| Phosphorus (P) is an essential nutrient and also a crucial limiting nutrient factor for plant growth in terrestrial ecosystem, especially in hilly red soil of mid-subtropical region. Vegetation restoration of degraded red soil is the essential model for the afforestation of difficult sites and ecological environment construction in South China, whose effect on soil phosphorus cycle and regulation mechanism remains unknown. More than 95% of total P in soil exists in slow acting forms, and only a small available portion of total P takes part in the biological cycle. Therefore, in order to understand the basic rules of soil P transformation, it is quite important to study P availability and its influence factors which can also serve as the guidance for vegetation restoration and forest management.Three types of undisturbed and homogeneous landform forests, namely, Pinus elleottii forest, Liquidambar formosana forest, P. elliotti-L. formosana forest and naturally restored grassland (control) were randomly selected from the long-term reforestation experimental sites in Jiangxi Province. Three plots with the size of 20m×20m squares were set in each vegetation type. Using field investigation and laboratory analysis as research methods, the key indexes of soil quality evaluation, such as physical, chemical and biological properties of the forest rhizosphere and non-rhizosphere soil, and nutrient content of fresh plant leaves and litter were investigated. The purposes of this study are to clarify the differences in physical, chemical and biological properties of soils under different vegetation types and different soil depths in hilly red soil of subtropical region; to demonstrate the influence of plant rhizosphere on chemical property of soil; to analyze the physical, chemical and biological regulation mechanism of phosphorus available in hilly red soil of subtropical region; and to evaluate the advantages and disadvantages in artificial afforestation in hilly red soil of mid-subtropical region.The main research results were as follows:1. Vegetation restoration in degraded hilly red soil region has greatly changed soil P availability. In contrast to naturally restored grassland (control), the available P, microbial biomass P and organic P in 19-year-old forest soil were improved significantly, and surface gathering was found from each P. Soil available P and microbial biomass P in Liquidambar formosana forest and P. elliotti-L. formosana forest were significantly greater than those in Pinus elleottii forest. Meantime O-P, Ca-P and total P in planting sites were significantly lower than those in naturally restored grassland, but an opposite trend was noticed for Al-P and Fe-P. In addition, microbial biomass P and available P in rhizosphere soil of plantations were significantly greater than that in non-rhizosphere soil. Soil available P was positively correlated with Al-P, Fe-P, organic P and microbial soil biomass P, suggesting that soil organic P, microbial biomass P, Al-P, Fe-P were potential resources of soil available P in forest soil of red soil region. Revegetation in degraded red soil would be helpful in improving accommodating soil activated P, especially in increasing the integration of Al-P and Fe-P. Better improvement effect was achieved in the broad-leaved forest and broad-leaved and conniferous mixed forest than in coniferous forest.2. The physical and chemical properties of soil are key regulating factors of P availability. The physical property, pH, total N, total K, available K were not significantly changed. Hydrotic total acidity, organic C,ammoniac N and C:N values in Liquidambar formosana forest and P.elliotti-L.formosana forest were significantly higher than those in control, and surface gathering was also noticed from them all. Soil available P was positively correlated with hydrotic total acidity, organic C, total N, ammoniac N, nitrate N and stoichiometric ratio. All these indicate that the improvement of soil P availability was intensely correlated with soil C and N.3. The biological property of soil is a key regulating factor of P availability. The acid -phosphatase activity of soil, bacteria, inorganic phosphate-solubilizing microbes and organic phosphate-solubilizing microbes in Liquidambar formosana forest and P. elliotti-L. formosana forest were significantly higher than those in Pinus elleottii forest and control in 19-year-old forest soil, and surface gathering was present on all of them. Meanwhile, total P content in fresh leaves and litters of Liquidambar formosana were separately higher than that in Pinus elleottii forest, suggesting that quantity and quality of litters were critical factors for activity and abundance of microbes. Soil available P content was positively correlated with soil acid-phosphatase activity, quantity of bacteria, inorganic phosphate-solubilizing microbes and organic phosphate-solubilizing microbes and total P content in fresh leaves and litters. It was proved that the biological properties such as forest soil microbes, plant property and so on were key regulating factors of P availability.In summary, supply capacity of soil P in hilly red soil of vegetation restoration was significantly improved. Tree types and its dispose modes were extrinsic factors in variability of soil P activity while chemical and biological properties of soil are intrinsic factors for soil P supplement. Overall, soil P activity in planting sites was higher than that in control, and broad-leaved forest and broad-leaved and conniferous mixed forest were better than coniferous forest. Organic C, activity of N, acid-phosphatase activity of soil, abundance of bacteria, inorganic phosphate-solubilizing microbes and organic phosphate-solubilizing microbes were major biological and chemical factors in regulating soil P transformation. It can be inferred that some management such as increasing complexity of stand structure, reserving understory vegetation and forbidding litter collection were effective means to improve the supply capacity of phosphorus in planting sites of degraded red soil. |
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