Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 121

6.4 MICROBES-MEDIATED MITIGATION OF AS IN CONTAMINATED SOILS:

ASSOCIATED FACTORS AFFECTING MITIGATION

The mitigation of As by microbes will depend on the amount of available As, soil properties and
other environmental conditions (Brammer, 2009). Following major factors are having roles in As
contamination to crops.

6.4.1 Irrigation
The problem of As in plants and crops predominantly occurs in the areas where As-contaminated
groundwater is being used to irrigate agriculture lands. When soils kept submerged as in flooded
paddy fields, As gets mobilized into the soil solution mainly as arsenite (Xu et al ., 2008). This is
a result of reduction of strongly adsorbed As(V) to more weakly adsorbed As(III), leading to an
enhanced partition of As from the solid to the solution phase. Consequently the bioavailability
of As is enhanced to rice plants grown under submerged conditions than to those grown under
aerobic conditions. The As conversion in soil to different bioavailable forms is affected by water
management. Soil bacteria are also mobilizing As into irrigation water.

6.4.2 Habitat
Variation exists in the environmental conditions of affected habitats such as climate, sediment
sources, geomorphology and hydrology etc. The diverse environment of contaminated habitats
regulates the microbiota and their activities directly through the modification of their population
size, diversity and biogeochemistry under different toxicity levels (Kavamura and Esposito, 2010).
Kandeler et al . (1996) noticed that an increase in the contaminant level of heavy metals above
their threshold limits in the soil exerted a negative influence on microbial activities, resulting in a
decrease in the functional diversity of soil microorganisms and activities of soil enzymes in those
contaminated soils.

6.4.3 Soil properties
As chemistry in the soil is affected by soil chemical and biological properties. The survival,
propagation and functioning of microbes in the rhizosphere noticeably vary for a range of soil
processes (Hinsinger et al ., 2005; Watt et al ., 2006), These processes depend not only on the
host plants (Kamilova et al ., 2006; Nunan et al ., 2005) but also on the conditions and properties
of the medium, e.g., soil and water composition of habitat and concentrations of nutrients and
pollutants, as well as of other competing microorganisms are majorly influential in regulating
these processes (Gunatilaka, 2006; Kamilova et al ., 2006). Soil pH is one of the main factors
controlling the solubility of heavy metals; and solubility increases when the pH falls below
6.0 (Ross, 1994). Addition of lime with organic matter may raise the soil pH of acidic soils
(having pH < 6.5) and immobilize the metals (Clement et al ., 2003). Further this also depends
on the nature of the organic matter and microbial population creating the redox conditions for
As speciation.

6.4.4 Root exudates
The number of microorganisms in the rhizosphere is typically one order of magnitude larger than
in non-rhizosphere soil due to the continuous input of root derived organic substrates, resulting in
a more diverse, active and synergetic microbial community (Marschner, 1995). The production
of organic acids, such as oxalic acid by ectomycorrhizal fungi (Malajczuk and Cromack, 1982),
could also be of importance in determining metal uptake, since these acids can bind metals.
Root mediated rhizospheric interactions could be positive or negative. Positive interactions
involve root exudate mediated interactions with plant growth-promoting rhizobacteria (PGPR),