The global environmental hazard of glyphosate use.

Agricultural pesticides can become persistent environmental pollutants. Among many, glyphosate (GLP) is under particular scrutiny because of its extensive use and its alleged threats to the ecosystem and human health. Here, we introduce the first global environmental contamination analysis of GLP and its metabolite, AMPA, conducted with a mechanistic dynamic model at 0.5 × 0.5° spatial resolution (about 55 km at the equator) fed with geographically-distributed agricultural quantities, soil and biogeochemical properties, and hydroclimatic variables. Our analyses reveal that about 1% of croplands worldwide (385,000 km2) are susceptible to mid to high contamination hazard and less than 0.1% has a high hazard. Hotspots found in South America, Europe, and East and South Asia were mostly correlated to widespread GLP use in pastures, soybean, and corn; diffuse contributing processes were mainly biodegradation recalcitrance and persistence, while soil residue accumulation and leaching below the root zone contributed locally to the hazard in hotspots. Hydroclimatic and soil variables were major controlling factors of contamination hotspots. The relatively low risk of environmental exposure highlighted in our work for a single active substance does not rule out a greater recognition of environmental pollution by pesticides and calls for worldwide cooperation to develop timely standards and implement regulated strategies to prevent excess global environmental pollution.

PEST-CHEMGRIDS, global gridded maps of the top 20 crop-specific pesticide application rates from 2015 to 2025.

Available georeferenced environmental layers are facilitating new insights into global environmental assets and their vulnerability to anthropogenic inputs. Geographically gridded data of agricultural pesticides are crucial to assess human and ecosystem exposure to potential and recognised toxicants. However, pesticides inventories are often sparse over time and by region, mostly report aggregated classes of active ingredients, and are generally fragmented across local or government authorities, thus hampering an integrated global analysis of pesticide risk. Here, we introduce PEST-CHEMGRIDS, a comprehensive database of the 20 most used pesticide active ingredients on 6 dominant crops and 4 aggregated crop classes at 5 arc-min resolution (about 10 km at the equator) projected from 2015 to 2025. To estimate the global application rates of specific active ingredients we use spatial statistical methods to re-analyse the USGS/PNSP and FAOSTAT pesticide databases along with other public inventories including global gridded data of soil physical properties, hydroclimatic variables, agricultural quantities, and socio-economic indices. PEST-CHEMGRIDS can be used in global environmental modelling, assessment of agrichemical contamination, and risk analysis.

Glyphosate dispersion, degradation, and aquifer contamination in vineyards and wheat fields in the Po Valley, Italy.

Biodegradation of glyphosate (GLP) and its metabolite aminomethylphosphonic acid (AMPA) was numerically assessed for a vineyard and a wheat field in the Po Valley, Italy. Calculation of the Hazard Quotient suggested that GLP and AMPA can pose a risk of aquifer contamination in the top 1.5 m depth within 50 years of GLP use. Numerical results relative to soil GLP and AMPA concentrations, and GLP age, half life, and turnover time show that GLP was equivalently removed through hydrolysis and oxidation, but the latter produced AMPA. Biodegradation processes in the root zone removed more than 90% of applied GLP and more than 23% of the produced AMPA between two consecutive applications. Doubling organic carbon availability enhanced GLP and AMPA biodegradation, especially GLP hydrolysis to sarcosine. This work highlights that GLP and AMPA removal is controlled by soil water dynamics that depend on ecohydrological boundary conditions, and by carbon sources availability to biodegraders.

Analysis of glyphosate degradation in a soil microcosm.

Glyphosate (GLP) herbicide leaching into soil can undergo abiotic degradation and two enzymatic oxidative or hydrolytic reactions in both aerobic and anaerobic conditions; biotic oxidation produces aminomethylphosphonic acid (AMPA). Both GLP and AMPA are phytotoxic. A comprehensive GLP degradation reaction network was developed from the literature to account for the above pathways, and fifteen experimental data sets were used to determine the corresponding Michaelis-Menten-Monod (MMM) kinetic parameters. Various sensitivity analyses were designed to assess GLP and AMPA degradation potential against O2 (aq) and carbon (C) availability, pH, and birnessite mineral content, and showed that bacteria oxidized or hydrolyzed up to 98% of GLP and only 9% of AMPA. Lack of a C source limited the GLP cometabolic hydrolytic pathways, which produces non-toxic byproducts and promotes AMPA biodegradation. Low bacterial activity in O2 (aq)-limited conditions or non-neutral pH resulted in GLP accumulation. Birnessite mineral catalyzed fast GLP and AMPA chemodegradation reaching alone efficiencies of 79% and 88%, respectively, regardless of the other variables and produced non-toxic byproducts. Overall, O2 (aq) and birnessite availability played the major roles in determining the partitioning of GLP and its byproducts mass fluxes across the reaction network, while birnessite, C availability, and pH affected GLP and AMPA biodegradation effectiveness.

In-situ atrazine biodegradation dynamics in wheat (Triticum) crops under variable hydrologic regime.

A comprehensive biodegradation reaction network of atrazine (ATZ) and its 18 byproducts was coupled to the nitrogen cycle and integrated in a computational solver to assess the in-situ biodegradation effectiveness and leaching along a 5m deep soil cultivated with wheat in West Wyalong, New South Wales, Australia. Biodegradation removed 97.7% of 2kg/ha ATZ yearly applications in the root zone, but removal substantially decreased at increasing depths; dechlorination removed 79% of ATZ in aerobic conditions and 18% in anaerobic conditions, whereas deethylation and oxidation removed only 0.11% and 0.15% of ATZ, respectively. The residual Cl mass fraction in ATZ and 4 byproducts was 2.4% of the applied mass. ATZ half-life ranged from 150 to 247days in the soil surface. ATZ reached 5m soil depth within 200years and its concentration increased from 1×10-6 to 4×10-6mg/kgdry-soil over time. The correlation between ATZ specific biomass degradation affinity Φ0 and half-life t1/2, although relatively uncertain for both hydrolyzing and oxidizing bacteria, suggested that microorganisms with high Φ0 led to low ATZ t1/2. Greater ATZ applications were balanced by small nonlinear increments of ATZ biodegraded fraction within the root zone and therefore less ATZ leached into the shallow aquifer.

Kinetics of atrazine, deisopropylatrazine, and deethylatrazine soil biodecomposers.

Twenty-two experimental sets were used to determine the biodecomposition parameters of atrazine (ATZ), deisopropylatrazine (DIATZ), and deethylatrazine (DEATZ) by inverse solution of Michaelis-Menten-Monod kinetic equations. The averaged maximum specific growth rate (µ), Michaelis-Menten half-saturation concentration (K), and biomass yield (Y) ranged between 2.00 × 10-7 and 4.62 × 10-5 1/s, 3.43 × 10-6 and 1.39 × 101 mol/L, and 1.20 × 102 and 2.98 × 105 mg-wet-Bio/mol-Subs, respectively. Parameters grouped by reaction pathway appeared clustered by aerobic and anaerobic catabolic breakdown, and were poorly correlated between each other (R ranging from -0.27 to 0.63, p ≥ 0.05). The tested bacterial strains decomposed ATZ, DIATZ, and DEATZ relatively rapidly in laboratory conditions, with an half-life (t1/2) ranging between 3 and 6 days. Numerical modeling showed that ATZ, DIATZ, and DEATZ half-lives were particularly sensitive to their initial concentration and the initial microbial biomass concentration. This study suggests that these bacterial strains can effectively be used or enhanced for bioremediation of agricultural soils where atrazine has been applied as long as these bacteria already coexist in or can integrate with the local soil microbial population at a given location.

Implicit Analytic Solution of Michaelis-Menten-Monod Kinetics.

An analytic solution to enzyme kinetics expressed by the Michaelis-Menten-Monod mathematical framework is presented. The analytic solution describes the implicit problem with the independent variable, normally time, substituted with the concentration of the reaction product. The analytic solution provides the substrate, enzyme, and microbial biomass concentration instantaneously and over all time domains without the use of numerical integration schemes or iterative solvers required to overcome transcendental functions. Experiments of NO2 - nitrification by Candidatus Nitrospira defluvii at temperatures ranging between 10 and 32 °C were used for validation tests with the numerical solution by finite differences and the implicit analytic solution presented here. Results showed that both finite differences and analytic solutions matched the experiments particularly well, with a correlation coefficient greater than 0.99 and residuals smaller than 2.75%.