| Soil microbial biomass(SMB)has been defined as the part of the organic matter in soil that constitutes living microorganisms smaller than 5～10μm~3. It includes bacteria, fungi, actinomyces, algae, protozoa and so on. It is an active and easily changable component. Microbial biomass has been regarded as nutrient pool with which easily changes. It can provide energy for decomposition and mineralization of organic matter. SMB plays an important role in the cycling of carbon, nitrogen, phosphorus, and sulfur. The content of SMB in soil in the different ecosystems depends on climate, vegetation, soil types and cultivation practices. The content of SMB can indicate the change of soil fertility and cultivation. Recently years, more attention has often been focused on the content of SMB in the different soil types, land use, and application of different fertilizers. However, these studies mainly focused on the content of soil microbial biomass carbon (BC) or soil microbial biomass nitrogen (BN) in different soil types. And information about the relationships between microbial biomass carbon and nitrogen, and their effect on N immobilization and mineralization in soil, and the contents of dissolved organic carbon and nitrogen in the different land use patterns, and fertilizers and the relationship between dissolved organic carbon, nitrogen and BC, BN is limited.In this study, soil samples were collected from farmlands and woodlands in different areas of the south part of Loess Plateau. The incubation methods of the in situ in field and in laboratory were carried out to study the contents of BC, BN, dissolved organic carbon and nitrogen in different soils on the Loessial Plateau; and the relationship between BC, BN and dissolved organic carbon, nitrogen under different cultivation and fertilizer treatments was also evaluted. The main results showed as follows:1. This study was conducted to determine the contents of soil microbial biomass carbon (BC), and nitrogen (BN), K2SO4-extractable organic carbon (SOC), and nitrogen (SON) in three different soil types and under two different use patterns in the south part of Loess Plateau. The results showed that the contents of BC, BN, SOC and SON in the forest soils were higher than those in the arable soils; and the concentrations of these parameters in the forest litter were significantly higher than those in the 0~20 cm soil layers. Under the different agricultural cultivation, the concentrations of BC and BN in the Anthrosols were higher than those in the Isohumisols and the Argosols. The concentration of SOC in the Argosols was higher than that in the Anthrosols and the Isohumisols. The concentration of SON in the Isohumisols was higher than that in the Anthrosols and the Argosols. Soil types had significant effect on the concentration of BN, but had no significant effect on the concentrations of BC, SOC and SON. The ratios of BC to soil organic C (BC/TC), and BN to soil total N (BN/TN) were significantly higher than the ratios of SOC to TC (SOC/TC), and SON to TN (SON/TN). There were significant positive correlations between BC, BN and SOC, SON, indicating the close relationships between BC, BN and SOC, SON.2. The incubation in situ in field showed the concentrations of BC and BN in the wheat straw mulch were higher than those in the conventional and the plastic film mulch cultivation methods when no fertilization added; the concentrations of K2SO4-extractable organic carbon (SOC), and nitrogen (SON) in the conventional cultivation method were higher than those in the wheat straw mulch and the plastic film mulch cultivation method. With N fertilizer, the concentrations of BN and mineral N in plastic film mulch increased more than those in the other cultivation patterns. The application of N fertilizer increased the concentration of BC in the conventional and wheat straw mulch treatments, and also increased the concentration of SOC in the wheat straw mulch and the plastic film mulch treatment. Compared with different soil layers, the concentrations of BC, BN, SOC, SON and mineral N in 0~5 cm soil layer were higher than those in 5~10 cm and 10~20 cm. Moreover, during the incubation in the field, the concentration of BC in the samples decreased, while the concentration of BN increased significantly. But the content of SOC slightly decreased in the wheat straw mulch and the plastic film mulch treatments.3. The incubation experiment in laboratory showed that the effects of different combination of organic and inorganic fertilizers on the contents of BC, BN and SOC followed the order of adding straw>straw combined with N fertilizer>no fertilization and N fertilizer; and the content of mineral N in different treatments was adding N fertilizer>straw combined with N fertilizer>no fertilization>straw treatment. After adding straw and straw combined with N fertilizer, the highest content of BC was found in day 3 of incubation, and BN was in day 7 of incubation. Later on, the contents of BC and BN were decreased. But only adding straw, the content of mineral N was very low. The variation of content of BN was higher when only adding N fertilizer. The soil microorganism immobilized some nitrogen at the initial stage; but the immobilized nitrogen was mineralized quickly. The content of BN in treatment with no fertilization was very low; on the contrary, the content of mineral N in this treatment was significant higher than other treatments. The application of N fertilizer had limited effect on BC and SOC contents in the soil. Therefore, we can adjust N immobilization and mineralization by SMB in soil-plant systems by adding the different ratios of organic and inorganic N fertilizers in order to balance the nitrogen supplement, and reduce nitrogen loss from soil to the environment.4. The incubation experiment in laboratory indicated that the contents of BC, BN and SOC at 35℃was lower than those at 25℃, but the content of mineral N at 35℃was higher than those at 25℃. At 35℃, the rate of nitrogen mineralization was faster at 35℃than at 25℃.
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