Bological Nitrogen Fixation And Ecological Effects Of Nodule And Non-nodule Pioneer Plants In Hailuogou Glacier Retreating Area

The pioneer plants,known as the"polar pioneers",are dominant in the community of the early primary succession before the glacier retreat area,and have important application value in ecological protection and vegetation restoration.The Hailuogou Glacier retreat area,located at the eastern edge of the Qinghai-tibet plateau,has developed for nearly 130 years,forming a complete plant succession sequence from pioneer plants to the top community,and provides an ideal place to study plant primary succession and the function of pioneer plants.In this study,the nitrogen fixation capacity of noduled and non-noduled plants dominanting the community in the beginning of the primary succession at the Hailuogou Glacier retreat area,was monitored monthly(May to November)in situ to explore the contribution of these two taypes of plants to nitrogen input on ecosystems level at different succession stages.Moreover,temperature,precipitation,plant nutrient content and the community structure of nitrogen-fixing bacteria in plants were analyzed,so as to explore the factors affecting the nitrogen fixation rate.In addition,through analyzing the nutrient content,stoichiometry and the rhizosphere soil properties of different pioneer plants,to explore the influence of plant nitrogen fixation on vegetation succession and soil development.This study aims to promote the understanding of the ecological functions of pioneer plants,the processes and rules of the primary succession and to provide theoretical basis for remediation of degraded and damaged ecosystems.The main results of this study are as follows:(1)The leaf,twig,and root of the non-noduled plants(Salix rehderiana,Salix ernesti,)and noduled plants(Astragalus,Hippophae,Populus),living in the early primary succession at the Hailuogou Glacier retreat area,had the ability of nitrogen fixation.The main nitrogen-fixing organ of noduled plants was nodules.The nitrogen fixation rate of the root of leguminous Astragalus was between 0.15-6.21 mg N g^-1 d^-1and the root of Actinomycorrhizoma plant Hippophae was between 0.13-5.36 mg N g^-1 d^-1.The main nitrogen-fixing organ of non-noduled plants was leaf and their nitrogen fixation rate was relatively low,between 0.01-0.38 mg N g^-1 d^-1.(2)The nitrogen fixation rate in the growing season was highest in the 29yr-and39yr stage,reaching 2.8-3.5 kg N ya^-1 ha^-1,and was lower at the 4-9yr stage and 61yr stage of the succession.Input from noduled nitrogen fixation was high in the 4-9yr stage of succession while declined through succession,and non-noduled nitrogen fixation was dominant in the 29-61yr stage.Due to the large amount of inputs from the noduled nitrogen fixation in the early stage of succession,the nitrogen cycle has largely recuperated by 29 years after the glacier retreated.Our study provided a important insights into the sources and dynamics of nitrogen in primary succession and enriches the theoretical framework of forest ecological restoration.(3)Temperature and precipitation had a significant effect on the nitrogen fixation at the 4-9yr stage(P<0.01)and the effect was weaked at the 55-61yr stage;Plant nitrogen fixation rate was not related to the nutrient contents of soil,but closely related to the nutrient contents of plant(P<0.01).Therefore,temperature,precipitation,nutrient content of plant tissues and structure and composition of endophytes had important effects on nitrogen fixation.(4)A large number of nitrogen-fixing microorganisms were detected in both non-noduled and noduled pioneer plant tissues,but their structure was affected by plant species and tissue types.Rhizobium and Frankia were dominant in the root of Astragalus and Hippophae,respectively.Bacillus,Bradyrhizobium,Azospirillum and Frankia were popular among non-noduled plants.(5)Plant nutrient content and utilization were effect by species and types of pioneer plants.The concentration of nitrogen and phosphorus in the leaves of noduled plants was significantly higher than that of non-noduled plants.Due to the relatively high nitrogen fixation rate,noduled plants had significantly higher nitrogen and phosphorus concentration than non-noduled plants(P<0.01),and they had low nitrogen nutrient reabsorption utilization rate and high phosphorus nutrient reabsorption utilization rate.However,non-noduled plants adopt a relatively conservative strategy in nitrogen nutrient utilization,which made them to keep the rapid growth in the nutrient-poor environment.With the accumulation of nitrogen in soil,the nitrogen nutrient reabsorption utilization rate decreased with the succession.The foliar N:P ratio,between14-16,indicates that in the early stage of succession(0-39 years),the plants were not limited by N or P,but there might be limited by P in the 61-yr stages.Our results confirm that plant nitrogen fixation can overcome the limitation of N in early succession,and emphasize the important role of noduled-plants in the regulation of C,N and P stoichiometry of vegetation.(6)This study showed that plants could promote the soil development through their roots,but it was affected by the characteristics of vegetation.The rhizosphere soil of noduled plants Astragalus and non-noduled plants Salix rehderiana had higher enzyme activity and organic acid content and the facilitative effects on soil development was more significant;The facilitative effects of noduled plants Hippophae were no different with non-noduled plants Salix ernesti,Populus.Moreover,soil enzyme activity was significantly related to microbial biomass and nutrient content,indicating that soil enzyme activity as an indicator of soil biological activity and nutrient content.