Influence of plant roots on rhizosphere soil solution composition of long-term contaminated soils

Abstract

Understanding the root mechanisms of hyperaccumulating or phytostabilizing plants in response to heavy metal exposure is important for the successful implementation of phytoremediation technologies. Root induced dissolved organic carbon (DOC) and organic acids (OAs) were investigated under greenhouse conditions in the rhizosphere after growing two plant species, Indian mustard ( Brassica juncea) and sunflower ( Helianthus annuus L.) in long-term heavy metal contaminated soils. Two alkaline soils (pH > 8) were highly contaminated while the other two acidic soils (pH < 6.5) were moderately contaminated with heavy metals. Cropping Indian mustard and sunflower on the alkaline soils increased the mean DOC concentrations in soil solution. Correspondingly, OAs which constituted a fraction of the total observed DOC, also increased due to plant cropping in these highly contaminated soils. The most marked increase was observed for lactate which increased 3–10 fold relative to the controls. Increased Cu and Pb solubility, through formation of metal–DOC complexes at soil pH > 8, was mainly attributed to increases in rhizosphere DOC. Soluble Cu increased from 47 (control) to 493 and 524 µg L − 1 and soluble Pb increased from 40 (control) to 149 and 148 µg L − 1 for sunflower and mustard, respectively in soil the most contaminated alkaline soil. MINTEQA2 speciation indicated that in this soil, the increased Cu and Pb in soil solution existed as metal–DOC complexes but soluble Zn existed mainly as a free ion (> 60% of the total Zn). In contrast, in a moderately contaminated acidic soil (380 mg kg − 1 Pb; pH < 6.5), Pb and Zn solubility was slightly decreased after plant culture and governed by soil solution pH rather than the DOC concentration, thus demonstrating that the influence of root induced DOC on metal solubility is a function of pH as well as total metal loading.