Medicago sativa

How it works

Mechanisms: rhizodegradation

Contaminants

Contaminant	Evidence	What was shown
Polychlorinated biphenyls	field	A 180-day field trial on farmland with weathered PCBs: alfalfa alone lowered soil PCBs by 8.1%; inoculated with Rhizobium meliloti by 33.8%, and with Rhizobium plus the mycorrhizal fungus Glomus caledonium by 43.5% (Teng et al. 2010). In pots, with Aroclor 1248 at 100 mg/kg, 38% or less of the initial PCBs was recovered from planted pots against more than 82% from controls (Chekol et al. 2004). Teng et al., Influence of arbuscular mycorrhiza and Rhizobium on phytoremediation by alfalfa of an agricultural soil contaminated with weathered PCBs: a field study, Int. J. Phytoremediation 12:516, 2010 Chekol et al., Phytoremediation of polychlorinated biphenyl-contaminated soils: the rhizosphere effect, Environ. Int. 30:799, 2004
Petroleum hydrocarbons	field	At ten refinery sites (13.7 g/kg hydrocarbons on average), alfalfa and ryegrass with nitrogen fertiliser and tillage: 72–90% decontamination in a year; with natural vegetation alone the same result took five years (Panchenko et al. 2023). In a greenhouse, with 20 g/kg crude oil, degradation after one year ranged among genotypes from 33 to 56%, against 46% for the unvegetated control: only two of twenty genotypes did better than bare soil (Wiltse et al. 1998). Panchenko et al., Natural and technical phytoremediation of oil-contaminated soil, Life 13:177, 2023 Wiltse et al., Greenhouse evaluation of agronomic and crude oil-phytoremediation potential among alfalfa genotypes, J. Environ. Qual. 27:169, agris.fao.org, 1998
Polycyclic aromatic hydrocarbons	greenhouse	In pots, 60 days: 69–85% of spiked pyrene gone from rhizosphere soil against 59–80% outside it, six percentage points more on average; at 492 mg/kg pyrene shoot biomass fell to 34% (Fan et al. 2008). Fan et al., Promotion of pyrene degradation in rhizosphere of alfalfa (Medicago sativa L.), Chemosphere 71:1593, 2008

Contaminant

Evidence

What was shown

field

A 180-day field trial on farmland with weathered PCBs: alfalfa alone lowered soil PCBs by 8.1%; inoculated with Rhizobium meliloti by 33.8%, and with Rhizobium plus the mycorrhizal fungus Glomus caledonium by 43.5% (Teng et al. 2010). In pots, with Aroclor 1248 at 100 mg/kg, 38% or less of the initial PCBs was recovered from planted pots against more than 82% from controls (Chekol et al. 2004).

Petroleum hydrocarbons

field

At ten refinery sites (13.7 g/kg hydrocarbons on average), alfalfa and ryegrass with nitrogen fertiliser and tillage: 72–90% decontamination in a year; with natural vegetation alone the same result took five years (Panchenko et al. 2023). In a greenhouse, with 20 g/kg crude oil, degradation after one year ranged among genotypes from 33 to 56%, against 46% for the unvegetated control: only two of twenty genotypes did better than bare soil (Wiltse et al. 1998).

Polycyclic aromatic hydrocarbons

greenhouse

In pots, 60 days: 69–85% of spiked pyrene gone from rhizosphere soil against 59–80% outside it, six percentage points more on average; at 492 mg/kg pyrene shoot biomass fell to 34% (Fan et al. 2008).

Fan et al., Promotion of pyrene degradation in rhizosphere of alfalfa (Medicago sativa L.), Chemosphere 71:1593, 2008

Limits

The gain due to the plant is often small against unplanted soil (a few percentage points for pyrene, 8% for PCBs without inoculum) and depends on genotype and associated microbes. In field trials on hydrocarbons it is hard to separate the plant's effect from that of fertiliser and tillage. In heavily contaminated soil yield falls to a third (Wiltse et al. 1998). Forage grown on PCB soil must not be fed to livestock.

Where it has been tried

Brescia-Caffaro

PhytoremediationGreenhouse, with soil from the site

Festuca arundinacea, Cucurbita pepo subsp. pepo, Medicago sativa, Phalaris arundinacea

Greenhouse rhizoremediation trial on farm soil from the SIN contaminated with aged PCBs (mg/kg range), in 6 kg pots to approach field conditions. Ten treatments in triplicate: seven species (grasses and trees) and five growing conditions (single species, intercropping, redox cycle by flooding, added compost or ammonium thiosulfate), with unplanted controls, for 18 months. The same experiment produced soil half-lives for PCBs and for dioxins and furans.

Outcome After 18 months total PCBs had fallen by 14–20%. The best treatments (about 20%) were Festuca arundinacea with compost, Festuca arundinacea grown with Cucurbita pepo, and Medicago sativa with Rhizobium and mycorrhizal fungi. PCB half-lives ranged from 1.3 to 5.6 years. For dioxins and furans only Festuca arundinacea gave a significant reduction (11–24% depending on the congener; half-lives 2.5–5.8 years). In a later column test, soil grown with Medicago sativa released about twice as much PCB in leachate, bound to particulate organic carbon.

Microbial bioremediationLaboratory, with soil from the site

Medicago sativa, Centaurea nigrescens, Dactylis glomerata

Study of the bacteria associated with the roots of three spontaneous plants (Medicago sativa, Centaurea nigrescens, Dactylis glomerata) growing in a former farm field of the SIN after cultivation was banned. Communities described by 16S rRNA gene sequencing; 165 rhizosphere isolates able to grow on biphenyl tested for plant growth promotion and degradation potential.

Outcome Soil fraction and plant species explained 21% and 18% of the variation in the bacterial microbiome. The bphA gene (biphenyl dioxygenase) was present in the soil metagenome. Eleven isolates tested on tomato confirmed the plant growth promotion and protection effect.

Polychlorinated biphenyls

Vergani L. et al. Bacteria associated to plants naturally selected in a historical PCB polluted soil show potential to sustain natural attenuation. Frontiers in Microbiology 8:1385, 2017

PhytoremediationGreenhouse, with soil from the site, 2015–2016

Festuca arundinacea, Phalaris arundinacea, Cucurbita pepo subsp. pepo, Medicago sativa, Brassica juncea, Salix caprea, Athyrium filix-femina

The University of Insubria, the University of Milan and ERSAF took about 2,500 kg of soil from the site and grew plants on it in a greenhouse for 18 months (May 2015 – November 2016) in 6 kg pots: ten treatments in triplicate with seven species, intercropping, compost, redox cycles and ammonium thiosulphate, with unplanted controls. It is a semi-field trial, not an open-field one.

Outcome After 18 months total PCBs had fallen by 14–20%. The best treatments (Festuca arundinacea with compost or intercropped with pumpkin; alfalfa with Rhizobium and mycorrhizal fungi) reached about 20%. For dioxins and furans tall fescue alone gave reductions of 11–24% depending on the congener, with half-lives of 2.5–5.8 years. The authors name the short duration, against the persistence of these compounds, as a limit.

Caffaro di Torviscosa (formerly Grado and Marano Lagoon)

PhytoremediationField scale

Helianthus annuus, Medicago sativa, Raphanus sativus var. oleiformis, Lolium multiflorum

Sunflower, alfalfa, fodder radish and Italian ryegrass grown in the field on pyrite cinders capped with 0.15 m of clean soil, comparing ploughing with subsoiling to 0.3 m. Ploughing brought about 30% more contamination into the topsoil.

Outcome Removal in the shoots was poor: at most 33 mg/m² of trace elements, 62% of it zinc and 18% copper. Fodder radish and sunflower gave the most biomass. Fine roots in the shallow layers were a large sink for the elements, useful for phytostabilisation.

Vamerali T., Bandiera M., Mosca G. In situ phytoremediation of arsenic- and metal-polluted pyrite waste with field crops: effects of soil management. Chemosphere 83, 2011

Hinkley groundwater contamination

PhytoremediationField scale

Alfalfa cultivation as part of treating the contaminated water. Wikipedia lists the method without dates, areas or results; to be checked against the regional water board's documents.