermore, in all conditions, the relative initial wetness of the agricultural soil was correlated positively with the enhanced glyphosate mobilization, consistent with field‐scale reports that herbicide applied to soils following rain events were prone to runoff.

Heavy rainfall following a summer drought stimulates soil redox dynamics and facilitates rapid and deep translocation of glyphosate in floodplain soils.

We present field data on the effects of heavy rainfall after drought on the mobility of glyphosate and redox conditions in a clayey floodplain soil. By applying glyphosate together with deuterated water as conservative tracer in combination with time resolved in situ redox potential measurements, the spatial and temporal patterns of water infiltration and pesticide transport as well as the concomitant changes of the redox conditions were revealed. Our findings demonstrate that shrinkage cracks in dry soils can serve as effective transport paths for atmospheric oxygen, water and glyphosate. The rain intensity of a typical summer storm event (approx. 25 mm within one hour) was sufficient to translocate deuterated water and glyphosate to the subsoil (50 cm) within 2 hours. Soil wetting induced partial closure of the shrinkage cracks and stimulated microbial activity resulting in pronounced dynamics of in situ soil redox conditions. Redox potentials in 40 to 50 cm depth dropped permanently to strongly reducing conditions within hours to days but fluctuated between reducing and oxidizing conditions in 10 to 30 cm depth. Our findings highlight the close link between the presence of macropores (shrinkage cracks), heavy rainfall after drought, redox dynamics and pesticide translocation to the subsoil and thus call for further studies addressing the effects of dynamic redox conditions as a limiting factor for glyphosate degradation.

Effect of mulch on glyphosate mobility in soil column

Glyphosate is a broad-spectrum herbicide that often used in controlling weed growth in oil palm plantation. Glyphosate has a high polarity and solubility in water and is quickly absorbed by soil particles. Oil palm waste such as oil palm trunks (OPTs) and empty fruit bunches (EFBs) and the use of glyphosate in agricultural land can contribute to environmental pollution. Apart from the commercial mulch that often used in nurseries, mulch from OPT and EFB can be used to absorb glyphosate to prevent weed growth. The glyphosate concentration of soil and water samples was determined using High Performance Liquid Chromatography (HPLC). Two doses of glyphosate were used which are 5.5 mL/L (2255 mg a.i/L) and 11 mL/L (4510 mg a.i/L). Two water volumes are used which are 250 ml and 500 ml. Glyphosate residual concentration values was high in Serdang soil series at layer three while in Selangor soil series at layer one. The highest concentrations of glyphosate residues were 32.57 mg/L and 34.42 mg/L at Serdang and Selangor soil series respectively. Samples of soil in the mulch of EFB and OPT show low concentrations of glyphosate residue compared to commercial and non-mulching column. Results show that EFB and OPT indicate low residual concentration values at each layer in the soil column for both doses.

Influence of cow-dung amendment on glyphosate mobility in soil

Sorption and desorption characteristics of glyphosate on native soil and soil amended with cow dung were investigated. Sorption was pH dependent and decreased with increasing pH. Glyphosate partition coefficient ranged from 275 to 16,200 L kg−1 in native soil, but for soil amended with cow dung it was significantly lower, viz 180–1530 L kg−1. Glyphosate partition coefficient showed strong negative correlation with cation exchange capacity and positive correlation with surface area. Phosphate in cow dung suppressed glyphosate sorption and enhanced desorption. This study revealed that cow-dung addition to soil could raise the mobility of glyphosate towards groundwater contamination due to the presence of phosphate.

Modeling the mobility of glyphosate from two contrasting agricultural soils in laboratory column experiments

Glyphosate (GLP) currently is one of the most widely used herbicides worldwide. The persistence of GLP and its major metabolite, aminomethylphosphonic acid (AMPA) in the environment has been described by other authors. This study was aimed at comparing the GLP and AMPA behavior in sandy and loamy sand soils after spiking with enhanced (445 µg g−1) concentrations of GLP in herbicide KLINIK® (Nufarm, Austria) and bioaugmentation followed by 40 days weathering and a consistent three-stage leaching in a laboratory column experiment. Soil samples were obtained from mineral topsoil (0–10 cm) within former agricultural lands where soil parent material was formed by glacigenic deposits. The total amount of GLP and AMPA collected during three leaching stages was significantly (p<.05) higher from columns with sandy soil, compared to loamy sand soil. Bioaugmentation resulted in considerably lower concentrations of AMPA in leachates, especially in the sets with sandy soil (p=.01). Leachates were tested using FTIR spectroscopy and Daphnia magna. Statistical analysis of the changes in Ntot, Ctot, K+, Mg2+, Al3+, Ca2+, Mn2+ and Fe3+ concentrations in soils after the leaching experiment revealed that the loamy sand soil was likely to be more sensitive to the addition of GLP and bioaugmentation than sandy soil.

Sorption of radiolabelled glyphosate on biochar aged in contrasting soils

Glyphosate mobility from terrestrial to aquatic environments has raised concerns about it. Utilizing soil’s inherent properties along with sorption properties of aged biochar, we hypothesized that selective application of biochar would be more effective in economic terms for glyphosate sorption on contrasting soils. To test this hypothesis, batch experiments and liquid scintillation counting for 14C labeled glyphosate were used. The sorption behavior of glyphosate was examined in four contrasting Australian soil types (Oxisol, Vertisol, Entisol, and Inceptisol) amended with aged biochar to determine glyphosate concentrations by measuring 14C activity using liquid scintillation counting. Freundlich parameters were calculated for soil-soil/biochar combinations. The pattern of glyphosate sorption was Oxisol > Vertisol > Entisol > Inceptisol. Oxisol adsorbed approximately five times more glyphosate compared with Inceptisol. Oxisol soil system adsorbed maximum amount of glyphosate principally due to the presence of iron-aluminum oxides exhibiting variable charges which got increased due to the presence of aged biochar. Considering all the soil/soil-biochar systems, Inceptisol soil system showed the least adsorption of glyphosate. A significant contribution of char was observed only in the Entisol soil system and the finding is valuable as char can be applied in Entisol soil systems to control glyphosate mobility.

Adsorption-desorption and leaching potential of glyphosate and aminomethylphosphonic acid in acidic Malaysian soil amended with cow dung and rice husk ash.

This study investigates adsorption-desorption and the leaching potential of glyphosate and aminomethylphosphonic acid (AMPA) in control and amended-addition of cow dung or rice husk ash-acidic Malaysian soil with high oxide mineral content. The addition of cow dung or rice husk ash increased the adsorptive removal of AMPA. The isotherm data of glyphosate and AMPA best fitted the Freundlich model. The constant Kf for glyphosate was high in the control soil (544.873mgg-1) followed by soil with cow dung (482.451mgg-1) then soil with rice husk ash (418.539mgg-1). However, for AMPA, soil with cow dung was high (166.636mgg-1) followed by soil with rice husk ash (137.570mgg-1) then the control soil (48.446mgg-1). The 1/n values for both glyphosate and AMPA adsorptions were < 1 indicating their strong affinity for adsorbents. Desorption of both glyphosate and AMPA occurred only in the control soil. The compounds were not detected in soils with added cow dung or rice husk ash. The addition of cow dung or rice husk ash increased glyphosate mobility. However, ground water ubiquity scores for both control and amended soils were < 2.8. This indicated glyphosate is a transitional herbicide; therefore, its leaching potential in the soil is low, despite the addition of cow dung or rice husk ash. Addition of these wastes decreased the mobility and leaching potential of AMPA. The addition of cow dung or rice husk ash could be beneficial in increasing adsorption and enhancing degradation of these compounds.

Spatial glyphosate and AMPA redistribution on the soil surface driven by sediment transport processes - A flume experiment.

This study investigates the influence of small-scale sediment transport on glyphosate and AMPA redistribution on the soil surface and on their off-site transport during water erosion events. Both a smooth surface (T1) and a surface with "seeding lines on the contour" (T2) were tested in a rainfall simulation experiment using soil flumes (1 × 0.5 m) with a 5% slope. A dose of 178 mg m-2 of a glyphosate-based formulation (CLINIC®) was applied on the upper 0.2 m of the flumes. Four 15-min rainfall events (RE) with 30-min interval in between and a total rainfall intensity of 30 mm h-1 were applied. Runoff samples were collected after each RE in a collector at the flume outlet. At the end of the four REs, soil and sediment samples were collected in the application area and in four 20 cm-segments downslope of the application area. Samples were collected according to the following visually distinguished soil surface groups: light sedimentation (LS), dark sedimentation (DS), background and aggregates. Results showed that runoff, suspended sediment and associated glyphosate and AMPA off-site transport were significantly lower in T2 than in T1. Glyphosate and AMPA off-site deposition was higher for T2 than for T1, and their contents on the soil surface decreased with increasing distance from the application area for all soil surface groups and in both treatments. The LS and DS groups presented the highest glyphosate and AMPA contents, but the background group contributed the most to the downslope off-site deposition. Glyphosate and AMPA off-target particle-bound transport was 9.4% (T1) and 17.8% (T2) of the applied amount, while water-dissolved transport was 2.8% (T1) and 0.5% (T2). Particle size and organic matter influenced the mobility of glyphosate and AMPA to off-target areas. These results indicate that the pollution risk of terrestrial and aquatic environments through runoff and deposition can be considerable.

Desorption rate of glyphosate from goethite as affected by different entering ligands: hints on the desorption mechanism

Environmental contextGlyphosate is a heavily used herbicide that is mobilised in soil and sediments through adsorption–desorption processes from the surface of mineral particles. We demonstrate that the desorption rate of glyphosate from goethite, a ubiquitous mineral, is nearly independent of the concentration and nature of the substance that is used to desorb it. The results elucidate the desorption mechanism and are relevant to understand and predict the environmental mobility of glyphosate. AbstractThe desorption kinetics of glyphosate (Gly) from goethite was studied in a flow cell using attenuated total reflectance Fourier-transform infrared spectroscopy. Because Gly forms an inner-sphere surface complex by coordinating to Fe atoms at the goethite surface, the desorption process is actually a ligand-exchange reaction, where Gly is the leaving ligand and water molecules or dissolved substances are the entering ligands. A series of possible entering ligands that can be found in nature was tested to evaluate their effect on the desorption kinetics of Gly. Contrarily to expectations, the desorption rate was quite independent of the entering ligand concentration. Moreover, the identity of this ligand (phosphate, citrate, sulfate, oxalate, EDTA, thiocyanate, humic acid, water) had only a small effect on the value of the desorption rate constant. By analogy with the reactivity of transition metal complexes in solution, it is concluded that the rate is mainly controlled by the breaking of the Fe–Gly bond, through a dissociative or a dissociative interchange mechanism. The results are relevant in understanding and predicting the environmental mobility of Gly: irrespective of the identity of the entering ligand, Gly will always desorb from iron (hydr)oxides in nature at nearly the same rate, simplifying calculations and predictions enormously. The importance of studying desorption kinetics using mineral surfaces and environmentally relevant molecules is also highlighted.

Stabilized Organic Matter Quantities May Strongly Affect the Mobility of Glyphosate into Soil

&lt;p&gt;Glyphosate [N-(phosphonomethyl) glycine] is an enzyme inhibiting herbicide, which is widely used in the world. Here, we investigate the glyphosate adsorption potential in tropical soils with pH close to 5. The herbicide was determined via HPLC with detection by UV-Vis. Our results suggest that glyphosate interacts especially with the stabilized organic matter. Thus, even with large amounts of soil organic matter, the soils may not be as effective in the adsorption of the herbicide if they do not also possess high percentage of humic substances. These data are worth far as it allows us to speculate on glyphosate interaction mechanism with the humic substances in slightly acid medium, in addition, also enable us to propose the use of that fraction of the organic matter to immobilize part of the herbicide in the soil and inhibiting its leaching into water bodies.&lt;/p&gt;

Adsorption and mobility of glyphosate in different soils under no-till and conventional tillage

Glyphosate (N-(phosphonomethyl) glycine) is a post-emergence, non-selective, foliar herbicide. Around 200millionliters of this herbicide are applied every year in Argentina, where the main agricultural practice is no-till (NT), accounting for 78.5% of the cultivated land. In this work, we studied the adsorption of glyphosate in different soils under long-term management (more than 16years) of NT and conventional tillage (CT). Samples were taken from different regions of Argentina corresponding to: Paraná soil (PAR), a silty clay loam soil (<37% clay), Manfredi (MAN) and Pergamino (PER), both silty loam soils (<26% clay). We found that the adsorption was very high in all the soils, and it was particularly influenced by the soil clay content and CEC and negatively related to pH and phosphorus. In general, the adsorption coefficient (Kf) was higher in the CT samples. We also studied the vertical transport of glyphosate in undisturbed columns (15cm long) and compared the effect of NT and CT. Less than 0.24% of the applied pesticide leached in all soils. No significant difference was found between the total amount of leached glyphosate between soils or tillage practice. The highest glyphosate concentration (67.53% of the initially applied doses) was found in the top 5cm of the columns. The strong retention of glyphosate to the soil matrix, as confirmed by the high Kf values obtained in the isotherm studies, was the dominant factor influencing glyphosate mobility through the soil profile.

Effect of Soil Aeration and Phosphate Addition on the Microbial Bioavailability of Carbon-14-Glyphosate

The adsorption, desorption, degradation, and mineralization of C-glyphosate [-(phosphonomethyl)glycine] were examined in Catlin (a fine-silty, mixed, superactive, mesic Oxyaquic Argiudoll), Flanagan (a fine, smectitic, mesic Aquic Argiudoll), and Drummer (a fine-silty, mixed, superactive, mesic Typic Endoaquoll) soils under oxic and anoxic soil conditions. With the exception of the Drummer soil, soil aeration did not significantly alter the adsorption pattern of C-glyphosate to soils. Herbicide desorption was generally enhanced with anaerobiosis in all the soil types. Anoxic soils demonstrated slower microbial degradation and mineralization kinetics of C-glyphosate than oxic soils in all the soil types studied. Phosphate additions significantly reduced the adsorption of C-glyphosate to soils irrespective of soil aeration and confirmed the well-established competitive adsorption theory. The addition of soil phosphate stimulated degradation only in anoxic soils. The results from this research highlight the importance of soil redox conditions as an important factor affecting the bioavailability and mobility of glyphosate in soils.

Effects of pig slurry application on soil physical and chemical properties and glyphosate mobility

Pig slurry applied to soil at different rates may affect soil properties and the mobility of chemical compounds within the soil. The purpose of this study was to evaluate the effects of rates of pig slurry application in agricultural areas on soil physical and chemical properties and on the mobility of glyphosate through the soil profile. The study was carried out in the 12th year of an experiment with pig slurry applied at rates of 0 (control), 50, 100 and 200 m³ ha-1 yr-1 on a Latossolo Vermelho distrófico (Hapludox) soil. In the control, the quantities of P and K removed by harvested grains were replaced in the next crop cycle. Soil physical properties (bulk density, porosity, texture, and saturated hydraulic conductivity) and chemical properties (organic matter, pH, extractable P, and exchangeable K) were measured. Soil solution samples were collected at depths of 20, 40 and 80 cm using suction lysimeters, and glyphosate concentrations were measured over a 60-day period after slurry application. Soil physical and chemical properties were little affected by the pig slurry applications, but soil pH was reduced and P levels increased in the surface layers. In turn, K levels were increased in sub-surface layers. Glyphosate concentrations tended to decrease over time but were not affected by pig slurry application. The concentrations of glyphosate found in different depths show that the pratice of this application in agricultural soils has the potential for contamination of groundwater, especially when the water table is the surface and heavy rains occur immediately after application.

The herbicide glyphosate and its metabolite AMPA in the Lavaux vineyard area, western Switzerland: Proof of widespread export to surface waters. Part II: The role of infiltration and surface runoff

Two parcels of the Lavaux vineyard area, western Switzerland, were studied to assess to which extent the widely used herbicide, glyphosate, and its metabolite aminomethylphosphonic acid (AMPA) were retained in the soil or exported to surface waters. They were equipped at their bottom with porous ceramic cups and runoff collectors, which allowed retrieving water samples for the growing seasons 2010 and 2011. The role of slope, soil properties and rainfall regime in their export was examined and the surface runoff/throughflows ratio was determined with a mass balance. Our results revealed elevated glyphosate and AMPA concentrations at 60 and 80 cm depth at parcel bottoms, suggesting their infiltration in the upper parts of the parcels and the presence of preferential flows in the studied parcels. Indeed, the succession of rainy days induced the gradual saturation of the soil porosity, leading to rapid infiltration through macropores, as well as surface runoff formation. Furthermore, the presence of more impervious weathered marls at 100 cm depth induced throughflows, the importance of which in the lateral transport of the herbicide molecules was determined by the slope steepness. Mobility of glyphosate and AMPA into the unsaturated zone was thus likely driven by precipitation regime and soil characteristics, such as slope, porosity structure and layer permeability discrepancy. Important rainfall events (>10 mm/day) were clearly exporting molecules from the soil top layer, as indicated by important concentrations in runoff samples. The mass balance showed that total loss (10–20%) mainly occurred through surface runoff (96%) and, to a minor extent, by throughflows in soils (4%), with subsequent exfiltration to surface waters.

Ecotoxicological effects of different glyphosate formulations

Glyphosate is an active substance of the most used herbicides worldwide. Nevertheless, questions on safety of glyphosate-based herbicides are periodically raised and recent studies indicate that glyphosate may not be as safe as assumed, mostly due to the additives/surfactants in its formulations. The aim of this study was to evaluate the effects of isopropylamine salt of glyphosate and two glyphosate-based herbicides, Roundup Max™ (containing surfactant polyethoxylated tallow amine, POEA) and Roundup Quick™ (without POEA), on non-target species. Special focus was on the evaluation of long-term effects of high concentrations (simulating accidental pollution, e.g. transportation spills) of glyphosate formulations on soil health. Laboratory ecotoxicity testing was conducted with (i) two aquatic organisms – crustaceans Daphnia magna and marine bacteria Vibrio fischeri, (ii) five bacterial strains (Escherichia coli MG1655, Pseudomonas putida KT2440 and three bacterial isolates from the soil) and (iii) terrestrial plants Raphanus sativus and Hordeum vulgare. Laboratory toxicity results showed that among the non-target test species, D. magna and V. fischeri were the most sensitive to glyphosate formulations: acute EC50 values ranged from 4 to 49mgL−1. Direct relation between the toxicity of the tested formulations and the presence/absence of the surfactant POEA was not evident. Long-term outdoor experiments (April to September 2012) showed that the number of heterotrophic microbes in Roundup-spiked (up to 1000-fold the recommended field rate) soils during two months after the treatment were significantly higher than in the control soils, especially in case of Roundup Quick™. Residual toxicity of the treated soils to terrestrial plants decreased more rapidly in Roundup Quick-spiked soils. It was shown that in temperate climate conditions the recovery of soil health in case of (accidental) pollution by glyphosate formulations is slow and may even exceed the duration of the vegetation period. The mobility of glyphosate in the soils proved very low thus risks to aquatic ecosystems due to application of glyphosate-based herbicides may occur rather in case of direct contamination of surface water.

Glyphosate bioavailability in soil

Biodegradation of glyphosate in sod-podzol soil by both the indigenous micro flora and the introduced strain Ochrobactrum anthropi GPK 3 was studied with respect to its sorption and mobility. The experiments were carried out in columns simulating the vertical soil profile. Soil samples studied were taken from soil horizons 0-10, 10-20, and 20-30 cm deep. It was found out that the most of the herbicide (up to 84%) was adsorbed by soil during the first 24 h; the rest (16%) remained in the soluble fraction. The adsorbed glyphosate was completely extractable by alkali. No irreversible binding of glyphosate was observed. By the end of the experiment (21st day), glyphosate was only found in extractable fractions. The comparison of the effect of the introduced O. anthropi GPK 3 and indigenous microbial community on the total toxicant content (both soluble and absorbed) in the upper 10 cm soil layer showed its reduction by 42% (21 mg/kg soil) and 10-12% (5 mg/kg soil), respectively. Simultaneously, 14-18% glyphosate moved to a lower 10-20 cm layer. Watering (that simulated rainfall) resulted in a 20% increase of its content at this depth; 6-8% of herbicide was further washed down to the 20-30 cm layer. The glyphosate mobility down the soil profile reduced its density in the upper layer, where it was available for biodegradation, and resulted in its concentration in lower horizons characterized by the absence (or low level) of biodegradative processes. It was shown for the first time how the herbicide biodegradation in soil can be increased manifold by introduction of the selected strain O. anthropi GPK 3.

Glyphosate mobility in soils by phosphate application: Laboratory column experiments

The supposition that phosphate accumulation in soil might result in reduced glyphosate retention capacity and thus increased leaching risk is mostly obtained from many batch experiments, rather than column experiments that may simulate actual field conditions better. However, we hypothesize that: (1) phosphate application might induce system pH decrease, and (2) the overall glyphosate mobility by phosphate application might depend on the relative contribution of two competing processes: increase in glyphosate adsorption from pH decrease versus reduced glyphosate adsorption from competitive adsorption between phosphate and glyphosate for sorption sites available. To test these two hypotheses, laboratory batch and column experiments using a miscible displacement approach were conducted on two Primosols and one Anthrosols,to investigate, respectively, (1) pH-dependent glyphosate adsorption onto the three studied soils, and (2) glyphosate leaching and mobility in soil columns as influenced by phosphate application. Our results showed that glyphosate adsorption consistently decreased with increase in system pH. We found that the extent of phosphate-induced pH decrease was, in decreasing order, Sandic Primosols, Alluvic Primosols, and Stagnic Anthrosols. The effect of phosphate application on glyphosate mobility varied with soil type, with the mobility being reduced in Sandic Primosols, less reduced in Alluvic Primosols, and in Stagnic Anthrosols the mobility seemed to be raised. These differences may be attributed to the relative change of the dominant factor that controlled the glyphosate mobility by phosphate application. In Sandic Primosols the increase in glyphosate adsorption from pH decrease probably dominated over the reduced glyphosate adsorption from the increased repellence between pre-sorbed phosphate and glyphosate; while in Stagnic Anthrosols the later likely became the dominant factor as this soil contained the highest content of soil oxides among the studied soils, which is considered to urge the repellence. We conclude that phosphate application can cause system pH change with various extents in the soil, which subsequently contribute to glyphosate mobility in different degree.

Physiological and molecular bases of glyphosate resistance in Conyza bonariensis biotypes from Spain

SummaryThe physiological and molecular bases of glyphosate resistance in one susceptible (S) and four resistant (R) Conyza bonariensis biotypes (sampled in orchards from Andalusia, Spain) were investigated. Resistance index (RI) values of the four R biotypes ranged between 2.9 and 5.6. The main physiological difference between S and R biotypes was the dissimilar mobility of glyphosate in the whole plant. In R biotypes, the herbicide was translocated less from leaves to culm and root, and more from culm to leaves compared with the S biotype. The upward mobility of glyphosate via xylem suggests that the herbicide may be sequestered to the apoplast or the vacuole. The hypothesis of an insensitive 5‐enolpyruvylshikimate‐3‐phosphate synthase (EPSPS) was provisionally discarded on the basis of shikimate accumulation in R plant tissues after glyphosate treatment. At the molecular level, the relative abundance of EPSPS mRNA prior to glyphosate treatment was approximately double in two R biotypes compared with the S standard and the other R biotypes. Moreover, the two R biotypes having both no translocation and doubled EPSPS mRNA levels had also the highest RI. These results suggest that two factors may be related to glyphosate resistance in the R biotypes: (i) impaired translocation and (ii) high basal EPSPS transcript levels. The comparison between these findings and earlier results on glyphosate resistance mechanism in Conyza canadensis biotypes from the USA, suggests that similar agronomic factors (repeated application of glyphosate, no crop and herbicide rotation, no tillage) have selected similar traits on different genetic pools of the resistance‐prone Conyza genus.