| Shrub is a dominant vegetation type in arid and semiarid areas, which play an irreplaceable role in wind prevention and sand fixation, water and soil conservation and maintain ecosystem stability in region level.One of the important ecological service function for shrub is fixed carbon function. In this study, taking the five natural dominant desert shrub ecosystems in west Ordos region of Inner Mongolia autonomous region, China as research objects and using the method combing field investigation with laboratory analysis, we studied the distribution pattern and established the best shrub biomass estimated models.Based on the carbon content of each part in five desert shrubs, the current fixed carbon amounts and seasonal dynamic were analysed. At last, we estimated carbon balance and defined the carbon source/sequestration question of five desert shrubs in west Ordos region though detemining shrub photosynthetic fixed carbon and soil carbon emission though respiration. The results will provide an important reference for carbon cycling and scientific evaluating fixed carbon function in desert ecosystem. The main conclusions as follows:(1) The differences among dry/fresh weight ratio of single shrub and the differences among parts of shrub reached a significant level (P<0.05).The differences among root-shoot ratio of five shrubs reached a significant level (P<0.05). The root-shoot ratio of five shrub species was respectively that Reaumuria songarica was 1.05, Zygophyllum xanthoxylum was 1.01, Helianthemum songaricum was 0.92, Ammopiptanthus mogolicus was 0.90, and Tetraena mongolica was 0.49.The roots and branches were main contributors to total biomass of desert shrubs and the proportion of roots and branches biomass to total biomass reached above 80%. With the increase of root diameter class, the root biomass was increasing. The R2 value of single shrub biomass models were all more than 0.80 and reached a significant level (P<0.05). The prediction accuracy of five desert shrubs’biomass models were good. The research results can provide important basic data and theoretical basics for the response of shrub productivity, carbon storage and carbon cycle to global climate changes in desert area of western China.(2) The organs average carbon contents’distribution of five desert shrubs in west Ordos region was random. The change range of each organ was in 29.78%～56.05%, but the average carbon content of aboveground part of shrubs was higher than belowground parts. The average carbon content of five desert shrubs in summer was higher than in spring and the differences reached significant level (P<0.05). We determined the weighted average carbon content of five desert shrubs in summer season: Ammopiptanthus mogolicus 44.25% and 42.31%, Zygophyllum xanthoxylum 41.65% and 40.32%, Tetraena mongolica 42.39% and 41.37%, Helianthemum songaricum 48.78% and 44.35%, Reaumuria songarica 39.25% and 39.86%.(3) The carbon storage was similar among five desert shrub ecosystems and its carbon storage was in the range of 40.28～55.51 t·hm-2, in which the carbon storage was in the range of 39.40～54.48 t·hm-2. The soil carbon storage was increasing with the soil depth. The biomass carbon density in plant layer was shrub layer>herb layer>litter layer. The biomass carbon density of shrub layer was more with the distance increasing to Yellow River and the biomass carbon density of the nearest site of A. mogolicus and the farthest site of H.songaricum was respectively 92.16% and 62.42% of the plant layer. However, the herb layer showed the opposite rule as the shrub layer. The root biomass carbon of grass layer was the main part of shrub ecosystem and its biomass carbon density was 5.36%～45.18% of plant layer carbon density. Except P. songarica shrub ecosystem, the carbon storage of belowground part in other four desert shrubs was more than aboveground part (P<0.05). The carbon storage of single shrub showed that branches>roots>leaves. The thick root sand branches were the main contributors to carbon storage of single shrub and showed obvious differences among shrub species (P<0.05). The roots and leaves carbon storage was 20.00%～33.53% and 2.02%～ 24.54% of the total plant layer.(4) The capabilities of daily carbon fixation in terms of per individual shrub were in order of Ammopiptanthus mogolicus> Zygophyllum xanthoxylum> Tetraena mongolica> Reaumuria songarica> Helianthemum songaricum. According to annual fixation carbon capacities, the five desert shrubs were divided into three categories. The～ highly annual carbon fixation group including A. mogolicus and Z. xanthoxylum(6.216～ 8.892 t·hm-2·a-1), medium annual carbon fixation group including T. mongolica and R. songarica(1.742～3.962 t·hm-2·a-1), lowly annual carbon fixation group including H. songaricum(0.386 t·hm-2·a-1). Stomatal conductance, transpiration rate and air temperature were key factors affecting photosynthetic rate. The daily net photosynthetic rate in leaves of five desert shrubs presented a double-peak curve, except Z. xanthoxylum, T. mongolica and R. songarica in autumn represented a single peak curve and an obvious "midday depression" phenomenon occurred. The five desert shrubs were belonging to sunny plants, in which T. mongolica had certain adaptability to bright light and the light use efficiency of A. mogolicus, Z. xanthoxylum and R. songarica were relative higher than others. The study will provide the certain reference for carbon fixation potential and carbon sequestration estimation in desert area.(5) The monthly soil carbon emission rate of the five desert shrubs in growing season (May to July) was much higher than that in the non-growing season (October).The daily variation of soil carbon emission rate displayed a single peak curve with the maximum rate occurring between 11:30-13:30. Scatter plots between soil carbon emission rate and soil temperature showed clockwise ring distribution. The determination coefficient of the best regression equations reached extremely significant level (R2=0.627-0.905) and there was a positive correlation between soil carbon emission rate and soil temperature. The soil water content in five desert shrub forests was low and its change was small. The soil carbon emission rate associated with soil water content had the same diurnal variation and there was significant negative correlation between soil carbon emission rate and soil water content. Under the typical Intergovernmental Panel on Climate Change (IPCC) series of emission scenarios (SRES), i.e., B1, A1B and A2, the soil carbon emissions of five desert shrub forests soil are expected to be increased by 6.94%-14.66% and the most temperature-sensitive shrubs’soil was Zygophyllum xanthoxylum.(6) The shrub ecosystem carbon budget of Ammopiptanthus mogolicus, Zygophyllum xanthoxylum and Tetraena mongolica were positive value, those net carbon exchange respectively were 1.004 ·hm-2·a-1、0.693 t·hm-2a-1 and 0.252 thm-2a-1, which showed income items was more than expenditure interns and the shrub ecosystems had a higher "carbon sequestration"capacity. While the shrub ecosytems carbon budget of Helianthemum songaricum and Reaumuria songarica were negative value, which net carbon exchange respectively were-2.605 t·hm-2·a-1 and-1.303 t·hm-2·a-1, which showed expenditure intems was more than income items and the shrub ecosystems belonged to a lower "carbon source"... |
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