Dynamic changes in the restoration process
The total net dry biomass C T and total number of species N of the plant communities in Plots I (amended with organic manure), II (amended with chemical fertilizer) and III (uncontaminated site) obtained in the period 2010 - 2016 are given in Table 1. Basic data for plant species are presented in Appendix 1. The values of h , s, f ,s /C T andf /C T in Table 1 were calculated, respectively, by Eqs. 2-6. As a record of the restoration process history of the plant community, the data show the dynamic changes in its ecological and thermodynamic states. The effect of the applied restoration method has been partly discussed in previous papers (Wu et al., 2017; 2018). Some of the main results are briefly summarized below:
(1) The applied methods had not only enhanced the biomass growth and metal uptake of the plant community but also enriched its species composition (Table 1, Appendix 1). In the later period, bothC T and N of the plant community in Plot I (amended with organic manure) were all several folds greater than those in Plot II (amended with chemical fertilizers) and their differences were highly significant (P < 0.01). In addition to supply of nutrients and improvement of soil properties, the remediation effect of the organic manure was mainly attributed to its incorporation with plant roots, microorganism and substrates, forming an integrated system of chelation, oxidation, adsorption and absorption in reduction of the metal toxicity in the rhizosphere. The significant differences inC T and N between years 2015 and 2016 in Plot I indicated that the plant community was still undergoing fast development. The species enrichment in both Plots I and II after land preparation was attributed to the natural germination of the native metal-tolerant species from the soil seed inventory while the difference in N between plots was mainly caused by the applied fertilizers.
(2) The highest N value 66 was observed in Plot III for the plant community naturally developed under uncontaminated soil conditions (Table 1). The C T value in Plot III was much lower than that in plot I and the difference was attributed to the significant contribution of the transplanted P. fortunei andK. bipinnata in plot I. The total metal uptake quantities for all measured metals were very low in Plot III (Wu et al., 2018) and the correlation between its biomass quantity and metal uptake was insignificant (R2 < 0.20), indicating that metal toxicity was not the factor limiting the plant growth. The species number N in Plot III remained unchanged in years 2015 and 2016 and the difference in C T between these two years was insignificant (Table 1), showing that the plant community had reached a relatively steady state.
It is noted in Table 1 that the thermodynamic factors, h ,s , f and s/C T, all increased with time in both Plots I and II with f/C T as a unique exception. Unlike h , which is independent of N , boths and f are functions of C T andN . If s reaches its maximum, s =s m = C Tln(N ) (Eq. 19 in theoretical section), Eq. 6 becomes
f/C T = ln(N m) -s/C T = ln(N m) – ln(N ) (8)
showing that f/C T will decrease whiles/C T will increase with increasing N . The observed changes in f/C T ands/C T were thus expected results, indicating thatN is the key factor that governs the internal energy flow and distribution. Since the inverse relationship betweenf/C T and s/C T holds in nature, Eqs. 6 and 8 can be applied to analyze the productivity-species richness relationship.