| The elements of N together with C are called the two basic life elements in the ecosystem, which have a mutual affinity and restraint each other in the nature. The ultimate goal of rational application of N fertilizer in a corn field is that the soil fertility can be improved only through the reasonable arrangement of N supply, which can improve the microbial characteristics to participate into the transformation of soil organic C, ultimately a fertile soil with some good biochemical properties is gained. The idea advocated above has a theoretical exploration value and a practical guiding significance in the current situation. In view of them, a long-term fertilization experiment, a dynamic plot experiment and a laboratory incubation experiment are all adopted to explore the effects of different amounts and forms of N on the soil microbial properties and the characteristics of organic C components.Firstly, the three selected long-term fertilization experimental fields served as our research objects, we divided them into high, medium and low fertility plots in accordance with their basic fertilities. The five N application levels were set to compare the differences among the microbial characteristics and organic C components. The conclusion was as follows:(1) The plot with high basic fertility was beneficial to the accumulation of microbial biomass N(MBN). With the increasing of N dosage, the microbial assimilation of N(MBN) was enhanced in the three tested areas. When the applied amount of urea reached 312 kg/hm2, both of the MBN from the areas with medium and low fertility had a slight drop. However, the soils with different basic fertility did not have a regular impact on their microbial biomass C(MBC). The activities of sucrase, urease and alkaline phosphatase had a positive response to the increase of N dosage. The difference among the basic fertility could affect the expression of catalase activity. Compared with the treatment of no N, the protease activity could be inhibited by a small amount of N supply, but it could be improved by the increased N application;(2) The influence of different N levels on the soil organic carbon(SOC) content form the soils with three different basic fertilities had different effects. In the area with low fertility, the increase of N application could decrease the oxidation stability of SOC, which was bad for the maintenance of fertility, However the appropriate dosage of N in the range of 0~240 kg/hm2 could enhance the content of readily oxidizable organic C(ROC) in the soil with high fertility; The N application could be beneficial to the mineralization of organic C in the light and heavy fractions from a plot with high fertility, indirectly make its SOC younger.To further clarify the dynamic response of the soil biochemical properties from different corn growth stages on the dosage of N based on the same fertility condition, the split plot experiment method was adopt to study the related contents. Results indicated that:(1) No N was advantageous to the maintenance of fertility, but the reasonable N was more beneficial to a stable output of soil fertility; The high amount of N could slightly enhance the soil total N and K contents, but the total P content had a loss in its fluctuation process;(2) The supply of N fertilizer was conducive to enhance the activities of urease and catalase, but the alkaline phosphatase activity suffered an inhibition to different degrees in this process; Without or small amounts of N could promote the transformation of soil organic N, which could be beneficial to enhance the the protease activity;(3) With the corn growth period continuing, the MBC form a corn field climbed up first and declined later, ultimately stabilized to a certain level. In general, the N application was advantageous to maintain the storage of MBC, but in this process the MBN had a decrease and an increase later, whose stability reached the best state when treated by no N;(4) A moderate amount of N was beneficial to the increase of ROC and its proportion in the TOC, on the contrary the water soluble organic C and its share in the TOC were both reduced;(5) The N range of 0~200 kg/hm2 could make soil humic acid(HA) components experience the mineralization and decomposition, and after the N dosage being enhanced to 250 kg/hm2, the HA components had a steady increase; With the increase of N dosage, the dynamic rule of C contents from the fulvic acid(FA) and humin(Hu) components performed an initial increase and slow decrease later. In this process, the FA component could been accumulated, and the inert C of Hu suffered a loss only under the condition of N reaching maximum amount(250 kg/hm2). Obviously, driven by the high N supply, the microorganisms tended to the humification; Based on the organic fertilizer, N supply could promote the formation of extracting humic acid(HE); Under the influence of 150 and 200 kg/hm2 N levels, the growth in HE provided by the newly formed FA, and the N dosage increased to 250 kg/hm2, its accumulation caused by the HA;To elucidate the mechanism and rule of the N dosage affecting the soil biochemical properties, the production environment corn removed and the indoor pure culture was adopted. The albic soil mixed with straw served as our tested object, through the adjustment of N dosage and the combination of different N forms to study the differences in the composition of organic C functional groups, microbiological characteristics and the dynamic change of organic C components. The results showed that:(1) The SOC and total N contents of mixture applied by N had a loss because of the mineralization. Although the available N, P and K contents had been enhanced at the beginning of culture(15 d), and eventually they were consumed gradually;(2) The urea could improve the urease activity significantly, especially the treatment of high N dosage, but with the decrease of remaining urea, the urease activity was gradually leveled off. At the same time, the activities of alkaline phosphatase, invertase and catalase from the mixture were enhanced first and lowered later, but the proteinase activity could be improved significantly. The moderate N supply(200 mg/kg) was beneficial to improve the activities of four kinds of enzyme mentioned above;(3) N addition could relieve the downward trend of MBC, and the reduced amount of N application was more advantageous to enhance the efficiency of microbial N fixation;(4) N addition in the range of 0~200 mg/kg could decrease the water soluble organic C(WSOC) content of mixture, but the high N dosage(400 mg/kg N) could effectively alleviate the loss of WSOC; The increase of N dosage could reduce the loss of ROC, and the stability of SOC could be controled well through a suitable reduction of N supply;(5) The stability of HE could be reached only by the appropriate dosage of N; The decreasing tendency of HA extracted from the soil amended with corn straw could be intensified after the application of N; N addition could help the mixture upwards the humification; Affected by no N or low-dose N supply, the Hu extracted from the mixture was gradually increased. However, the applied N dosage reached more than 200 mg/kg, the Hu component could be gradually decomposed;(6) The composition of soil organic C functional groups could be affected to a greater degree by the low-dose N supply. Under the effect of N application, while the change of aromatic C content was difficult to summarize, the alcohol hydroxyl(C-OH) could be oxidized and had a trend towards the carboxyl C. The N supply of 200 mg/kg was more helpful to the transformation of HA into FA.The experimental results from the different combinations of two N forms showed that:(1) The SOM and available nutrients in the mixture were gradually decreased with the culture regardless of the N forms When the NO3--N served as a dominant N form, the loss of total N was the most in mineralizing the same amount of straw, however, the NH4+-N could be more helpful to stabilize the total N level;(2) The treatments from the equal proportion and NH4+-N as the main N form could be beneficial to extend the duration of urease activity; Affected by the N application, the levels of K supply in the mixture could be enhanced;(3) The activity of alkaline phosphatase could be inhibited by the treatment that NH4+-N serving as the main N form; The equal proportion of NH4+-N and NO3--N could be more beneficial to the stability of sucrose activity and the inprovement of protease and catalase activities;(4) The microorganisms utilized the N form mainly containing NH4+-N, their MBC were more likely to be consumed, and the equal proportion of NH4+-N and NO3--N could be more conducive to the stability of microbial biomass(MBC and MBN);(5) The WSOC content in the mixture had a gradual decrease due to the supply of N, among which the effect from the equal proportion of NH4+-N and NO3--N was the most obvious; Based on the NH4+-N serving as the main N form, the consumption degree of microbes to ROC was the most, however, the equal ratio of NH4+-N and NO3--N was more conducive to the stability of ROC;(6) The equal proportion of NH4+-N and NO3--N could promote the microbial degradation of Hu component extracted from the mixture; The N supply that NH4+-N being dominant was more likely to reduce the microbial destruction of HE and promote the transformation of HA to FA component; The mineralization and decomposition of HA component extracted from the mixture was not affected by N forms, in the process, the complexity level of HA molecular was reduced;(7) The equal proportion of NH4+-N and NO3--N was beneficial to the formation of FA component, however, the HA component was utilized only a little based on the NO3--N serving as the main N form. |
