| 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 higherψs than trees, shrubs and perennial herbs (P<0.05), whileψs differed little for trees, shrubs and perennial herbs. The trend ofψs variation among lifeforms was shrubs (-3.483MPa) |
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