Root Turnover:  An Important Source of Microbial Substrates in Rhizosphere Remediation of Recalcitrant Contaminants

Abstract

The growth dynamics and phenolic content of mulberry (Morus sp.) fine roots (<1 mm diameter) were determined and examined in relationship to rhizosphere remediation of recalcitrant soil contaminants. Root turnover measurements of rhizotron-grown plants showed that 58% of the fine roots produced during a 6-month growing season (June-November) died at the end of the season. The concentration of phenolic compounds in fine roots increased approximately 2-fold during the later stages of the season, and the total phenolic content of dead fine roots reached a maximum value of 38 mg/g dry weight. The late-season increase in total phenolics was primarily due to accumulation of three different flavones (morusin, morusinol, and kuwanon C). These three flavones were shown to support the growth of the bacterium Burkholderia sp. LB400, a degrader of polychlorinated biphenyls (PCBs). Thus, it has been established that, upon death, the fine roots of mulberry can serve as a source of substrate for PCB-degrading bacteria. These results establish for the first time that the chemical content and turnover rate of fine roots should be considered an important aspect of rhizosphere remediation.

Phytotoxicity of branched cyclohexanes found in the volatile fraction of diesel fuel on germination of selected grass species

“…A plant can exude as much as 40% of its photosynthate into the soil (Kumar et al, 2006) and it is these sugars, organic acids and larger organic compounds that are used as sources of energy to support 10-100 times more microorganisms per gram of rhizosphere soil than unplanted soil (Lynch, 1990). In addition, the rapid decay of fine plant root material in the rhizosphere provides an important source of microbial nutrition (Leigh et al, 2002) and root growth stimulates oxidative degradation of petroleum hydrocarbons by promoting soil aeration. This complex interaction of roots, root exudates and the microorganisms populating the rhizosphere have been shown to enhance degradation of petroleum hydrocarbon contaminants (Binet et al, 2000, Ferro et al, 1998, Gaskin & Bentham, 2010, Lu et al, 2010, Peng et al, 2009.…”

Phytoremediation of Weathered Petroleum in Groundwater by Arroyo Willows in Nutrient Amended On-Site Mesocosms

“…The rhizosphere enlarges microbial populations and encourages metabolic activities by chemical and physical means. Roots provide elevated concentrations of labile carbon through sloughing of cells and root exudation (Leigh, 2002).…”

“…Plants also provide organic matter to the soil, either after they die or as living plants through the loss of root cap cells and the excretion of mucigel, which is a gelatinous substance that is a lubricant for root penetration through the soil (Cunningham et al, 1996). In addition to providing substrates to degradative bacteria, root turnover also provides oxygen essential for the activity of dioxygenase and monooxygenase enzymes that catalyze the first step in aerobic degradation of aromatic contaminants (Leigh et al, 2002). Root turnover is considered a major contributor to soil aeration through formation of air channels created when roots die and decay (Leigh et al,, 2002).…”

“…In addition to providing substrates to degradative bacteria, root turnover also provides oxygen essential for the activity of dioxygenase and monooxygenase enzymes that catalyze the first step in aerobic degradation of aromatic contaminants (Leigh et al, 2002). Root turnover is considered a major contributor to soil aeration through formation of air channels created when roots die and decay (Leigh et al,, 2002). Leigh et al (2002) demonstrated that seasonal fine root death releases several flavones which act as substrates for polychlorinated biphenyl (PCB) degrading bacteria.…”

“…Root turnover is considered a major contributor to soil aeration through formation of air channels created when roots die and decay (Leigh et al,, 2002). Leigh et al (2002) demonstrated that seasonal fine root death releases several flavones which act as substrates for polychlorinated biphenyl (PCB) degrading bacteria.…”