Internal energy ratios as ecological indicators for description of the phytoremediation process on a manganese tailing site

Abstract

A plant community is an open thermodynamic system. In accordance with the first and second laws of thermodynamics, the total internal energy U of a plant community before reaching equilibrium will increase under natural conditions because of solar energy intake and nutrient uptake. As state functions of a thermodynamic system, enthalpy H, entropy S and Gibbs free energy G are related to U by G + TS = H = U + PV. Increase in U will lead to increases in H, S and G since given volume V and temperature T or pressure P, dG + TdS = dH = dU > 0. The experimental data collected from the plant communities restored on a manganese tailing site confirmed the potential trend of increase in H, S and G of the plant communities with increase in their biomass quantity and biodiversity. The traditional criterion “ΔG < 0” would then be no longer valid as a spontaneity index for description of the ecological process of a growing and developing plant community. Since G/H = 1-T(S/H), given T, G/H will be negatively related to S/H. The number of plant species N is an essential property of a plant community, and as a function of N, the S/H ratio is expected to increase with increase in species richness under natural conditions, resulting in d(S/H) ≥ 0 and d(G/H) ≤ 0. The change in G/H can thus be applied as a thermodynamic parameter to judge the spontaneity of plant community growth and development. The results obtained from the present study showed that both G/H and S/H ratios were useful indicators for description of the ecological restoration process.