| The process of N transformation in the soil mostly comprise of mineralization, immobilization and nitrification. All the transforms had a closer connection with each other, then made the N transformation become a more coherent system. The mineralization and immobilization could make a significant change to the substrate of the nitrification. Nitrification in acid soil is a hot topic in the research of N transformation because the mechanism was still unknown. As a main contributor of the nitrification, soil microbe got the ability to use the N to complete the nitrification, and the activity of the microbe could reflect the strength of nitrification. And the way microbe to assimilation the N substrate are different in autotrophic and heterotrophic microorganism. Many factors could effect the nitrification in the soil, such as soil pH, roots of the plant, water content, the concentration of oxygen, the temperature of soil and so on. Many of them influence the nitrification in two ways, the first is to change the activity of the microbe, and the second one is to change the concentration of the N substrate. The pH of the soil and the concentration of N substrate are two main factors to effect the nitrification in soil, but they can not explain the spatial variability of the nitrification in acid soil in justified way. Soil organic colloid, inorganic colloid and oxide colloid constituted the soil surface area, and they have significant impacted on the distributed of microbe in soil and the element transformation for their colloidal properties of the surface.15N isotope tracer method is an important method to the research of N transformation process, after decades of development, many scientists have create some model to calculate the rate of the N transformation. With the15N label to the soil, different incubate time points was taken to measure the abundance of15N in soil, then using the model the N transformation in the soil tested can be calculated.This paper focus on the studies of the performance impact of the soil surface to the nitrification in acidic soil. We chosen a representative typical acidic yellow soil and slightly acidic purple soil as samples, with adding5%humic soil and5%hematite, the content of organic matter and mineral of the soil was changed. Three treatments have been set, the control, sample with5%humic soil and5%hematite, and this experiments using the method of15N double-subject. At the beginning of the incubate, solution of15N H4NO3and NH415NO3were add into the soil studied, then adjusting water content to60%field water capacity, incubating at25℃for a week at last. We chose the0.5hour,1day,3days,5days,7days as the sampling time. At each time we tested the concentration of soil nitrate and ammonium and abundances of15N. By means of15N isotope tracer, using15N transformation model to simulate the study of soil N transformation rate. This study focus on the relationship between nitrification, mineralization and immobilization. Changes in the three transformations were discussed to explain the mechanism of nitrification in acid soil. The results showed us:We found a big difference in N transformation rates between different soil types. Purple soil (pH=5.8) pH was higher than the yellow soil(pH=4.2). A higher pH environment was more suitable for autotrophic nitrification, which carried out purple soil autotrophic nitrification rate was seven times higher than the yellow soil. With the increase of C/N ration, a significantly lower rates of N mineralization was found. The rates of mineralization of labile organic N to NH4+and mineralization of recalcitrant organic N to NH4+, yellow soil was2.3and1.3times higher than the purple soil, respectively.There are relationship between different N transformation, microbial in acidic soil may through the way to make use of the immobilization NH4+to increase the substrate of nitrification for itself, Soil autotrophic nitrification and the immobilization of NH4+to organic N was significant positive correlation (R2=0.921, P<0.01) in the soil studied. Heterotrophic microorganisms could use organic N substrates as the substrate for nitrification, with the increase of mineralization of labile organic N to NH4+, the heterotrophic nitrification rates continue to reduce (R2=0.883, P<0.01).Effects of humic substrate on N transformation process was studied. On the one hand, in the increase of the concentration of substrate heterotrophic nitrification, thereby enhancing heterotrophic nitrification rate; on the other hand, soil organic matter increased the C/N ratio, which could influence the mineralization of organic N.5%hematite changes the iron mineral content of the soil as well as some important physical and chemical properties, and affects the soil in N transformation rates further. Iron oxide colloids at lower pH conditions could have a positive charge, this property reflecting the weakening of the adsorption of NH4+, and enhancement adsorbed of NO3-, in conjunction with the action of microorganisms, specifically reflecting microbial immobilization of NH4+in the yellow soil and purple soil were weakened2.5and9.5 times, respectively; and the immobilization of NO3-was much higher than the beginning in two types of soil. Hematite through the poison of iron on the microbial activities, may effect of nitrification rate in the soil. |
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