Contents Of Active Organic Carbon, Nitrogen And Its Relationship With Nitrogen Mineralization Of Soil On The Loess Plateau

The microbial biomass carbon or nitrogen (BC, BN), and soluble organic carbon or nitrogen (SOC, SON) is defined as the active organic carbon, nitrogen in soil. BC and BN are the nutrients which are fixed in microbial biomass; and they are the impetus for nutrient transformations and cycles of soil nutrients, and they relate with the supplying and storing of soil nutrients. The SOC and SON are organic molecular compounds which can dissolve in water, acid and alkali solutions, and can be filtered through 0.45μm sieve pore. They play an important role in substance transformations and erengy metabolisms in both land and water ecosystems. As most of the nitrogen in soils is in organic form, the nitrogen mineralization is one of the key processes which reflect the capability of nitrogen supplying in soil ; and it is also one of the hotspots in the field of soil ecologies all over the world. In past few years, many of the researches had focused on nitrogen mineralization, or nutrients in soil microbial biomass, or soluble organic nutrient. Information about the relationship between nitrogen mineralization and soil active organic carbon, nitrogen, especially the microbial biomass carbon, nitrogen, is limited.In this study, the chemical and biochemical analysis methods and the nitrogen mineralization incubation methods were used to determine the contents of soil microbial biomass carbon, nitrogen, and SOC and SON in different soils from the south part of the Loess Plateau, and to evaluate the mineralization of nitrogen from the soils; and the relationships between BC, BN, SOC, and SON and the nitrogen mineralization of the were also estimated. The main results showed as follows:1.Contents of the soil SOC, SON, BC, and BN in forest soils of Isohumisols on the loess plateau were from 41.64 mg/kg to 74.72 mg/kg(average 59.30 mg/kg), 2.55 mg/kg to 7.55 mg/kg(average 4.97 mg/kg),168.72 mg/kg to 364.26 mg/kg(average 239.1 mg/kg), 31.89 mg/kg to 92.01 mg/kg(average 54.94 mg/kg), respectively. The contents of the SOC, SON, BC, BN in forest soils were higher than those in the farmland soils. There was an increased trend of the contents of SOC and SON with the ages of the trees. But we could not find significant change of BC and BN with the ages of the trees. The ratio of BC and BN to the soil organic carbon and total soil nitrogen were significantly higher than that of SOC and SON to soil organic carbon and total soil nitrogen.2. Under aerobic incubation condition, the amounts of N mineralization (AMN) from forest soils of Isohumisols on the loess plateau with different ages of trees during day0 to day14, day15 to day28 and day0 to 28 incubation were 6.78～15.69 mg/kg,3.11～5.29 mg/kg and 9.89～20.98 mg/kg, respectively; the mean amounts of mineralized nitrogen was 8.72 mg/kg,4.22 mg/kg and 12.94 mg/kg, respectively. Under anaerobic incubation condition, the AMN was from 1.01 to 5.46 mg/kg, with average of 2.97 mg/kg. Under both aerobic and anaerobic incubation conditions, the increasing trend of the AMN with the ages of trees was not obvious.3. Under aerobic incubation condition, from day 0 to day 14, the mean AMN in soils of Argosols, Anthrosols and Isohumisols were 24.44,16.83 and 14.31 mg/kg, respectively; from day 15 to day 28, which was 12.91,11.65 and 10.92 mg/kg, respectively; from day 0 to day 28, which was 37.35, 28.48 and 25.23 mg/kg, respectively. The order of the mean AMN was Argosols>Anthrosols>Isohumisols for the each incubation period. Under anaerobic incubation condition, the order of the mean AMN was Anthrosols> Argosols>Isohumisols. Under aerobic incubation condition, from day 0 to day 14, day 15 to day 28, and day 0 to day 28, the AMN in forest top soils of Isohumisols was higher than that of farmland soil. The average AMN in forest top soils from day 0 to day 14 was 2.3 times as high as that in farmland soils; 1.8 times from day 15 to day 28; and 2 times from day 0 to day 28. Under aerobic incubation condition, from day 0 to day 28, there was the most significant positive relationship between the AMN and the mineralization rate in forest and farmland soils of Isohumisols. Under anaerobic incubation condition within 7d, the average AMN in forest soils and in farmland soils was 11.16 mg/kg and 2.34 mg/kg, respectively. There was the most significant difference of AMN between forest soils and farmland soils.4. Compared the nitrogen mineralization under aerobic and anaerobic incubation conditions, we found that there was a significant positive relationship between the AMN within 0～14, 0～28 d under aerobic incubation condition and within 7d under anaerobic incubation condition, the coefficients was 0.920 and 0.930, respectively; but the AMN under aerobic incubation condition was higher than that under anaerobic incubation condition for the same soil.5. There was a significant positive relationship between the SOC, BC and the AMN after 28 d under aerobic incubation; there was a significant positive relationship between BN and the AMN after 28 d under aerobic incubation condition, while the relationship between SON and the AMN after 28 d under aerobic incubation condition did not reach significant level.