| Lateral migration of meandering river is the major driving force that shapes geomorphic units within the riparian zone.Meandering channel,point bars,and oxbow lakes generated from cutoffs distribute along the river corridors and create a mosaic of landscape which,not only connect uplands and rivers and thus regulate and trap organic carbon transport between them,but also are important carbon reservoirs themselves.Located in the Qinghai-Tibet Plateau,Zoige Basin has developed many meandering rivers.The basin is abundant in peat soils that contain a large percentage of organic matter,accounting for?88%of the total peat organic carbon in the Qinghai-Tibet Plateau.Since 1970s,residents have dug ditches to drain wetlands for grazing,significantly changed hydrological connectivity within the catchment,making distribution of geomorphic units more complex.Focusing on the influence of fluvial geomorphology and human activities on the transport of organic carbon,we conducted a four-year study within the meandering river of Zoige Basin,and have acquired the following results.Along the main channel of White River,concentration of dissolved organic carbon(DOC)maintained a stable level downstream,while concentration of particulate organic carbon(POC)that mainly transported from peat soils decreased with inclined total suspended sediments(TSS).However,because of the increasing trend of TSS,POC flux increased downstream,and remained the major component of riverine organic carbon flux.This pattern is stable between different studied years.Total organic carbon exported from White River was 1.1×10~4 t,which is a lower amount in comparison with other north peatland catchments,as a result of the influence of alpine climate.Affected by valley and channel geometry,along White River,the number of point bars decreased downstream whilst the average area increased,suggesting a larger amount of trapped POC by point bars.Within individual point bars,organic carbon distributed homogeneously with an average level of 1.08%,and did not show significant differences or trends at both vertical and longitudinal directions.However,positions with pioneer plants had significantly higher carbon concentration compared with bare land and resulted local hotspots,because of the litters that transported from live plants.Organic carbon concentration of point bars was lower than on the floodplain,because point bars were at their earlier stage of terrestrialization and had shorter times for organic matter accumulation.Within a hydrologic year,at the studied concave bank,organic carbon derived from flow erosion and bank failure was about 1.49m~3 in volume.Organic carbon inputted into the channel complained by bank failure could be viewed as an instantaneous process,however it took days to years for soil blocks to be transported downstream,depending upon hydrologic regime and the ability of water to carry sediments.Composition of meandering river banks determined whether bank erosion could maintain its strength at river segment to catchment scales.In the studied area,grazing and channelization significantly altered the natural composition of river banks.Lake geometry demonstrated significant differences among oxbow lakes with varying connectivity.Lake length,area,length/width ratio,and curvature decreased from the most connected lakes(lotic lakes)to less connected lakes(semi-lotic lakes)and finally to non-connected lakes(lentic lakes).Water chemistry in oxbow lakes also demonstrated environmental gradient relative to lake-channel connectivity,i.e.,increased connectivity led to decreases in organic carbon concentration from 19.86mg/L to 15.10 mg/L,and eventually to 10.23 mg/L.This reflected the changing role of oxbow lakes from transport to product.This environmental gradient was however not stable.Flood pluses in the growing season and older lakes reconnection with newly formed lakes could alter the connectivity during baseflow periods and acted as disturbances to the gradient.Organic carbon fluxes in ditches were dominated by DOC during both baseflow periods and stormflow periods,although under intensive rainfall events flux of POC could temporarily exceed DOC at the time high discharge happened.During stormflows,the response of POC to discharge was rapid and caused sharp increases in concentration(maximum 30 mg/L),mainly because of the peat soils that rich in organic carbon mobilized and moved with saturation overland flow.On the other hand,dilution effect was observed in DOC concentr ation,because of rainwater mixing with overland flow induced by infiltration excess runoff,rainwater directly falls into the ditch and mixes with ditch water.In summary,at different spatial scales(individual landscapes,meander bends,river segment,and catchment),fluvial geomorphic processes that responsible for channel laterally migration and human activities such as ditches drainage directly determined some paths for organic carbon transport among different landscapes and within individual landscapes.Meanwhile,at different temporary scales(hours,days,months,within years,and between years)temperature,precipitation,soil,vegetation,and hydrology that determined by ecological background and grazing and channel projects regulated the timing,direction,and flux of organic carbon transport.Because different landscape units had varying degrees of acting in the organic carbon dynamics within meandering river corridor,their carbon reservoir thus had different rates,magnitudes,seasonal and spatial patterns,resulting the inhomogeneous organic carbon distribution within river corridor.This distribution pattern and the geomorphologic landscapes together created wetland ecosystem with different nutrient availability,different hydrological connect ivity,different biological community structure,and different level of disturbances. |
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