| The Minqin desert-oasis ecotone is a typical transitional zone with unique natural geographical features in China's arid regions.The distributed large area of Nitraria tangutorum nebkhas plays an important role in maintaining the ecological environment,reducing sandstorm damage and stabilizing the oasis environment of Minqin oasis.Soil respiration is a major component of the carbon cycle of terrestrial ecosystems and is a critical process in the interaction between regional climate change and the global carbon cycle.At present,the nebkhas of Nitraria tangutorum distributing in the ecotone of Minqin oasis are seriously degraded,which will inevitably lead to the change of ecosystem carbon cycle through the change of shrub vegetation distribution and pattern of character.The vulnerability of vegetation and its habitat in the ecotone of desert-oasis and its extreme sensitivity to water make the study of the mechanism of soil carbon release and the impact on the regional ecosystem carbon cycle more prominent and urgent.Therefore,quantifying the spatial pattern and studying on the characteristics of soil respiration in different developmental stages of Nitraria tangutorum nebkhas are comprehensive and urgent steps forward to understand the function of the regional carbon cycle.Based on field investigation,four typical developmental stages of Nitraria tangutorum nebkhas including embryonic stage,developing stage,stable stage and activating stage were been selected as samples to observe in the Minqin desert-oasis ecotone.From the spatial pattern and vegetation characteristics of the Nitraria tangutorum nebkhas,soil respiration of nebkhas at different time scales?day,month,year?were monitored to study the spatial heterogeneity of soil respiration in the developmental stages of Nitraria tangutorum nebkhas,then to explore the response characteristics and differences of soil respiration at hydrothermal conditions of developing process,finally,to assess the carbon budget of the Nitraria tangutorum nebkhas by using the methods of simulation and inversion.The main results and conclusions of this study included:?1?The correlation between vegetation and morphological characteristics during the different developmental stages of nebkhas was elucidated,and the main research indicators for judging nebkhas at different developmental stages were extracted from many indicators.The overall growth status and vegetation coverage of the nebkhas gradually proceeded with different stages,which was increased firstly and then decreased.The status of vegetation growth was the best in the developing stage,the coverage of vegetation nebkhas was the highest in the stable stage,but both of those were the worst in the activating stage?P<0.01?.The correlation between vegetation and morphological characteristics changed with the different developmental stages.The correlation coefficient between the horizontal scale and the height of the nebkhas increased with the process of development stages.Four main indicators affecting the developmental stages of Nitraria tangutorum nebkhas were extracted by using cluster analysis method,such as:nebkhas volume?V?,vegetation coverage?C?,deadwood rate?DR?and sand crust?CT?.These indicators were consistent with the actual observation and the distribution characteristics of Nitraria tangutorum nebkhas,the cumulative proporation explained of those items can reach more than 86%,which would be used to further determine the developmental stage of nebkhas.?2?Clarified the spatial pattern of the developing process of Nitraria tangutorum nebkhas;and revealed the mechanism of maintaining its ecological pattern.There was not shown obviously aggregation phenomenon in each developmental stage of nebkhas.The overall performance showed a uniform distribution on the small scale<10m and a random distribution on the medium or larger scales>10m or>30m.The correlation between small scale and large scale nebkhas was increased in the negative relationship or not associated with each other,indicating that the composition and structure of the Nitraria tangutorum nebkhas community would more unstable with the development of the developmental stages.?3?The difference between the average soil respiration value of 8:00?11:00 am during the growing season?May-October?and the value of 9:00?11:00 am during the dormant period?November-April of next year?of the Nitraria tangutorum nebkhas were better represent the average value of the whole day,which of the relative difference percentage was between 2.14%to 8.09%.Although there was a certain relative difference,but its differences offset each other on the average value of long time scales.The soil respiration rate of the Nitraria tangutorum nebkhas had obvious spatial and temporal heterogeneity:On time scale,the daily change of the soil respiration rate of Nitraria tangutorum nebkhas were mainly represented"one-peak curve"at different growing period;while,on space scale,there was spatial heterogeneity of soil respiration rate at different positions on the nebkhas,whereas there was a significant difference?P<0.01?at the different label positions.The average soil respiration rate shifted back and forth along the sides of the nebkhas main axis.The CV of soil respiration in the nebkhas showed a trend opposite to the soil respiration rate on different time scales:The CV of soil respiration was the lowest in summer with an average of 9.01%and was the highest with an average of25.63%in winter.The difference of soil respiration CV in each developmental stage was obvious.?4?Elucidated the soil respiration characteristics of Nitraria tangutorum nebkhas and its response mechanism to related ecological factors.The soil respiration rate of the Nitraria tangutorum nebkhas showed a single-peak curve during the inter-annual year.The soil respiration rate reached to the highest in July and August,lowest in winter.The mean values of soil respiration rate were significantly different among different phases of nebkhas,which ranked from high to low was stable stage 0.48?mol·m-2·s-1>developing stage 0.43?mol·m-2·s-1>activating stage 0.31?mol·m-2·s-1>embryonic stage 0.28?mol·m-2·s-1.The relationship between soil respiration rate and water content of 0?20 cm soil depth was closely related.The contribution rate of water content to respiratory rate in each stage was higher than50%,while,the related effects gradually enhanced from the embryonic stage to the activation stage,indicating that the respiratory rate was most dependent on soil moisture during the activating stage.?5?Revealed the relationship between vegetation growth status and soil respiration of Nitraria tangutorum nebkhas.Soil respiration,vegetation cover,aboveground biomass,and root biomass of soil depth 0?20cm showed a significant positive correlation?P<0.01?,especially the contribution of underground roots of 0?20cm to soil respiration reached to 47%.The average soil respiration Q10 value of Nitraria tangutorum nebkhas fluctuated between 1?3,in which the stable stage was the highest 2.7 and the activating stage was the lowest 1.1,basal respiration of the developmental stages fluctuated between 0.08?1.Three factors included vegetation cover,soil moisture and temperature had been established through the stepwise regression model.The optimal regression equation indicated that the effect of soil respiration of nebkhas caused by the result of the vegetation growth status and the surface water content and temperature.?6?Estimated annual carbon emissions from individual nebkhas to the desert-oasis ecotone ecosystem during the whole year using the accumulative method,such as:stable stage16.78 Kg·C·ha-1,developing stage 14.49 Kg·C·ha-1,activating stage 15.16 Kg·C·ha-1,and the embryonic stage 6.58 Kg·C·ha-1. |
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