Regulatory Mechanisms Of Key Factors On N₂O Production And Reduction In Typical Farmland Soils

Soil is one of the most important source of N₂O emission.Nitrification and denitrification are two main pathways for soil N₂O emission.The specific contributions they made vary from different soil conditions.Soil net N₂O emission equals to N₂O gross production minus consumption.Due to the high N₂ background concentration in atmosphere,it is an ongoing challenge to directly measure the de-nitrogen emission from soil.Most of the current researches related to soil N₂O emission focused on only the apparent N₂O fluxes but not on the N₂O production and consumption processes because of the lack of effective method for N₂ determination.This thesis used a new method named He-sealed-direct N₂ measurement to investigate the effects of daily freeze-thaw period,N fertilization and soil pH,affecting the north-China plain soil and southern paddy soil N₂O emission.Our brief conclusions are as follow:1)Nitrogen fertilization not only changed the substrate concentration that result in the N₂O production,but also affected the N₂O consumption.We investigated how different fertilization levels affected the soil N₂O consumption by using the soil sampled from where has been researched for field experiments under different levels of fertilization in Luancheng experimental station.In this study,we observed:compared to the samples that didn't get fertilized,constant fertilization(400 kg N ha^-1yr^-1 for 15 years)significantly increased the nitrate concentration in surface soil,enhanced the N₂O emission rate and N₂O/?N₂+N₂O?ratio.Further analysis indicated that soil N₂O reductase activity has a negative correlation to soil nitrate concentration,while N₂O/?N₂+N₂O?ratio has a positive correlation to soil nitrate concentration.Those results confirmed that,on the one hand,excessive fertilization increased the soil N₂O production;on the other hand,it could enhance soil nitrate content and inhibit soil N₂O reductase activity,there would be more nitrate transformed into N₂O resulted from limited N₂O reduction process.Thus we should keep our eyes on how to control the low nitrate concentration in soil surface in typical North-China plain ecosystem in order to reduce the N₂O emission.2)Soil pH is another emission factor affecting soil N₂O production and N₂O consumption.Previous research results indicated:soil pH value has a negative correlation to soil N₂O/?N₂+N₂O?product ratio,soil acidification could promote the mineral nitrogen convert into N₂O.There are few studies have researched the specific microbiological mechanisms under this process.Our study investigated two kinds of soil,high natural pH soil?maize-wheat rotation agro-ecosystems of North China Plain,pH7.91?and low natural pH soil?rice-wheat rotation agro-ecosystems of Taihu Lake regions,pH 5.67?,we found that there are a significant negative correlation between the soil pH and N₂O emission in two kinds of croplands.This pattern can also be seen in soil pH and N₂O/?N₂+N₂O?product ratio.The N₂O-reducing bacteria has postive link to the N₂O/?N₂+N₂O?product ratio in Taihu soil,while the North-China plain doesn't have any correlation.Therefore,we conclude that soil pH plays a crucial role in soil N₂O production and consumption.Compared with high natural pH soil,the nosZ gene abundance in southern paddy soil are easier affected by pH variation.3)North-China Plain is located in temperate area.Daily freeze-thaw conditions could happened frequently in that area during winter.Most studies focused on the effects of seasonal freeze-thaw period,while researchers have seldom concentrated on the daily freeze-thaw period.We have conducted several incubation experiments by simulating daily freeze-thaw process,and we found:daily freeze-thaw would destroy soil aggregate and enhance soil substrates availabilities.This change elevates the soil N₂O emission as twice as the control treatment at 0.1632nmol g^-1 h^-1.This kind of intensive temperature disturbance could change the soil microorganism structure.There are two denitrifier,Bacillus and Pseudomonas,has been increased after the frozen treatment.After 8hours of the melt process has been initiated,the N₂O production rate under 50%water content treatment was obviously higher than another moisture condition.We didn't observe that there is any significant difference in N₂O emission under the various nitrate addition treatments(0-24?g N g^-1).These results indicate:daily freeze-thaw period has the same significant positive influence on the nitrous oxide emission and N₂O/?N₂+N₂O?ratio as seasonal freeze-thaw period.According to our results mentioned above,soil nitrate concentration,pH value and daily freeze-thaw period play crucial roles in cropland soil N₂O emissions and reduction.Appropriate nitrogen fertilization could make sure the nitrate concentration in surface soil remains at a low level.It can not only reduce the nitrogen loss resulted from denitrification,but also ensure N₂O emission contributes less to total nitrogen loss in soil.Excessive nitrogen fertilization would induce the soil acidification and affect N₂O reduction.Daily freeze-thaw cycle has the similar impact on increasing N₂O emission from soil as seasonal freeze-thaw period.It is a necessary condition should be considered in assesing annual N₂O fluxes of soil ecosystem.