Stoichiometry Of Soil Microbial Carbon Use Efficiency At Different Water Level Gradients In Tongzhuang River Of The Three Gorges Reservoir Area

Soil microbial carbon use efficiency relate to the disturbance intensity of soil,which increase with increasing disturbance.The riparian zone of the Three Gorges Reservoir area is affected by the annual impoundment and flood discharge or seasonal water level fluctuation of the Three Gorges Dam.Soil disturbance is enhanced and soil properties are significantly changed,which may affect soil microbial carbon use efficiency.Soil samples of different inundation duration were measured in the riparian of the Three Gorges Reservoir Area in order to determined soil carbon,nitrogen,phosphorus,microbial biomass,enzyme activity,microbial carbon use efficiency and those ecological stoichiometry at different water level gradients.Redundancy analysis was used to elucidate the major factors affecting microbial biomass,enzyme activity,and microbial carbon use efficiency.The main conclusions were as follows:1)The results of soil physicochemical properties showed that soil carbon,nitrogen,phosphorus,soil microbial biomass and enzyme activity were significantly higher in never inundation sites than those in ripatian zone,which explained that the soil quality in the ripatian zone was poor.Soil carbon,nitrogen and phosphorus had no significant difference between the different water level gradients in ripatian zone,while soil microbial biomass and enzyme activity had significant differences.Those indicated that soil carbon,nitrogen and phosphorus content were not sensitive to the flooding,but soil microbial biomass and enzyme activity were sensitive indicators for evaluating soil quality.In the riparian zone,microbial biomass and enzyme activities were highest in the MI,microbial biomass carbon,microbial biomass nitrogen and enzyme activities were lowest in the LI,microbial biomass phosphorus was lowest in the SI.Those indicated that the microorganisms were the most active in the MI zone,where soil quality was better.But soil quality was more serious in the LI zone for longer-term inundating.2)Soil ecological stoichiometry studies showed that the values of soil C/P,N/P,MBC/MBP and MBN/MBP were the highest in the SI zone,and there were no significant difference between LI,MI,and NI zones.For the phosphorus absorption in the citrus roots and water wave scouring,phosphorus and microbial biomass phosphorus decreased rapidly in the LI zone,which could be used as a nutrient limiting factor.Soil MBC/MBN was significantly highest in the LI zone,and it was no significant differences between other zones.Soil microorganisms in the LI zone were mainly fungi,other zones were mainly bacteria.It showed that increasing and flowing of the water level had a great influence on soil microorganisms.In the LI zone,PAG/PNAG was significantly highest,PNAG/PNP was significantly lowest,which indicated that the ecosystem nutrient cycle was controlled by the PANG enzyme.In the SI zone,PAG/PNAG and PNAG/PNP were the lowest,which indicated that the ecological nutrient cycle was affected by PAG enzymes.Ecological stoichiometry of the soil carbon,nitrogen,phosphorus,microbial biomass and enzyme changed with the different water level gradient,indicating that the internal stability of the riparian zone was poor.3)The efficiency with which microorganisms use carbon substrates is often termed carbon use efficiency.Soil microbial carbon use efficiency was estimated by the soil carbon,nitrogen,phosphorus,microbial biomass and enzyme activity.The variation range of CUE_C:Nwas 0.22-0.28,and CUE_C:Pwas 0.51-0.54.The variation range of CUE was within the range of terrestrial ecosystem changes.CUE in ripatian zone was significantly higher than never inundation sites,because the water wave fluctuation caused soil disturbance,leading to CUE increase.There was no significant difference in CUE_C:Nbetween the water level gradient soils in ripatian zone,while CUE_C:Pwas the highest in the SI zone.For the phosphorus absorption in the citrus roots and water wave scouring,phosphorus and microbial biomass phosphorus decreased rapidly in the LI zone.Soil disturbance was enhanced in the LI zone,which improved the soil microbial carbon use efficiency.4)Redundancy analysis showed that the first two sorting axes of RDA explained75.55%of soil microbial biomass,88.16%of soil enzymes,and 98.8%of CUE,indicating that the ranking results were well.The first two axes of RDA could reflect the major factors affecting soil microbial biomass,enzyme activity,and microbial CUE.Soil microbial CUE was mainly affected by soil SOC.The combination of SOC,PAG/PNAG,C/N,MBC/MBN,TP,PNAG/PNP,MBP and C/P factors could explain 97.1%change,indicating that the changes of soil microbial CUE was mainly affected by these eight factors.