Nitrogen Immobilization By Soil Microbial Biomass And Its Control In Different Soils

Soil microbial biomass (SMB) has been defined as a part of the organic matter in soil that constitutes living microorganisms smaller than 5000μm3. It includes bacteria, fungi, actinomyces, algae, protozoa and so on. It is an active and easily changeable 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 contents of SMB in different soils under the various ecosystems depend on climate, vegetation, soil types and cultivation practices. The content of SMB can indicate the changes of soil fertility and cultivation. Soil microorganism is very important in adjusting nitrogen mineralization and immobilization in soils. Plant growing needs the continuous mineralization of nitrogen from soil in view of plant nutrition; on the other hand, it is necessary to increase nitrogen immobilization in soil to improve soil fertility and ecological environment. In recent years, many researchers have studied the effects of different soil types, land use, and application of different fertilizers on the contents of SMB in different soils. However, these studies mainly focused on the contents of soil microbial biomass carbon and nitrogen (BC, BN) in different soil types, or the nitrogen absorption by crop and nitrogen loss after applied nitrogen. But the study on N immobilization and transformation in different treated soil after applied different forms of nitrogen is less.Organic manure combined chemical fertilizer is the effective way to improve crop yield and soil fertility. Fallow or natural leisure is not only a traditional mode to recovery soil fertility but also one method to improve environment in fragile ecological environment area. Those measures could affect the Soil microbe's quantities and activity undoubtedly, but the effect to Soil nitrogen immobilization need to study. So in this study, we took soil samples from a 17-year long-term fertilization experiment located in the southern part of the Loess Plateau and compared the transformation of exogenous NO₃^- or NH₄⁺ of long-term fertilization or Fallow soil during the incubation. The main results showed as follows:1. The treatments added manure and nitrogen, phosphorus and potassium fertilizers (MNPK) or fallow significantly increased the contents of organic matter and total N in the soils; and the N mineralization quantity and rate of these treatments were also significantly increased. The treatment of nitrogen, phosphorus and potassium fertilizers (NPK) increased inorganic N in the soils significantly, but had less effect on contents of organic matter, total N, and characteristics of nitrogen mineralization of the soils.2. Soils sterilizing with autoclaved method did not significantly increased the content of NO3- in soils, but significantly increased the content of NH₄⁺ in soils, the increases was in range of 9.14 to 19.36 mg/kg; and the increase rate of different treatments was in the following orders, MNPK >NPK >fallow and no fertilizer treatment. And the content of NH4+ in soils during sterilization incubation after sterilizing was also increased significantly; and the increase rate of NH4+ in soils during sterilization incubation had a significant relationship with the content of organic matter in the soils.3. Although the contents of organic matter and soluble organic carbon in the soils were different, there were no significant differences in N retention by biotic and abiotic factors in the soil when nitrate form of nitrogen was added. The content of NO3- in the same soil with and without sterilization during incubation remained relatively constant, indicating abiotic and biological factors had no obviously effects on NO3- transformation in the condition of this incubation.4. When ammonium was added into the soils, N immobilization by biotic factors occurred since the 3 days after the incubation. At the end of incubation, about 41% of the NH4+-N added was immobilized by soil microbes, and about 23% of NH4+-N added was fixed or lost by abiotic factors. The long-term fertilization and fallow had no significant effects on N retention by abiotic factors; however, the long-term addition of chemical and organic fertilizers and fallow significantly increased the N retention by biotic factor in comparison with the treatments without fertilizer or only addition of chemical fertilizers.5. when wheat straw, which was difficult to decompose, was used as organic carbon source, the nitrogen immobilized by soil microbial biomass was lower in compared with using glucose as organic carbon; and the immobilization of ammonium by microbial biomass was much higher than that of nitrate. When glucose was supplied as energy source, the immobilization of the two forms of nitrogen (ammonium and nitrate) was significantly increased; and there were no significant differences in nitrogen immobilization for the two forms of nitrogen. It needs to study the effects of different organic carbon sources in adjusting the nitrogen immobilization and release in soil to control nitrogen supply, and reduce its losses.