Distribution Of Soil Microbial Diversity In Different Size Of Aggregates During The Process Of Revegetation On Southern Area Of Ningxia

Unreasonable land use and management has led to serious soil degradation on the Loess Plateau, which is a threat to the land resources of the people survival. Therefore, improving soil quality and speeding up the vegetation restoration has become an important goal of ecological restoration of the Loess Plateau. This study selects natural revegetation (degraded Stipa bungeana Trin, Cymbopogon citratus, Thymus mongolicus, Artemisia sacrorum, Stipa bungeana Trin and Stipa grandis P. Smirn) and artificial revegetation (for contrast with Crop land, Natural grassland, 15-year-old Caragana korshinskii Kom and 25-year-old Caragana korshinskii Kom) on Ningxia Loess Plateau as the research objects , and analyses the soil physical properties, nutrients, microbial biomass C, N, P, soil microbial structural diversity (phospholipid fatty acid), soil enzyme activity (alkaline phosphatase, urease and invertase) , in order to explore microbial diversity changes in different soil aggregates fractions on revegetation. The main results are as follows:(1) It's shown that under natural revegetation and artificial revegetation, the percentage of soil aggregates fractions is the highest in 0.25-1mm, the least in 2-3 mm of soil aggregates and with the size of soil aggregates increase. The percentage of soil aggregates fractions are increased in 20-40cm soil layer than in 0-20cm soil layer, and significantly higher in the 3 ~ 5mm and> 5mm soil aggregates than in 0-20cm soil layer. In addition, soil bulk density showed a decreasing trend.(2) Organic carbon, total nitrogen and AV-K are lowest in crop land, and AV-P in 0-20cm soil layer higher than natural grassland and 15 and 25 years Caragana korshinskii Kom, but lower in 20-40cm soil layer. Ammonium nitrogen is higher in Caragana korshinskii Kom 0-20cm soil laye than in crop land and natural grassland, and nitrate nitrogen is higher in crop land. Microbial biomass and enzyme activity are the lowest in crop land. With the revegetation years increase, organic carbon, total nitrogen and AV-K has significantly increased, while AV-P has decreased and ammonium nitrogen and nitrate nitrogen keep stable.(3) Degraded Stipa bungeana Trin community shows the worst cumulation of soil nutrients under natural revegetation area. Soil microbial shows the worst growth and reproduction in Stipa bungeana Trin and Thymus mongolicus communities. Soil alkaline phosphatase and invertase are higher in Artemisia sacrorum, Thymus mongolicus, Stipa grandis P. Smirn, Stipa bungeana Trin communities than Cymbopogon citratus and Stipa bungeana Trin community. Soil urease is lower in Artemisia sacrorum and Thymus mongolicus community than other communities(4) The analysis of different size aggregates PLFA under natural revegetation and artificial revegetation shows that the differences of microbial diversity mainly lie in Gram-negative bacteria and protozoa. Crop land and degeretation Stipa bungeana Trin communities have the lowest PLFA species, and compared with the most, they lack of 16:1ω7, i17: 0, a17: 0,17:1ω8c, i18: 0, 20:4ω6 and so on, and 16:1ω7 is characterized the Gram-negative bacteria, 20:4ω6 is characterized the protozoan. Fatty acids 9Me 14:0, a14: 0, 16:1ω9, 16:0, a16: 0,18:1ω9c, 18:1ω7 in all samples has the highest levels, and is the dominant fatty acids. In terms of total phospholipid fatty acids, the bacteria has the most percentage in total phospholipid fatty acids, followed by actinomycetes and fungi, and protozoa, the least. Under other samples except Thymus mongolicus community and natural grassland, gram-negative bacteria has the largest percentage of total PLFA in different size of aggregates, followed by aerobic bacteria and Gram-positive bacteria, methane oxidizing bacteria and anaerobic bacteria the lowest. And in Thymus mongolicus community, the Gram-negative bacteria change to Gram-positive bacteria, that is to say, the chemoautotrophism microbial change to chemoheterotrophy microbial in this soil.(5) Under the same communities or the same artificial revegetation, total PLFA is shown higher intermediately and lower in both sides with the size of aggregates (<0.25 mm,0.25-1 mm,1-2 mm,2-3 mm,3-5 mm and >5 mm) increase, and has the highest in 1-2 mm and 2-3 mm aggregates, but decreases in 3-5 mm and >5 mm. The percentage of bacterial in total PLFA shows the similar trend, but the highest is different in different layers. Actinomycetes and fungi shows the similar with the bacterial in different size of aggregates under different artificial revegetation. Protozoa shows the lowest content, but generally appears in1-2mm, 2-3mm and 3-5mm aggregates through some soils, and the percentage of protozoa generally higher in 1-2mm aggregate than other aggregates.(6)According to some physiologecal indicators of PLFA, the Stipa grandis P. Smirn and Thymus mongolicus communities contain the most microbial diversity and information, and the input of organic carbon volume is large. Crop land and Stipa bungeana Trin communities contain the lowest. Compared with 0-20cm soil layer under every sample, in 20-40cm soil layer there is a changing that the chemoautotrophism microbial change to chemoheterotrophy microbial. There is the same changing in <0.25mm, 0.25-1mm, 1-2mm aggregates under the same samples. The stress is lower in 2-3mm and 3-5mm aggregates than other size aggregates. In addition, through the principal component analysis of phospholipid fatty acids in different size aggregates, and the comprehensive scores , it can be seen that the highest score generally appears in 1-2mm, 2-3mm and 3-5mm aggregates, and the lowest is in <0.25mm and> 5mm aggregates. The results of cluster analysis may be different under different plant community and artificial revegetation.