Submitted to: International Journal of Phytoremediation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 15, 2005
Publication Date: August 1, 2005
Citation: Zhang, L., Angle, J.S., Delorme, T.A., Chaney, R.L. 2003. Degradation of alyssum murale biomass in soil. International Journal of Phytoremediation. 7(3):169-176.
Interpretive Summary: One of the hypotheses regarding why hyperaccumulator plant species have evolved this property of hyperaccumulation is prevention of other plant species competition for the space where the hyperaccumulator grows. This phenomenon is called 'allelopathy', and has been observed to result from particular organic compounds present in leaves or roots which restrict growth of other species. In the present test, we grew Alyssum murale on high Ni or low Ni soils, then incorporated the biomass from both high and low Ni biomass into the surface of high Ni serpentine and low Ni Maryland soil. The Ni from the biomass was immediately extractable by water or a neutral salt solution which has been shown to extract levels of Ni proportional to Ni levels accumulated by crop plants, hence a test for plant available metals. With increasing time after mixing with soil, the extractable Ni declined rapidly to near background levels. Rapid reaction of Ni with soils is known to occur when soluble salts of Ni are mixed with soils. The forms of Ni in Alyssum murale leaves has been shown to be Ni-malate, Ni-malonate, Ni-citrate and Ni sulfate, all of which could allow the Ni to be adsorbed rapidly by soil Fe and Mn oxides or humic materials. These results suggest that reaction of leaf Ni with soil strongly limited the potential for allelopathy to result due to the leaf Ni reaching the soil surface. Thus, if leaf Ni is capable of causing allelopathy, it would take many years of leaf fall to achieve inhibition of growth of non-tolerance plant species on the soil surrounding Alyssum murale plants.
The Ni-hyperaccumulating plant Alyssum murale accumulates exceptionally high concentrations of nickel in its aboveground biomass. The reason for hyperaccumulation remains unproven, however it has been proposed that elemental allelopathy might be important. High Ni leaves shed by the plant may create a 'toxic zone' around the plant where germination of competing plants is inhibited. The efficacy of this argument will partially depend upon the rate at which leaves degrade in soil and free metals are released, and the subsequent rate at which metals are bound to soil constituents. To test this hypothesis, A. murale was grown on both high and low Ni soils to achieve high (11,957 g Ni/kg) and low Ni biomass (445 g Ni/kg). Shredded leaf and stem biomass were added to a serpentine soil from Oregon that was originally used to grow high Ni biomass and a low Ni control soil from Maryland. Biomass Ni was readily extractable suggesting near immediate release. Extractable nickel in soil declined rapidly due to Ni binding in soil. These results suggest that Ni released from biomass of Ni hyperaccumulators may affect its immediate niche, but only after multiple years of biomass addition to the surface of soil.