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The periodical fluctuation of water-level-fluctuating zone in Three Gorges Reservoir led to the variation of the soil ecological environment between wet and dry state.This variation inevitably produce a greater change in the soil seed bank,which further affects the standing vegetation recovery and growth.In this paper,we studied the dynamic variation of soil seed bank in water-level-fluctuating zone of Three Gorges Reservoir at the beginning after charging water was studied by substituting flooding time gradient with vertical space gradient.As soil seed bank density of clear-cutting land increased gradually with time,the density of abandoned farmland decreased.With the vertical space gradient increasing,density of soil seed bank in water-level-fluctuating zone in May,July,September showed gradual increasing trend respectively.The above results suggested that the longer flooding dures,the smaller soil seeds storage amount is.Co-occuring dominant species in different months,including Ficus tikoua,Youngla japonica,Cyperus microiria,Digitaria chrysoblephara,Veronica didyma,Mazus miquelii,Gnaphalium affine,Cardamine lyrata,Pouzolzia zeylanica,Veronicastrum stenostachyum,Cyperus rotundus,Erigeron annuus,Eragrostis ferruginea and Boehmeria nivea,were main components of soil seed bank.These species had a strong adaptability to the fluctuating environmental changes.The life-form composition of soil seed bank in different months was very similar.They were mainly composed of seeds of annual and perennial herbs,while being relatively lack of the seeds of shrub and arbor.Moreover,the life form composition proportions of soil seed banks of different altitude space were different.Combining with water ecological function of the soil seed bank,we can divided it into hygrophytes,hygromesophytes,mesophytes and mesoxerophytes.They were mainly mesophytes based,the proportion of mesoxerophytes in high-altitude section and that of hygromesophytes in low altitudes of were relatively high,while the proportion of hygrophytes in different altitude section in different months changes only a little.This indicated water flooded caused mesophytes and hygromesophytes gradually increasing in the soil seed bank,which was also the change trend of plant water ecological function type in water-level-fluctuating zone.The plants in soil seed bank had higher biological diversity as well as the plant species was high variable in different elevation sections and was lack of absolute dominant species.The vegetation communities were obviously in its early succession stage.In conclusion,from the results of the soil seed bank of water-level-fluctuating zone in Three Gorges Reservoir,vegetation restoration and reconstruction of the potential were unrealistic.This suggested that the changes of habitat,spesies composition and seed reserves of the soil seed bank were drastic at the beginning after charging water.In order to restore the vegetation of water-level-fluctuating zone more quickly,more manual intervention need to be introduced rather than only depending on the original ones.

The quality and quantity of litter and its decomposition rate represent,to some extent,the nutritional status of the soil.In order to accurately estimate the contribution of litter decomposition to the annual net return of soil carbon pool and understand the biochemistry of litter-soil continuum,the relationships between litter substrate quality and soil nutrients were studied for different-aged Pinus massoniana stands.The results showed that C,C/N ratio,C/P ratio,lignin/N ratio,lignin/P ratio of litter substrate collected from three different-aged P.massoniana stands were significantly different.The lignin/N ratio of leaf litter in nearly-mature P.massoniana stand was 33.65% and 39.24% higher than that in middle-aged and mature stands,respectively.However,N,P,K and lignin contents of litter substrate did not differ among the different-aged stands.The contents of N,P,and K varied with tissue types showing barkbranchesleafdebris,but the contents of C/N ratio and C/P ratio followed an opposite trend showing barkbranchesleafdebris.The contents of organic matter,total nitrogen,available phosphorus in 0—20 cm soil layer tended to increase from the nearly-mature,to the middle-aged,and mature stands and to decrease from 0—5 cm soil layer,to 5—10 cm,and 10—20 cm soil layer.The total phosphorus was lowest in the middle-aged stands and largest in the mature stand.The soil pH value varied from 4.55—5.55 showing middle-aged standmature standnearly-mature stand.Redundancy analysis(RDA) between litter substrate quality and soil nutrients indicated that the leaf litter substrate quality was significantly correlated with soil nutrients.The effects of litter nitrogen,phosphorus,cellulose,hemicellulose,lignin,lignin/N ratio,and C/N ratio on soil nutrients in different-aged P.massoniana stands were significant.Litter nitrogen content was significantly correlated with total soil nitrogen content.Soil pH,bulk density,and N content were significantly negatively correlated with litter C/N ratio,but positively correlated with lignin/N ratio.Soil surface organic matter,N and P contents were found to depend on nutrients contents in litter substrate.High litter substrate quality resulted in great soil nutrient contents.

Roots play an important role in individual plant carbon budgets and ecosystem-level carbon and nutrient cycling.Considerable research has concentrated on the vertical distribution and seasonal dynamics of fine root biomass.However,the spatial distribution of root biomass has been poorly studied.Our objectives are to determine how fine root biomass changes with horizontal distance,and to examine the spatial distribution of fine root biomass in stands of different ages. The horizontal and vertical distribution of root(0—10mm diameter) biomass for three Pinus massoniana plantations of different ages,20,30 and 46 year-old,in the Three Gorges Reservoir area,Hubei Province,China,was studied.Ten cm diameter soil cores were collected in July,2010 at different horizontal distances(0.5,1.0,1.5 and 2.0m) from the stem.Each soil core was separated into five sections at depths of 0—10,10—20,20—30,30—40 and 40—60 cm.Roots(0—10mm) were first separated into living and dead roots.Then both categories were classified into two diameter classes,that was,two groups for fine roots(0—1mm and 1—2mm),and two groups for coarse roots(2—5mm and 5—10mm).Roots were dried at 80℃ to a constant mass and weighed. Our results indicated the average fine root biomass of P.massoniana in the Three Gorges Reservoir area was 0.56 t/hm2,which is within the range of worldwide fine root biomass(2—5mm diameter),but lower than the average fine root biomass of subtropical evergreen needleleaf forest.The total root biomass(≤ 10mm diameter) for P.massoniana forest for the 20,30 and 46 year-old stands was 2.40,4.72 t/hm2,and 2.94 t/hm2,respectively,and differences between them were statistically significant(P0.05).However,the fine root biomass(0—2mm diameter) declined insignificantly with an increase in forest age(P0.05).In all three stands,living root biomass was much higher than dead root biomass.Fine root biomass in all the stands peaked at 1.0 m from the stem,but fine root biomass in stands of different ages changed differently as the horizontal distance increased.Fine root biomass was mainly concentrated in the upper soil layer and decreased as soil depth increased.For all ages of stands,47.53%—71.73% of living roots were observed in the 0—20cm soil layer.The majority of coarse roots(2—10mm diameter) were at a depth of 20—60cm.We concluded smaller roots were more sensitive to environmental change than larger roots,based on an analysis of the interactions between the spatial distribution of fine root biomass and soil depth,distance from stem and stand age.Spatial distribution of fine root biomass was very significantly influenced by soil depth(P0.01),but effects of stand age and horizontal distance from the tree stem were not significant(P0.05).Also,all these factors had no significant influence on the spatial distribution of coarse root biomass(P0.05).The results indicate more consideration should be given to research on the spatial distribution of roots and the effects of these integrated factors on root distribution.