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.