Thermodynamic predictions for the effects of root exudates on metal speciation in the rhizosphere

Abstract

Root exudates are comprised of many organic and amino acids which are capable of complexing micronutrient ions such as Fe, Cu and Zn. If present in high enough concentrations, root exudates may have a dramatic affect on the speciation of the soil solution. Our objectives were to estimate the concentrations of root exudates in the rhizosphere, and determine how they would alter the soluble fraction of Fe, Cu and Zn in hypothetical rhizosphere soil solutions. Root and exudate data of two plant species (Pinus radiata and Hordeum vulgare) were collected from previous literature and three cases were used to calculate exudate concentrations in the soil solution. Root exudate data of Pinus radiata included both amino and organic acids while Hordeum vulgare only included amino acids. Calculated soil solution concentrations of root exudates were used as input into the equilibrium computer program, GEOCHEM. When we confined exudate concentrations to the rhizosphere (1 mm from root) and assumed a 99 percent microbial utilization rate, we found that total soluble Fe(III) and Cu(II) was increased by several orders of magnitude over the pH range of 4 to 8, due to the presence of root exudates from Pinus radiata. Total soluble Zn(II) was also higher in the presence of exudates at pH values above 6. The difference in the amount of total soluble metal between plant species was mainly attributed to the presence of organic acids. We concluded that root exudates are important in increasing the soluble fraction of Fe(III), Cu(II), and Zn(II) in the rhizo‐sphere. We also found that the estimated oxalate concentrations in the rhizosphere were high enough to cause precipitation of calcium oxalate, possibly an important Ca solid phase in the rhizosphere.

Key words:

• Fe(III)
• Cu(II)
• Zn(II)
• complexation
• amino acids
• organic acids
• oxalate

Notes

Journal Article No. 4854, Oklahoma Agric. Exp. Stn., Stillwater, OK 74078.