| Soil degradation is a major threat to sustainable use of soil ecosystem, because it decreases actual and potential level of vegetation cover. Local land use and land-cover change can play a pivotal role in environmental and ecological changes and furthermore contributes to global change. Consequently, improving the physical, chemical and microbial properties of degraded soil is very important for sustainable use of soil ecosystem. The results showed:1. After abandonment of agricultural land, with vegetation succession plant species diversity and evenness increased gradually, and tended to be the highest contents under 10-yr fallow. after that they decreased, whereas plant species richness increased gradually, and above-ground biomass was higher under 3-yr fallow than that of under 5-yr fallow, after 5-yr fallow above-ground biomass increased gradually.2. With vegetation succession organic carbon (OC), total nitrogen (TN), ratio of organic C/ total P (C/P), ratio of total N/total P (N/P), microbial biomass carbon (MBC), N (MBN), total FAMEs, fungal biomass, ratio of fungal to bacterial biomass, actinomycetes biomass and Gram-negative bacterial biomass in the topsoil (0-10cm) increased gradually, whereas there were no trends in metabolic quotient, MBC/OC and MBN/TN. Soil organic C and total N in the topsoil showed a positive correlation with microbial biomass C, N, and total FAMEs, suggesting that there was close relationship between soil fertility and microbial biomass, that total FAMEs and microbial biomass were potentially good indices of soil fertility in study area. The chemical and microbial properties measured in soil were significantly higher in the virgin vegetation than those of arable land, with various fallows being intermediate. Organic C and TN in the 10-30cm at various fallows were higher compared with arable land, while the accumulation of microbial biomass C, N mainly occurred in the topsoil. Therefore, the results showed that soil fertility may be improved, while total P may become the limiting factor relative to C and N with vegetation succession after abandoned arable land.3. With soil depths, the OC, TN, MBC, MBN, MBC/OC, MBN/TN, total FAME, and Gram-negative bacterial biomass readily decreased, metabolic quotient, actinomycetes biomass and Gram-positive bacterial biomass gradually increased except for arable land, 3-yr fallow and virgin vegetation facing east which were higher in the 10-30cm than other depths, suggesting that microbes inhabiting the deeper soil horizons were more carbon limited than surface-dwelling microbes.4. Plant species diversity and richness index showed a positive relationship with C/N, fungal biomass, ratio of fungal/bacterial biomass. Above-ground biomass showed a positive relationship with organic C, total N, C/N, total FAMEs and fungal biomass in soil, a negative relationship with metabolic quotient, no significantrelationship with microbial biomass C, N. Therefore, the results showed that that plant above-ground biomass affected the active microbial biomass, but not microbial C, N, that C/N in soil increased readily with plant species diversity, that microbial communities changed and fungi became gradually the dominated species of microbial community with succession.5. The effects of fertilization combined with crop rotation on the chemical properties and microbial biomass was investigated in the central Loess Plateau, China. In order to create a more uniform experimental environment and avoid the influence of different crop residues on effects of fertilization, wheat/potato (W/P) rotation was selected in fertilization treatment. The results showed that fertilization could increase organic carbon, total nitrogen, MBC and MBN content compared to the control. It was concluded that appropriate land management such as fertilization combined with crop rotation would be helpful to improve or maintain soil fertility.6. Microbial biomass, organic carbon, total nitrogen, and microbial quotient (MBC/Corg) in soil were determined during the secondary forest succession in north Ziwulin region in the middle of Loess Plateau. The results showed that with secondary forest succession organic carbon (Corg), total nitrogen (TN), microbial biomass carbon (MBC) and N (MBN) in soil increased quickly, and tended to be the highest contents under SF17 (17-year secondary forest), after that they decreased and gradually remained at a quite constant level, suggesting that accumulations of organic C, total N, MBC and MBN in soil occurred mainly at the early succession stages (before SF17). Soil microbial biomass was markedly correlated with the organic carbon and total nitrogen content of soil (p<0.01), Furthermore, microbial quotient showed significant correlation not only with MBC and organic C, but also with succession duration during the secondary forest succession. Therefore, the results suggested that changes in microbial biomass in soil were related not only to the quantity, but also to the quality of soil organic matter during secondary forest succession, and that changes in above-ground plant species during succession were critical to improve degradation soil physical, chemical and microbial properties in north Ziwulin of Loess Plateau.
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