模拟条件下土壤硝化作用及硝化微生物对不同水分梯度的响应

RESPONSE OF NITRIFICATION AND NITRIFIER TO CHANGE IN SOIL MOISTURE CONTENT UNDER SIMULATED CONDITIONS

以江苏滨海县一植稻土壤为研究对象,在微宇宙培养条件下设置了不同水分处理（最大持水量的30%、60%、90%和淹水2 cm深）,研究了硝化作用及硝化微生物对水分变化的响应特征。结果表明：淹水处理显著降低了土壤的氧化还原电位（Eh）,但所有处理土壤Eh变化范围为330~500 m V,土壤整体处于氧化态。在每7天向土壤加入10 mg kg-1NH＋4-N的连续培养过程中,各个水分处理均观察到明显的NH＋4-N降低和NO-3-N累积的现象,60%WHC处理下土壤硝态氮累积最显著和迅速,90%WHC处理次之,随培养时间延长,30%WHC和淹水处理也观察到明显的硝化作用。淹水处理中氨氧化细菌（Ammonia-oxidizing bacteria,AOB）的数量显著高于非淹水处理,且淹水处理中AOB在DGGE图谱上的条带更加清晰明亮,而氨氧化古菌（Ammonia-oxidizing archaea,AOA）的群落组成和数量在不同水分处理间无明显变化。表明该土壤中AOB对水分条件变化响应灵敏,是该土壤的硝化作用、尤其是淹水条件下硝化作用发生的主要原因。

As rice is one of the most important crops in the world, paddy ecosystems have attracted worldwide atten- tion because of their importance in food supply and their eco-functions as wetland. In paddy fields, frequent alternation of wetting and drying required in rice production leads to drastic fluctuation of soil redox potential, thus affecting various mi- crobe-driven aerobic and anaerobic biogeochemical element recycling processes in the soil, including respiration nitrifica- tion and denitrification etc. Since the 1980s, excessive application of chemical fertilizers in various paddy field system management practices in China has brought about a series of environmental consequences, and most of the relevant re-search efforts in the country have been focused on emission of N2O, nitrification and denitrification, nevertheless, little is known about the underlying microbiological mechanism. Nitrification is an aerobic microbe catalyzed aerobic process, con- sisting of two steps, i.e. ammonia oxidation and nitrite oxidation. Ammonia oxidation, the rate-limiting step, is mainly catalyzed by ammonia-oxidizing bacteria （AOB） and recently discovered ammonia-oxidizing archaea （AOA）. It is gener- ally held that soil moisture ranging between 50% and 70% of water holding capacity （WHC） is the most suitable moisture condition for nitrification, and soils, too dry or too wet, are unfavorable to nitrification. Compared with upland soil, paddy soil is subjected to frequent alternation of drying and wetting, which leads to drastic fluctuation of soil redox potential, thus affecting microbe-driven biogeochemical element recycling processes and metabolic activity and functional succession of soil microbes. Being the major factors affecting soil redox potential, moisture and oxygen theretbre become the key fac- tors in the study on nitrogen recycling in paddy soil. To investigate responses of nitrification and nitrifying microbes to change in soil moisture and their underlying micro- biological mechanisms, soil sample was collected from a paddy field of alkaline paddy soil, typical of Binhai County, Jian- gsu Province, and soil microcosm experiments were set up with 4 moisture gradients including 30% WHC （ Treatment 1 ） , 60% WHC （ Treatment 2 ） , 90% WHC （ Treatment 3 ） , and 2 cm of overlaying water （ Treatment 4）. Results show that soil Eh was significantly lowered in Treatment 4, and did not differ much between the other three treatments. It varied from 330 to 500 mV, in all the treatments, indicating that on the whole the soils in all the treatments remained in oxidation state that satisfied nitrification basically even under flooded conditions. During the 60-day incubation period, 10 mg kg ＇ NH~ -N was added into soil once every 7 days. Apparent decrease in concentration of NH4＋ -N was observed in all the mois- ture treatments over time, accompanying rapid accumulation of NO3-N, which was the most obvious and rapid in Treat- ment 2 （60% WHC） and then in Treatment 3 （90% WHC）. Even though accumulation of NO3-N was slow in Treatment 1 and Treatment 4, active nitrification was also observed with the incubation going on. Compared with Treatments 1 , 2 and 3, Treatment 4 was much higher in abundance of ammonia oxidizing bacteria （AOB）. The dominant AOB bands in DGGE profiles were more distinct and brighter in Treatment 4 than in the others, and the number of AOB bands in the DGGE pro- files increased significantly in the late incubation period, that is, from 30 days to 60 days, while AOA （ ammonia oxidizing archaea） did not differ much either in abundance or in community structure between the 4 treatments. In addition, AOA dropped sharply in abundance in all the treatments after 15 days of incubation, which may be attributed to the inhibitive effect of continuous nitrogen addition on AOA. All the above described findings indicate that different ammonia oxidizing microbes have different requirements for water and oxygen, and the AOB in this type of soil, being very sensitive to chan- ges in soil moisture condition, are responsible for the nitrification in the tested soil, especially, in waterlogging conditions.