| After the completion of the Three Gorges Reservoir,the water level operation mode of “Winter Storage Summer Drainage” was adopted to form a water-level fluctuation zone with a vertical drop of 30 m.Most of the original vegetation could not tolerate long-term,high-intensity and anti-seasonal flooding stresses under the particular hydrological rhythm.As a result,the problems of original vegetation disappeared,biodiversity decreased,soil erosion and ecosystem degradation became more and more serious.In order to solve the above problems,it is necessary to restore the vegetation in the Three Gorges Reservoir area.Taxodium distichum and Taxodium ascendens are both deciduous trees of the genus Taxodium,which are suitable for high altitude water-level fluctuation zone due to the characteristics of water-tolerant.Under the flooding in the Three Gorges Reservoir area.Hypoxia was the main limiting factor for the survival of T.distichum and T.ascendens,it may lead to blocked energy metabolism,insufficient ATP synthesis.As a result,the active absorption and transportation of nutrient elements(especially C,N and P)are restricted,which lead to the changes of the content and availability of various nutrient elements in the soil.Generally,plants can develop healthily and stably when all nutrients are present at an appropriate ecostoichiometric ratio.To explore ecological stoichiometric characteristics in soil and plants of T.distichum and T.ascendens system in the Three Gorges hydro-fluctuation belt,the study selected T.distichum and T.ascendens that planted in the vegetation reconstruction demonstration base of the Three Gorges Reservoir area in Zhong County,Chongqing.The field was divided into three transects based on field data measurements and the current water level operation rhythm of the Three Gorges Reservoir area: deep submergence(DS,165 m),moderate submergence(MS,170 m)and shallow submergence(SS,175 m).Sampling ofbranches,leaves,fine roots,and topsoil were taken in July 2018(T1,vigorous growth period),September 2018(T2,late growth period),July 2019(T3,vigorous growth period),and September2019(T4,late growth period),respectively,then,the contents of C,N and P elements,as well as stoichiometric ratios were determined and analyzed.The results were as follows:(1)The height,canopy,basal diameter,and and diameter at breast height of T.distichum and T.ascendens were all inhibited by increasing flooding time and intensity,whereas growth was maintained,and the trend of growth was increased with the extension of sampling time.The height,canopy,and basal diameter of T.distichum were higher than that of T.ascendens,but its diameter at breast height was significantly lower than that of T.ascendens,which may be related to the difference in the adaptability of species.There was no obvious rule of soil temperature of T.distichum and T.ascendens at the same sampling time,but they were in the appropriate temperature range.The soil temperature of T.distichum and T.ascendens showed T1>T2 and T3>T4 at different sampling time.The ORP value decreased with the increase of water flooding time and intensity,and showed a trend of T1>T2 and T3>T4,where the ORP value of DS group was lower than 350 mV at T2 and T4 times.Except for T2 time,the soil water content and total porosity in MS and DS groups showed a trend higher than that in SS group,while the bulk density was opposite to the trends of the water content and porosity.The p H values of the three submergence treatment groups were not significantly different,all of which were weak acidity and showed as July>September.(2)The element content in the branches,leaves and fine roots of T.distichum and T.ascendens showed C>N>P in each submergence treatment group.The C content in branches,leaves and fine roots of T.distichum and T.ascendens in the same submergence treatment group was not significantly different in the four periods,and the ratio was about 1:1:1,which was much higher than its corresponding topsoil.The order of N content in the four periods was leaf>fine root>branch>soil,and the P content is leaf and fine root>branch and soil.C/N and C/P ratios were all represented as branches>fine roots>leaves.The N/P ratios of leaves and fine roots remained basically the same,relatively constant,and were less than 14,indicating that the growth of the two species may be restricted by N element in the water-level fluctuation zone.(3)There was no significant difference in C content of T.distichum and T.ascendens in each submergence treatment group.The N content of each component of T.distichum showed the trend of SS>MS>DS group,while the N content of branches and leaves of T.ascendens was opposite to that of T.distichum.The overall trend of P content in the components of the two tree species was basically similar.The order of P content in branches were MS>SS>DS group,the leaves were SS<MS<DS group,and fine roots were SS>MS>DS group.Except for C and N elements in the soil of T.ascendens,the contents of C,N,and P in the soil of both species were both higher in DS andSS groups than in MS group.The contents of C and N in the soils of each submergence treatment group were far lower than the average contents of C and N in the global and China terrestrial soils,and P content was equivalent to the average soil in China.The soil C/N ratios were all much less than 30,which indicated that the leaching risk of nitrate was high in the water-level fluctuation zone of the Three Gorges Reservoir region.Except for the C/P ratio in the four periods of the MS group,the soil C/P ratio of the two tree species was high in July and low in September.The soil N/P ratio was similar to the C/P ratio,and was much lower than the average value of the soil N/P ratio in China.(4)Redundancy analysis(RDA)indicated that only the fine root P content of T.distichum and T.ascendens showed a strong positive correlation with soil P content during the growth period of T1-T4.Judging by Garnier theory,the growth of fine roots of T.distichum and T.ascendens were mainly restricted by element P.There was a strong positive correlation between branches and fine roots N/P ratio of T.ascendens during T1-T4 growth period,indicating that the growth and metabolism of and T.ascendens were integrated.(5)On the whole,the homeostasis of T.distichum and T.ascendens was strong during the growth period.C element of both tree species had homeostasis in four periods.Except for N element of T.ascendens branches at T2 and T.distichum fine roots at T4 did not have homeostasis,the other organs all had homeostasis.The element P had weakly homeostasis,and the fine roots element P of T.distichum did not even have homeostasis in four growth periods.The homeostasis of element N and P was lower than that of element N/P ratio.The homeostasis of C,N,P elements and their ratios in the aboveground part was stronger than that in the underground part.In conclusion,the distribution and trade-off of photosynthetic products and nutrients of T.distichum and T.ascendens between plants and soil were shown that the two tree species by balancing the C elements of each component,maintaining the cooperative changes of N and P elements and the relative stability of N/P ratio to maintain the homeostasis.In this way,they can adapt to and respond to the changes of the water-level fluctuation zone of the Three Gorges Reservoir region.In addition,the damage to the fine roots of T.distichum and T.ascendens should be reduced as much as possible during the vegetation restoration and reconstruction in the water-level fluctuation zone,and the decomposition of litters should be rationally used to maintain the nutrient balance in the area,so as to reduce the loss of elements,thereby reducing the loss of elements. |
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