Studies On Nitrogen Transformations And Special Microbial Characteristics In The Temperate Semiarid Steppe

In the semiarid Leymus chinensis steppe of northern China, large quantity of nutrients were exported from the ecosystems because of irrational land uses, which also resulted in grassland degradation, soil impoverishment and lower productivity. Both fertilization and root cutting are the important management methods and contribute greatly to improve and maintain the nutrient balance of grassland ecosystem. With the development of molecular biology, it provides a new approach for evaluating soil nitrogen cycling by researching the function of the main functional groups of soil microorganisms in the process of soil nitrogen transformation at gene level. In this study, effects of fertilization and root cutting on soil nitrogen forms and transformations were studied in the Saibei Management District, Hebei province from 2014 to 2015. The main research contents include(1) influence of fertilization and root cutting on abundance of nitrogen transformation functional genes and community structure of ammonia oxidizing microorganisms,(2) approaches and mechanism of soil nitrogen transformation affected by grassland management measures, and(3) assessing the impact of management measures on Leymus chinensis grassland by determining the variations of soil respiration, enzymatic activity, soil physical and chemical properties and vegetation characteristics. The major results are as following:1. Management methods changed the transformation of soil nitrogen. Both nitrogen fertilizer application and root cutting increased soil nitrification and mineralization dramatically between May and July. Soil ammonium nitrogen content in the plots(N2P, N1P) applying nitrogenous fertilizer mixed with phosphate fertilizer was lower than that(N2, N1, P) applying nitrogenous fertilizer only significantly. Higher nitrogen fertilizer rate(N2, N2P) resulted in increasing of soil nitrate nitrogen content when compared with other treatments(N1, N1 P, P, WC). It will increase the risk of environmental pollution because of the nitrate leaching loss.2. Abundance of the key microbial functional genes focus on soil nitrogen transformation was influenced by grassland management methods. AOB-amo A and nif H genes copies were both sensitive to fertilization and their copies were increased significantly(p<0.05) as nitrogenous and phosphate fertilizer were applied mixed together. Root cutting had no effects on the copies of AOB-amo A and nir K but the copies of AOA-amo A were enhanced by 53.91% and that of nif H were reduced by 43.99%. The partial correlation analysis between genes copies and soil factors showed that AOA-amo A gene is mainly affected by soil p H(r=0.74, p=0.155) and total nitrogen content(r=0.70, p=0.170). Factors influenced AOB-amo A genes were variety, such as cumulative mineralization(r=0.94, p<0.05), total nitrogen content(r=0.96, p<0.01), nitrate nitrogen content(r=-0.92, p<0.05), C:N(r=-0.88, p<0.05), N:P(r=-0.95, p<0.05), water content(r=-0.93, p<0.05), nitrogen application rate(r=-0.89, p<0.05) and phosphorus application rate(r=-0.91, p<0.05). The copies of nif H gene were influenced largely by cumulative mineralization(r=0.76, p=0.072) and total phosphorus content(r=-0.92, p<0.05), but nir K gene’s copies were affected by cumulative nitrification(r=0.88, p<0.05) and organic matter content(r=-0.92, p<0.05).3. Community structures of soil ammonia oxidizing archaea(AOA) and bacteria(AOB) were alerted by grassland management methods. Crenarchaeota and proteobacteria were dominant in AOA and AOB community respectively. Root cutting impacted ammonia oxidizing microorganisms greatly, this enhanced AOA richness index obviously but reduced proteobacteria’s richness index. Nitrogen fertilizer application heightened soil AOB richness significantly(p<0.05). The RDA analysis between the community structure and soil factors showed that AOA community was mainly affected by p H(p=0.044), phosphorus fertilizer rate and total phosphorus content, but AOB community was influenced largely by organic matter content(p=0.004) and nitrogen fertilizer rate(p=0.038).4. Grassland vegetation characteristics influenced by management measures dramatically. Frequency of Gramineae was increased by 43.1% after fertilizer application, but Root cutting resulted in decreasing of Gramineae frequency by 43.8%(p<0.05). Phosphorus fertilizer application raised the frequency of Artemisia eriopoda Bunge and higher nitrogen application lowered it significantly(p<0.05), but root cutting also lowered it by 24.2%. Leymus chinensis’ s frequency was diminished by phosphorus application but not influenced by nitrogen application and root cutting. Nitrogen application heightened the sward height significantly(p<0.05) but phosphorus fertilizer application contributed less to sward height. Underground biomass of grassland in shallow soil was decreased by more than 30% in the study area after mixed application of higher level nitrogen and phosphorus fertilizer. Forb biomass was enhanced significantly by the nitrogen fertilizer application(p<0.05), but biomass of Artemisia eriopoda Bunge and Leymus chinensis were not influenced by fertilizer. The nitrogen content of the grasses were increased by nitrogen fertilizer application but not affected by phosphorus fertilizer applization and root cutting. Phosphorus content of Artemisia eriopoda Bunge and Leymus chinensis were enhanced when grassland had been applied with nitrogen and phosphorus fertilizer mixed. Phosphorus content of Gramineae and forbs were not affected by root cutting. Higher nitrogen fertilizer application enhanced the grass nitrogen content. The ratio of N to P of Gramineae, Artemisia eriopoda Bunge and Leymus chinensis was increased by applied mixed nitrogen and phosphorus together.5. Soil respiration, enzyme activities and physical and chemical properties were affected by grassland management methods. Nitrogen fertilizer application enhanced soil total nitrogen content up to 20.9% in the grassland. Soil total phosphorus content was increased by applied nitrogen and phosphorus fertilizer together but not influenced by phosphorus fertilizer application. Fertilizer and Root cutting had not effects on soil p H, SOM and EC. During the growing season, soil respiration showed decrease slightly after an increase and tended to peak in September and the lowest in October. Higher nitrogen fertilizer application increased soil respiration and the maximum monthly CO2 emission was 108.01 g CO2/m2 and the minimum one was 44.50 g CO2/m2 observed in October. Soil temperature was the key factor influenced soil respiration rate and both of them had significant positive correlation relationship(R2=0.5943, p=0.031). Root cutting enhanced activities of soil catalase, alkaline phosphatase, urease and invertase significantly, but higher nitrogen fertilizer application resulted in the decrease of soil urease activity.