Chromium availability in ultramafic soils from New Caledonia

Area of Science:

• Environmental Science
• Soil Science
• Geochemistry

Background:

• Ultramafic rocks are significant sources of chromium (Cr) in soils.
• Understanding chromium speciation and mobility is crucial for assessing soil toxicity and environmental risk.

Purpose of the Study:

• Investigate chromium sources and availability in soils formed on ultramafic rocks.
• Determine the factors influencing chromium speciation and concentration in soil solutions.
• Assess the potential for chromium toxicity to crops.

Main Methods:

• Mineralogical analysis of soil samples.
• Selective chemical extractions to determine Cr lability.
• Field collection of soil solutions using ceramic cups and lysimeters.
• Colorimetric determination of Cr(VI) and ICP-AES for total Cr.
• Speciation modeling using MINEQL+ V 4.5 software.

Main Results:

• Cr primarily sourced from Cr-substituted goethite and chromite.
• High concentrations of extractable Cr (up to 90 mg kg(-1)) were found, with low KCl-extractable Cr indicating Cr(VI) dominance.
• Soil solution Cr concentrations were generally low (<20 microg l(-1)) under natural vegetation due to Fe-oxide retention.
• Higher Cr concentrations were observed in aerated colluvial soils with high Mn-oxides, facilitating Cr(III) oxidation to Cr(VI).
• Significant Cr increases (up to 700 microg l(-1)) occurred in phosphorus-fertilized fields due to Cr(VI)-phosphate exchange.

Conclusions:

• Soils on ultramafic rocks can contain substantial amounts of bioavailable chromium.
• Soil aeration, Mn-oxide content, and phosphorus fertilization critically influence chromium mobility and speciation.
• Elevated soil solution chromium, particularly Cr(VI), poses a significant risk of crop toxicity, especially under P-fertilization.