| Under the background of global climate change,precipitation pattern,snow cover thickness and climate warming change the soil moisture,freeze-thaw cycles and freeze-thaw intensity in the freeze-thaw process.These freeze-thaw conditions change the stability of soil aggregates,nutrient availability and microbial activity,which involve the key processes of carbon and nitrogen turnover.However,how different environmental factors affect the greenhouse gas flux,dissolved organic matter content,composition and bioavailability of soil during freeze-thaw process is still unclear.In this thesis,the surface soil of typical broad-leaved Korean pine mixed mature forest(BKPF)and secondary birch forest(WBF)in Changbai Mountains of Northeast China was studied.Based on long-term field observation,indoor freeze-thaw test of soil column was constructed.Determination of net soil gas flux,soil active carbon and nitrogen index,dissolved organic matter(DOM)content in leachate,components and fluorescence parameters under freeze-thaw disturbance.The effects of freeze-thaw intensity(-5/5oC and-10/10 oC,0/0oC as control,12h/12h)and freeze-thaw cycles(1,7,14,21)and their interactions on net greenhouse gas flux and DOM fluorescence components,relative contribution rates and their characteristics of leachate in forest soils with different humidities(30%,60%,90% WFPS)were studied.The influence mechanism was analyzed by three-dimensional fluorescence spectrum-parallel factor analysis and the change of soil active carbon and nitrogen index.The main results of this study are as follows: Soil greenhouse gas flux under freeze-thaw disturbance was significantly affected by freeze-thaw intensity and soil moisture(P<0.01),and its influence degree changed with the number of freeze-thaw cycles.Humidity is the main factor affecting soil N2 O flux.The N2 O flux and accumulative flux increased significantly with the increase of soil moisture(P<0.05),N2 O emission of the same soil moisture level increases with the increase of freeze-thaw intensity(P<0.05).Freeze-thaw intensity is the main factor affecting CO2 flux.The CO2 flux and cumulative flux increase with the increase of freeze-thaw intensity at three soil moisture levels(P<0.01).CH4 fluxes were different due to forest stand,soil moisture and freeze-thaw intensity.Contrast showed methane absorption.Except for medium humidity WBF soil freeze-thaw treatment,other freeze-thaw treatments showed methane emission.During freeze-thaw process,the content of soil active carbon and nitrogen was significantly affected by freeze-thaw intensity and humidity,and there were differences between stands and freeze-thaw cycles.The content of DOC and ammonium nitrogen extracted by water was the highest under high humidity,and its increased significantly under high intensity freeze-thaw(P<0.01).Nitrate nitrogen content was affected by soil moisture and freeze-thaw intensity because of differences in stand,microbial carbon and nitrogen content was not significantly affected by freeze-thaw intensity,and soil moisture affected by differences in stand.Microbial carbon and nitrogen content decreased first and then increased with the increase of freeze-thaw cycles.The activities of soil enzymes in the two stands were significantly affected by freeze-thaw intensity(P<0.05),which mainly showed that high-intensity freeze-thaw significantly reduced the activities of BG and NAG enzymes.The correlation analysis showed that N2 O and CO2 emission fluxes were significantly correlated with DOC and microbial nitrogen content in water extraction under freeze-thaw disturbance(P<0.05).CH4 flux was significantly correlated with different nitrogen and enzyme activities(P<0.05).The accumulative fluxes of three gases are significantly correlated with ammoniation and mineralization of nitrogen,and these relationships change with the change of soil moisture,freeze-thaw intensity,freeze-thaw cycles.DOC:TN in soil environment was negatively correlated with microbial C/N ratio(P<0.01).Soil moisture and freeze-thaw cycle times significantly affect microbial C/N ratio(P<0.01),and there are significant differences in microbial metabolic entropy under different freezing-thawing conditions(P<0.01).In conclusion,different soil moisture and freeze-thaw intensities significantly affect the forms and states of soil active nutrients,and the changes of these conditions profoundly affect the metabolic degree and types of soil microorganisms,leading to changes in gas flux.The DOM content of soil leachate in the two stands was lower under moderate humidity(P<0.05),and was significantly affected by high intensity freeze-thaw(P<0.05).The DOM content increased first and then decreased with the increase of freeze-thaw cycles.Three fluorescence components identified in the DOM included:humic acid-like DOM,fulvic acid-like DOM,and protein-like DOM.the DOM of WBF soil leachates was dominated by humic acid-like substances(37.56%)and the DOM of fulvic acids and humic acids was highly disturbed by freeze-thaw and had low stability.The DOM of BKPF soil leachates is mainly consisted of fulvic acid-like substances(41.28%)with a high degree of humus which is less disturbed by freeze-thaw.The redundancy analysis showed that under the experimental conditions,the soil DOM characteristics were largely determined by the forest stand,and that the concentrations of DOC and its three fluorescence components in the WBF soil appeared to be greater than those in the BKPF soil.Soil moisture significantly affects DOM aromaticity(P<0.001),DOM aromaticity of soil leachates in two stands showed a trend of medium humidity>high humidity>and low humidity;with the increase of freeze-thaw strength,the aromaticity of BKPF soil leachates DOM decreased significantly(P<0.05);the repeated freeze-thaw cycles significantly increased the degree of DOM humification in the soil leachates(P<0.05).The degree of soil humification is related to the form and proportion of DOM in soil.Under freeze-thaw disturbance the higher the aromatization,the more difficult the DOM in soil with lower DOM content of protein is to be leached.From the results,it can be concluded that following different freeze-thaw disturbances,there was an increase in the concentrations and bio-availability of DOM released into temperate forest soils with low moisture,probably causing DOM leaching during spring freeze-thaw periods particularly under WBF stand.Freeze-thaw intensity and freeze-thaw cycles significantly affect net gas flux,soil active carbon and nitrogen content and DOM characteristics of leachate at different humidity levels,and there are some differences between the two stands.Under freeze-thaw disturbance,microbial nitrogen content and dissolved organic matter content mainly affect soil N2 O and CO2 flux,nitrate nitrogen content and BG enzyme activity affect CH4 net flux.Soil moisture changes soil aeration and affects microbial activity and nutrient migration and transformation.The freeze-thaw intensity further aggravates the difference of soil water and heat.The number of freeze-thaw cycles changed soil microbial structure and increased soil humification.The freeze-thaw process changes the migration and transformation of DOM in soil by affecting the form and state of nitrogen in soil,and then affecting the structure and metabolism of soil microorganisms.The experimental results would provide a basis for in-depth studying the changes in the concentrations,components and stability of DOM released into temperate forest soils during spring freeze-thaw periods in the field. |
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