Under The Environmental Condition Of Wetting And Drying Cycles:Edaphic Aggregate Characteristics And Their Relationships With Floral Above-ground Productivity

Due to higher frequent and intensive wetting and drying cycles triggered by the Three Gorges Dam operation,the riparian zones along the Three Gorges Reservoir(TGR)is more influenced than those of natural rivers.During more than 10-year ecological succession,the wetting and drying cycle becomes a main driver of soil aggregate stability in the TGR riparian zones.Notably,soil structural stability measured by aggregate stability determines the succession of the riparian ecosystems,and aboveground plant production is the crucial material basis and energy source for the ecosystems.Studying soil aggregating characteristics and their relationships with aboveground plant productivity could provide practical suggestions for ecology restoration of such fragile ecosystems.This study was conducted at riparian zone of the Baijiaxia Brook in Kaizhou county of Chongqing,China,which is a typical riparian area in the TGR area.Field vegetation survey,Yoder wet sieving,TWINSPAN classification and Detrended Correspondence Analysis(DCA),One-way ANOVA,Duncan's multiple range tests,and Pearson correlation analysis were applied to answer the following three research questions:(1)how wetting and drying cycles influence soil aggregate size distribution and stability,(2)how soil aggregate size distribution and stability link with aboveground biomass(AGB)and aboveground net primary productivity(ANPP),and(3)what is the role of plant community properties in the linkage between soil aggregation and ANPP/AGB.The main conclusions were as follows.(1)In most of the sampling sites,microaggregates(MI,0.25?0.053 mm)and silt+clay(SC,<0.053 mm)were dominant,and decreased with elevation,while the mass proportions of large macroaggregates(LM,>2 mm)and small macroaggregates(SM,0.25?2 mm)were relatively low,and increased with elevation.Only in June,the average mass proportions of LM among all elevation zones demonstrated significant different,while in other sampling seasons,all soil aggregate fractions showed significant difference only between some elevation zones.Notably,the seasonal difference of LM showed the most significant difference compared with other fractions.In different growth seasons,soil aggregate stability in all sapling seasons increased with elevation,but the seasonal changes in soil aggregate stability were insignificant.Significant seasonal differences of soil aggregate stability(P<0.05)were observed only at the higher elevation.(2)Spatial patterns of AGB along the elevational gradient in the three growing seasons demonstrated similar,showing a unimodal response curve.And the maximums of AGB in each growing season were observed at the ?(165?175 m)and IV(175?180 m)elevation levels.Overall,changes in ANPP showed different trends among sampling intervals.In the first sampling interval,ANPP showed a decreased trend with the increase of elevation,but increased in the second sampling interval.In the whole sampling period,ANPP were almost equal except at the IV elevation level.In general,ANPP showed insignificant difference along the elevational gradient,except between I(145?155 m)and ? elevation zones in the first two sampling intervals,and also no statistically significant differences among all elevation zones in the total sampling interval(P>0.05).AGB correlated with soil aggregate fractions(except for SC)at significant level(P<0.05),and mean weight diameter(MWD)generally demonstrated a negative but insignificant relationship with AGB(P>0.05).In the first sampling interval,ANPP showed independent relationship with all soil aggregate fractions.In the second interval,ANPP significantly related with the changes of SM,MI(P<0.05).However,in the whole sampling period,ANPP negatively correlated with SC at significant level(P<0.01).(3)Significant synergy or trade-off relationships between below-and under-ground only occurred in specific growth seasons and plant community types.In June,significant synergy relationships but insignificant trade-off relationships between belowground function(soil aggregate stability)and aboveground function(AGB)were found.In July,the synergy relationships dominated.While in September,the trade-off relationships turned to be dominant.Plant species diversity(S)played important roles in linking belowground with aboveground functions,indicating diversification effects exists.In general,S demonstrated stronger effects on belowground function(soil aggregate stability)than aboveground function(AGB).In different growth seasons,the diversification in affecting aboveground and belowground function showed significant differences along spatial gradient.However,only at a certain spatial elevation level and in a certain growth season,S significantly affected the relationship between ANPP and the daily change of MWD.Plant functional groups diversity(G)played an important role in linking belowground and aboveground functions at a certain elevation level and in given growth seasons,while dominant species from different functional groups were the key in playing the role,indicating that the single species effects also existed.In June,annual forb,perennial graminoid(especially Cynodon dactylon(L.)Pers.),and plant functional group diversity played significant roles in belowground function(P<0.05).In July,the role of plant species in every plant functional group played no significant roles in linking belowground and aboveground functions.In September,only annual forb and annual graminoid significantly linked with the belowground function.Notably,Cynodon dactylon(L.)Pers.is a typical dominant and pioneer species in structuring community,and the significant role in linking belowground and aboveground functions only occurred in early growth stage.During wetting and drying circulation,MWD and AGB synergically related with each other in June and July,while in September,the trade-off relationships turned to be dominant.This may relate to the adaptation strategies of plant species to stress environment,and may also to plant root traits,mycorrhizas,plant litters,and interspecific relationships.In summary,both effects of single dominant and diversified species in linking between below and under-ground functions exist.During succession assemblage process of plant community in drawdown zone,and the functions delivered by the plant community showed multiple characteristics.