تغيير در قابليت استفاده و اجزاء مس در ريزوسفر آفتابگردان در خاك آلوده تيمارشده با كلات كننده ها

Change in Availability and Fractions of Copper in the Rhizosphere of Sunflower in a Contaminated Soil Treated With Chelators

ویژگی¬های زیستی و شیمیایی محیط ریزوسفر از جمله فراهمی و اجزاء شیمیایی فلزات در آن با توده خاك متفاوت است. این پژوهش به منظور بررسی تغییر در قابلیت¬استفاده و اجزاء مس در ریزوسفر آفتابگردان (Helianthus annuus L.) در یك خاك شنی آلوده به مس تیمار¬شده با كلات-كننده¬ها (EDTA، اسید سیتریك و عصاره كود مرغی) در شرایط گلخانه¬ای انجام شد.EDTA و اسید سیتریك در سطوح غلظتی صفر، 5/0 و 1 ¬میلی-مول بر كیلوگرم خاك و عصاره كود مرغی در سطوح غلظتی صفر، 5/0 و 1 گرم بر كیلوگرم خاك استفاده شدند. تعداد سه بذر آفتابگردان در هر ریزوباكس كاشته شد. بعد از 10 هفته گیاهان برداشت شدند و خاك¬های ریزوسفری و توده جدا شدند. مس قابل¬استفاده با استفاده از 7 روش عصاره¬گیری شیمیایی و جزءبندی مس در خاك¬های ریزوسفری و توده اندازه‌گیری شد. ویژگی¬های خاك ریزوسفری با خاك توده متفاوت بود. مقدار مس عصاره¬گیری شده به روش¬های مختلف در خاك¬های ریزوسفری كمتر از خاك¬های توده بود. دامنه تغییرات مس عصاره¬گیری شده با عصاره¬گیرها از 023/0 تا 83/1 میلی¬گرم بر كیلوگرم بود. در خاك¬های ریزوسفری و توده تیمارشده با كلات¬كننده¬ها، بیشترین مقادیر مس به ترتیب در اجزاء باقیمانده، پیوند¬شده با ماده آلی، پیوند¬شده با اكسیدهای آهن و منگنز، پیوند¬شده با كربنات¬ها و تبادلی بود. میانگین اجزاء مس پیوند¬شده با كربنات¬ها، پیوند¬شده با اكسیدهای آهن و منگنز و جزء باقیمانده در شرایط ریزوسفری بیشتر از توده خاك بود. میانگین جزء پیوند¬شده با ماده آلی در شرایط ریزوسفری كمتر از توده خاك بود. دامنه تغییرات اجزاء مس از 81/2 تا 95/12 میلی¬گرم بر كیلوگرم بود.

Introduction: Rhizosphere is commonly defined as the zone where root activity significantly influences the biological and chemical properties of the soil. Biological, physical and chemical characteristics of rhizosphere, especially metal availability and metal chemical forms are different than the bulk soil. Plant roots continuously release compounds such as sugars, amino acids, and carboxylic acids. Plant roots have the ability to transform metal fractions for easier uptake through root exudation in the rhizosphere. This study was conducted to investigate change in availability and fractions of Copper in the rhizosphere of sunflower (Helianthus annuus L.) in a sandy contaminated soil treated with chelators (EDTA, citric acid and poultry manure extract (PME)) in greenhouse condition. Materials and Methods: In this study, EDTA and citric acid were used at concentrations of 0, 0.5 and 1 mmol kg-1 soil and PME was used at concentrations of0, 0.5 and 1 g kg-1 soil. Three seeds of sunflower were planted in the rhizobox. After 10 weeks, plants were harvested and rhizosphere and bulk soils were separated. Dissolved organic carbon (DOC), microbial biomass carbon (MBC), available Cu (by using 7 chemical procedures including DTPA-TEA,AB-DTPA, Mehlich1, Mehlich3, CaCl2 0.01 M, rhizosphere-based method and distilled water) and Cu-fractions were determined in the rhizosphere and bulk soils. Results and Discussion: Rhizosphere soils properties were different with bulk soils. The results showed that the mean of DOC and MBC in the rhizosphere soils were higher than the bulk soils, but this difference was significant in some treatments. The mean value of pH in the rhizosphere soils was significantly (p <0.05) smaller than one in the bulk soils. The amounts of extracted Cu by different methods in the rhizosphere were lower than ones in the bulk soils. The amounts of exteracted Cu with extractants ranged from 0.023 to 1.83mg kg-1. The amounts of extracted Cu by distilled water, rhizosphere-based method and CaCl2 0.01 M were not detected by an atomic absorption spectrophotometer. The maximum and minimum amounts of Cu were exteracted by mehlich3 and mehlich1 methods, respectively. As chelators added to soil, increased the amount of available Cu. Cu extracted by mehlich1 was read only by an atomic absorption spectrophotometer when EDTA added to the soil. In the rhizosphere and bulk soils treated with chelators, the greatest amounts of copper were observed in the residual, bounded to organic matter, bounded to iron and manganese oxides, bounded to carbonates and exchangeable fractions, respectively. The exchangeable Cu fraction was not detected by an atomic absorption spectrophotometer. The mean values of Cu bounded to carbonates, bounded to iron and manganese oxides and residual fractions in the rhizosphere were higher than ones in the bulk soils. The mean value of Cu bounded to organic matter fraction in the rhizosphere was smaller than one in the bulk soil. The amounts of Cu fractions ranged from 2.81 to 12.95mg kg-1. In the rhizosphere soils, Cu exteracted by Mehlich 1 and Mehlich 3 were significantly correlated with Cu bounded to organic matter fraction (r = 0.99), and shoot dry mass was significantly correlated with Cu bounded to carbonates (r = -0.99). In the bulk soils, Root dry mass was significantly correlated with Cu bounded to iron and manganese oxides (r =- 0.99). Conclusions: In the present study, the effects of EDTA, citric acid and PME on bioavailability and fractions of Cu in the rhizosphere of sunflower (Helianthus annuus L.) plant grown on a sandy contaminated soil were investigated. The chelators and environment had significant impacts (P <0.05) on Cu extracted by different extractants. The maximum and minimum amounts of Cu were exteracted when the EDTA (1 mmol kg-1) was added to the soil and no chelators were added to the soil (control condition), respectively. The results of this research illustrated that availability of Cu in the rhizosphere soils are different from the bulk soils. Also, the present studyshowed that the Cu values extracted with seven different chemical extractants from the rhizosphere soils were significantly lower than ones in the bulk soils. Furthermore, Cu concentrations in various chemical soil fractions differed between rhizosphere and bulk soils. This shows that the sunflower root-that induced modifications of soil physical, chemical, and biological conditions not only leads to depletion in mobile fractions of soil Cu, but also modifies the soil Cu fractions which are commonly considered as more stable. Therefore, using the rhizosphere soil would be recommended in the study of available Cu.