Ecological environment is fragile in water-wind erosion region where soil erosion is
very serious. And this region is the main source of the coarse sediments of the Yellow River.
Sound vegetation restoration measures are of importance to the vegetation restoration of
degraded ecosystem in the study area. Vegetation-soil systems in different habitats were
studied using the field survey and laboratory experiment. In detail, characteristics of standing
vegetation and soil seed bank during vegetation restoration were identified. The effects of
vegetation restoration on soil quality and water and photosynthesis physio-ecological
adaptability were clarified. These results are helpful to reveal the interactional mechanism
between vegetation and soil quality and provide theoretical basis for vegetation restoration
and reconstruction in water-wind erosion region on the Loess Plateau. The main conclusions
are as follows:
(1) Species composition and species diversity of communities during vegetation
restoration were clarified. Standing vegetation was dominated by Compositae, Gramineae and
Leguminocae species, which accounted for 76% of total species numbers. For life forms,
perennial hebaceous plants were dominant, which occupied 64% of total species numbers.
Vegetation succession process was classified three stages.The dominant species were annuals
of Artemisia scoparia and Salsola ruthenica in the.first stage; Astragalus melitoloides in the
second stage; Stipa bungeana in the three stages (after 15a). And the role of Stipa bungeana as
constructive species remanined unchanged with vegetation succession after 15a. There was no
obvious tendency in richness index. Shannon-wiener and evenness index increased, while
ecological dominance declined with increased years. The results indicated that species
composition of community becomed even and stable along with vegetation succession. (2) Characteristic of soil seed bank and its effect on vegetation restoration were studied.
The results showed that density of soil seed bank ranged from 105 to 6 301 seeds m-2 with an
average of 2 095±448 seeds m-2. Soil seed bank had vertical distribution pattern that seed
density at 0-5 cm depth (accounting for 72%) was greater than that at 5-10 cm depth. The
seed bank was mainly composed of early successional species such as Artemisia scoparia and
Eragrostis pilosa and their dentities accounted for 72% of total density (41.37% and 20.81%,
respectively). Species of the later successional stages (i.e. target species) were absent.
Similarity between seed bank and standing vegetation was low in the whole. Their Sorensen
similarity coefficient ranges from 0 to 0.667. The tendency of diversity with increased years
was consistent with the standing vegetation. The results suggested that seed bank plays an
important role on vegetation in the early succession stage, while contributes little in the later
succession stages. Potential for vegetation restoration from soil seed banks is limited and it is
recommended to employ morderate human intervention to accelerate succession.
(3) Impact of vegetation succession of abandoned croplands on soil quality was
clarified.The results showed that content of >0.25 mm soil water stable aggregate and MWD
increased, while destruction rate of >0.25 mm soil water stable aggregate decreased with
increased years, suggesting that soil structure stability increase during vegetation restoration.
Soil water content at 0-200 cm and 200-500 cm depth showed the trend of a decrease at the
inital stage but then an increase. Soil water content distribution in the 500 cm soil profile
showed a great fluctuation. Soil organic matter, total N, available N and available P
demonstrated a trend of a drease at the initial stage but then an increase, i.e. partially “U”
shape. They decreased with increasing depth in the soil profile. The trend of quantity of fungi,
bacteria and actinomyces with increased years was similar to the soil chemical properties. Soil
microflora in abandoned croplands was maily composed of bacteria accounting for more than
70% followed by actinomyces and fungi. The trends of soil invertase, urease and alkaline
phosphates with increased years were consistent with the soil chemical properties. But soil
catalase had no obvious trend with them.
(4) Soil quality factors of different vegetation types were compared. The results showed
that soil structure and stability of cropland and Caragana korshinskii land were better than
other types. The mean soil water contents of different vegetation types were less than 10% at
0-500cm depth. They were ranked as follows: cropland (10.53%) > Caragana korshinskii land (9.46%) > abandoned cropland (8.31%) > Pinus tabulaeformis forestland (8.21%) >
Populus simonii forestland (4.50%) > Artemisia desertorum land (3.97%) ≈ Robinia
pseudoacacia forestland (3.95%). The soil nutrients, enzyme activities and microorganisms of
Robinia pseudoacacia forestland and cropland were higher, while those of Caragana
korshinskii land and Populus simonii forestland were lower in the whole.
(5) A Minimum Data Set (MDS) for soil quality assessment was established by the
methods of sensitivity analysis, principal components analysis and correlation analysis.
Effects of succession stages and different vegetation types on soil quality were quantitatively
evaluated based on integrated indictors and MDS. The results showed that MDS included soil
organic matter content, available phosphorus, invertase activity and fungi. Established MDS
could reflect the information of integrated indicators and results of assessment were of better
representative. The trend of integrated fertility index (IFI) changes was partially “U” shape,
namely, showing a decrease in 15 years following abandonment, but an increase after 15 years.
Considering different vegetation types, the values of IFI ranked in the order: Robinia
pseudoacacia forestland>cropland>abandoned farmland>Pinus tabulaeformis
forestland>Artemisia desertorum land>Caragana korshinskii land>Populus simonii forestland.
This reflected human activity could improve soil quality to some extent. Conclusively, soil
quality level in the study area was low.
(6) Plant water physio-ecological adaptability was analyzed. The results show that the
values of leaf osmotic potential (ψs) ranged from -1.9 MPa in Sonchus oleraceus to -4.3 MPa
in Stipa bungeana in the 40-year abandoned croplands; 70% of all species fell in the range
from -4.0 MPa to -3.0 MPa. Considering life forms, annual herbs had significantly higher ψs
than trees, shrubs and perennial herbs (P<0.05), while ψs differed little for trees, shrubs and
perennial herbs. The trend of ψs variation among lifeforms was shrubs (-3.483MPa)
perennial woody plants could have a superior capacity to extract water by root system from
dry soil environment than annual herbs. Leaf osmotic potentials and water contents
demonstrated a significant tendency to decrease as years passed from the leaving the
croplands, especially in Stipa bungeana, Lespedeza davurica, and Astragalus melitoloides.
The results implied that species with low ψs in the later succession stages would have
substantial advantages to survive the xeric habits. And plant species had evolved adaptive plasticity in response to varying environmental conditions during vegetation succession.
(7) Plant photosynthesis physio-ecological adaptability was analyzed. The results
showed that large inter-species differences existed in all habitats, probably reflecting the
adaptation of species to the environment and strong influence of their genetic nature. At the
species level, the maximum photosynthesis rate (Pmax), photosynthetic nitrogen use efficiency
(PNUE), and specific leaf area (SLA) showed significant decreasing trends as time elapsed
since abandonment, while water use efficiency (WUE) and nitrogen content (Nmass) were not
significantly correlated to years elapsed since abandonment. At the community level, the
trends were consistent with those at the species level except for SLA. Among the plant species
in the studied area, Pmax, while not correlated with SLA, was significantly positively
correlated with the WUE、PNUE and Nmass（P<0.05); the PNUE, while not correlated with
Nmass, was significantly positively correlated with the WUE and SLA（P<0.001); and the Nmass
was significantly positively related to SLA (P<0.001). Compared with other studies in other
regions, the species in the study area had lower Pmax、PNUE and SLA. Those lower values
may be a result of the plant species adapting to infertile soil and harsh habitats of the study