Effects of chromium contamination on the soil microbiome and phytoremediation potential of crop plants

Soil pollution as a result of heavy metal contamination has become of major concern, with rising levels as a result of industrial effluents and waste. This is the case within Dindigul, Tamil Nadu, India, where tannery pollution has negatively affected soil health and caused land abandonment by farmers, reducing incomes and the availability of food for the local population. The extent of tannery pollution in Dindigul was determined by comparing soil samples from 11 sites within an area known as the Tannery Belt to 6 control sites. The level of contamination varied across the Tannery Belt, with several areas deemed moderately to highly polluted with Cr, as well as with Cd, Cu and Zn and salts. Salts and soil properties including pH, electrical conductivity, organic matter, and soil moisture all demonstrated correlations with heavy metals contributing to detrimental impact on the overall soil quality.

Soil metagenomics analysis showed that Cr pollution affected soil microorganism communities, causing changes in abundance of individual bacteria, fungi, nematode and protozoa in polluted soils compared to controls. Bacteria Thermomicrobiales and Tistrellales, fungi Eurotiales and Capnodiales, nematode Rhabditida and protozoa Phytomyxea, all showed significant resistance to the presence of pollution within the contaminated soil samples and demonstrated a large increase in frequency within the samples between control and contaminated. Bacteria Tepidisphaerales, fungi Hypocreales, nematode Dorylaimia and Tylenchida and protozoa Gregarinasina all showed sizable reduction in the relative frequency levels between the control and contaminated soil samples.

Six crop plants were identified as potential candidates for the cost-effective and sustainable remediation of Cr. From a pot experiment exposing the six crops to different chromium levels, tomato, sunflower, and sorghum demonstrated the ability to maintain biomass in high levels of Cr contamination. Cr uptake within edible structures did not exceed permissible limits for these edible crops, suggesting that the risk to human health is minimal. Sorghum was identified as the most appropriate crop for phytoremediation at highly polluted sites such as the Tannery Belt, with a chromium uptake potential of 38% when exposed to 700 mg/kg Cr, whereas tomato and sunflower could be planted in moderately contaminated sites.