Effects of arbuscular mycorrhiza and phosphorus application on arsenic toxicity in sunflower (Helianthus annuus L.) and on the transformation of arsenic in the rhizosphere

Effects of arbuscular mycorrhiza (AM) and phosphorus (P) application on arsenic (As) toxicity were studied in a rhizobox system with As-contaminated soil collected from Shimane Prefecture, Japan. The treatments consisted of a combination of two levels of AM (Glomus aggregatum) inoculation (-AM and +AM) and two levels of P application (-P and +P at 30 mg P kg(-1)). Sunflower (Helianthus annuus L.) seedlings were cultured in rhizoboxes for 6 weeks. Rates of root AM infection in +AM treatments were about 40% regardless of P application. AM inoculation as well as P application reduced As toxicity symptoms, most clearly so in the +AM-P treatment. Plant growth was highest in the +AM + P treatment. Shoot As concentrations were slightly reduced by AM inoculation but enhanced by P application. Shoot P concentration in the +AM-P treatment was similar to that of +P treatments and was higher than in -AM-P. Analyses of rhizosphere soils at the end of the cultivation period indicated that P application increased water-soluble As (WS-As) in all compartments while AM inoculation increased WS-As in the central compartment only. Both the WS-arsenite [WS-As-(III)] and the dominant form, arsenate [WS-As-(V)], showed gradients toward the root surface. Dimethylarsine (DMAA) was detected in the +AM treatments only. To our knowledge, this is the first report of the occurrence of DMAA in the mycorrhizosphere. AM inoculation increased WS-P similarly as +P treatments did and promoted acid phosphatase activity in the soil. In conclusion, AM inoculation alleviated the effects of As toxicity by improving P nutrition without increasing As concentrations in the shoots. Moreover, AM appeared to be involved in the transformation of soil inorganic As into less toxic organic forms.