Phytoremediation approaches for restoration of pesticide-contaminated agricultural soils

Introduction

Picture a soybean field in Mato Grosso after 25 years of continuous cropping: the yields remain profitable, but soil analyses reveal persistent residues of atrazine at 0.8 mg kg⁻¹ and chlorpyrifos at 0.3 mg kg⁻¹-concentrations that exceed Brazilian intervention values and threaten the shallow aquifer that supplies downstream communities [1,2] . This scenario, increasingly common across the Cerrado biome, illustrates why pesticide contamination of agricultural soils has become a priority environmental concern in tropical developing nations. Conventional remediation technologies-soil excavation, chemical oxidation, thermal desorptionare prohibitively expensive for the vast areas involved in tropical agriculture, with costs ranging from US$50 to US$500 per cubic meter [3] . Phytoremediation, the use of living plants and their associated rhizosphere microorganisms to degrade, extract, or immobilize contaminants, offers a cost-effective and ecologically compatible alternative at roughly US$5-15 per cubic meter [4,5] . Several mechanisms operate simultaneously: phytoextraction (root uptake and translocation), rhizodegradation (microbial breakdown stimulated by root exudates), phytostabilization (immobilization in the root zone), and phytovolatilization (transpiration of volatile metabolites) [6] . Despite growing evidence for phytoremediation of heavy metals and petroleum hydrocarbons, fewer investigations have targeted pesticide-contaminated agricultural soils under tropical conditions, and almost none have compared multiple plant species against multiple pesticide classes in a single field experiment [7,8] . This research evaluated six plant species representing grasses, brassicas, and crops for their ability to remediate soils contaminated with atrazine (triazine herbicide), chlorpyrifos (organophosphate insecticide), and endosulfan (organochlorine insecticide)-three compounds with contrasting chemical properties and degradation pathways.

Research area description

The experiment was conducted at the EMBRAPA Cerrados research station, located 35 km Northeast of Brasília, Federal District, Brazil (15°36′S, 47°42′W, elevation 1,014 m). The station sits within the core Cerrado biome on a well-drained Latossolo Vermelho (Ferralsol, FAO classification) developed from Tertiary laterite. Topsoil (0-20 cm) properties were: clay 58%, silt 14%, sand 28%, pH (H₂O) 5.4, organic carbon 22.7 g kg⁻¹, CEC 8.6 cmol₂ kg⁻¹. The regional climate is tropical savanna (Köppen Aw) with a distinct wet season (October-March, ~1,450 mm) and dry season (April-September, ~100 mm). Mean annual temperature is 21.3 °C [9] .

Soil and water quality analysis

Baseline soil contamination was characterized by collecting 40 georeferenced samples (0-30 cm) across the experimental area in February 2023. Pesticide concentrations were determined by QuEChERS extraction followed by GC-MS/MS (Agilent 7890B/7000D) with detection limits of 0.005, 0.002, and 0.003 mg kg⁻¹ for atrazine, chlorpyrifos, and endosulfan, respectively. Soil biological activity was assessed through dehydrogenase activity (TTC reduction method), fluorescein diacetate (FDA) hydrolysis, and microbial biomass carbon (fumigationextraction). Groundwater monitoring wells (3 m depth) at four plot corners were sampled monthly for pesticide residues using solid-phase extraction and LC-MS/MS [10] .

Materials and Methods

The experiment used a randomized complete block design with four replications. Seven treatments comprised six plant species (vetiver grass, Chrysopogon zizanioides; sunflower, Helianthus annuus; Indian mustard, Brassica juncea; maize, Zea mays; sorghum, Sorghum bicolor; tall fescue, Festuca arundinacea) and an unplanted control. Individual plots (3 m × 4 m) were established on naturally contaminated soil supplemented with additional pesticide solution to achieve uniform starting concentrations of 2.0 mg kg⁻¹ atrazine, 1.0 mg kg⁻¹ chlorpyrifos, and 0.5 mg kg⁻¹ endosulfan-levels representative of moderately contaminated Cerrado cropland [11] . Pesticide solutions were applied via backpack sprayer and incorporated by rotary tillage to 20 cm depth. Species were planted at recommended densities two weeks after pesticide application to allow initial soil equilibration. Plots received drip irrigation to maintain 60-70% field capacity throughout the 90-day experimental period (March-June 2023). Soil samples (0-20 cm, three cores per plot) were collected at 0, 30, 60, and 90 days for residual pesticide analysis and biological assays. Plant biomass was harvested at 90 days and analyzed for tissue pesticide concentrations [12] . All six plant species significantly enhanced pesticide removal compared to the unplanted control (Table 1). Vetiver grass achieved the highest removal across all three pesticides, reducing atrazine by 78.4%, chlorpyrifos by 71.2%, and endosulfan by 64.8%. Indian mustard was the second most effective species for organophosphate and organochlorine removal. The unplanted control showed only 14.6-22.7% dissipation over 90 days, attributable to abiotic degradation and native microbial activity. The bar chart (Figure 1) visually confirms that vetiver grass and tall fescue-both perennial grasses with extensive fibrous root systems-outperformed the annual crop species (maize, sorghum, sunflower) for atrazine removal, likely due to greater rhizosphere volume and root surface area available for microbial colonization. Rhizosphere microbial activity was strongly enhanced by all plant species (Table 2). Vetiver grass produced the highest dehydrogenase activity (287.4 µg TPF g⁻¹ 24h⁻¹), 4.1 times higher than the unplanted control, consistent with its extensive root system providing exudates that fuel cometabolic degradation of pesticide residues. Atrazine degradation followed first-order kinetics across all treatments (Table 3). Vetiver grass reduced atrazine concentration from 2.00 to 0.43 mg kg⁻¹ in 90 days, achieving a degradation half-life of 38 days compared to 247 days in unplanted soil-a 6.5-fold acceleration. The steepest decline occurred between days 30 and 60, coinciding with maximum root biomass development and peak rhizosphere microbial colonization.

Results

Discussion

The superior performance of vetiver grass across all three pesticides aligns with its recognized status as a hyperaccumulator of organic contaminants, attributed to its massive, deep root system (reaching 3-4 m depth), high transpiration rate, and exceptional tolerance to chemical stress [13] . Vetiver's fibrous root architecture creates a far larger rhizosphere volume per unit area than taproot species like sunflower or mustard, explaining the 2-3-fold difference in rhizosphere dehydrogenase activity between these groups. The root exudates of vetiver-rich in organic acids, phenolics, and sugars-serve as cometabolites that induce microbial enzymes capable of cleaving the triazine ring of atrazine and the phosphoester bond of chlorpyrifos [14] . Indian mustard's strong performance against chlorpyrifos (67.8%) and endosulfan (59.2%) despite moderate atrazine removal reflects its different remediation mechanism: Brassica species are known for high root uptake and enzymatic transformation via cytochrome P450 and glutathione Stransferase pathways, which are more effective on lipophilic compounds (log Kow > 3) than on polar herbicides like atrazine [15] . The practical implication is that mixed plantings-combining vetiver for broad-spectrum rhizodegradation with mustard for enhanced phytoextraction of hydrophobic pesticides-could outperform monoculture approaches, an avenue worth pursuing in future field-scale trials [16] .

Conclusion

Phytoremediation using locally adapted plant species achieved 37-78% removal of three target pesticides from contaminated Cerrado soils within 90 days, far exceeding the 15-23% dissipation observed in unplanted controls. Vetiver grass was the most effective single species, reducing atrazine by 78.4%, chlorpyrifos by 71.2%, and endosulfan by 64.8%, with a 4.1fold enhancement of rhizosphere microbial activity driving cometabolic degradation. Indian mustard showed complementary strength against hydrophobic pesticides through phytoextraction. Degradation half-lives under vetiver were 6.5 times shorter than in unplanted soil, confirming that phytoremediation can meaningfully accelerate natural attenuation. These results establish vetiver grass as a priority species for large-scale remediation of pesticide-contaminated tropical agricultural soils, and suggest that polyculture approaches combining rhizodegradation specialists with phytoextraction species may deliver even greater removal efficiencies.

References

1. CONAMA. Resolução n° 420/2009: critérios e valores orientadores de qualidade do solo. Diário Oficial da União, Brasília; 2009.
2. Dores EFGC, Carbo L, Ribeiro ML, De-Lamonica-Freire EM. Pesticide levels in ground and surface waters of Primavera do Leste region, Mato Grosso, Brazil. J Chromatogr Sci. 2008;46(7):585-590.
3. Mulligan CN, Yong RN, Gibbs BF. An evaluation of technologies for the heavy metal remediation of dredged sediments. J Hazard Mater. 2001;85(1-2):145-163.
4. Salt DE, Smith RD, Raskin I. Phytoremediation. Annu Rev Plant Physiol Plant Mol Biol. 1998;49:643-668.
5. Pilon-Smits E. Phytoremediation. Annu Rev Plant Biol. 2005;56:15-39.
6. Gerhardt KE, Huang XD, Glick BR, Greenberg BM. Phytoremediation and rhizoremediation of organic soil contaminants: potential and challenges. Plant Sci. 2009;176(1):20-30.
7. Cherian S, Oliveira MM. Transgenic plants in phytoremediation: recent advances and new possibilities. Environ Sci Technol. 2005;39(24):9377-9390.
8. Romeh AA, Hendawi MY. Chlorpyrifos insecticide uptake by plantain from polluted water and soil. Environ Chem Lett. 2013;11(2):163-170.
9. Reatto A, Correia JR, Spera ST, Martins ES. Solos do bioma Cerrado: aspectos pedológicos. In: Cerrado: ecologia e flora. Brasília: EMBRAPA Informação Tecnológica; 2008. p. 107-149.
10. Anastassiades M, Lehotay SJ, Štajnbaher D, Schenck FJ. Fast and easy multiresidue method employing acetonitrile extraction/partitioning and dispersive solid-phase extraction for the determination of pesticide residues in produce. J AOAC Int. 2003;86(2):412-431.
11. Pires FR, Souza CM, Silva AA, Queiroz MELR, Cecon PR, Santos JB. Phytoremediation of herbicide-contaminated soils. Planta Daninha. 2003;21(2):335-341.
12. Anderson TA, Guthrie EA, Walton BT. Bioremediation in the rhizosphere. Environ Sci Technol. 1993;27(13):26302636.
13. Truong P, Van TT, Pinners E. Vetiver system applications: a technical reference manual. The Vetiver Network International; 2008.
14. Singh BK, Walker A, Morgan JAW, Wright DJ. Biodegradation of chlorpyrifos by Enterobacter strain B-14 and its use in bioremediation of contaminated soils. Appl Environ Microbiol. 2004;70(8):4855-4863.
15. Dixit P, Mukherjee PK, Ramachandran V, Eapen S. Glutathione transferase from Trichoderma virens enhances cadmium tolerance without enhancing its accumulation in transgenic Nicotiana tabacum. PLoS ONE. 2011;6(1):e16360.
16. Nunes da Silva M, Cardoso AR, de Pinho D, Gomes MV, Vasconcelos MW. Phytoremediation: a technology for the future. Environ Pollut. 2021;274:116521.