| The ridges surrounding paddy fields have been pointed out as possible causes for low water productivities and shallow groundwater pollutions.Yet,detailed experimental data of the paddy field in the Jianghan Plain to confirm these assumptions are lacked.The objective of this study was to quantify the characteristics of water-nitrogen transport and conservation among different locations(i.e.,inner field and paddy ridge)in the paddy inner field-ridge transition zones,and to identify the dominant pathways for water flow in the transition zones.Two typical transition zones were investigated in the aged more than 60 yr paddy field by soil samples analysis,CT scan method to quantify the distribution of macropore,field dye tracer experiment to analyze the typies of preferential flows and breakthrough curve of KNO3 at paddy-ridge in the Jianghan Plain,Hubei Province.And those experiments results showed:(1)The soil properties of are different at each site.The bulk density of plow pan in the field was the highest(1.51 g/cm3),aid the ridge between fields was the second,und the intermediate between ridge and ditch and ditches were lower.The distribution of particle show that clay contents was larger,and the sand was less.The content of organic matter was higher in surface soil,lower in subsoil layer and substratum,and the ridges'subsoil layer organic matter contents were higher than field.The saturated hydraulic conductivity(Ks)decreased with soil depth increasing for each site,and characterized by higher standard deviation in the upper soil layers.Highest Ks was observed in the paddy ridges,followed by the irrigation canal,and lowest in the inner field,Implied the water would prefer to transport through the paddy ridges.In addition,significant differences of the soil water retention curves(SWRC)were observed,highest soil water capacity was identified in the inner field,followed by the irrigation canal,and lowest in the paddy ridges.The macroporosity calculated based on the SWRCs revealed that more macropores were exited in the ridges than that in the inner field,which resulted in more preferential flow paths.Different soil K,and water capacity between the inner field and paddy ridge could be mainly attributed to the enhanced macroporosity in the ridges,which was mainly resulted by the soil animal activation and root penetration.Although the water seepage in the paddy ridge could be reduced by the "Hard Pan" located at the depth of 18?35 cm,the preferential flow could not be prevented as the macropore exited.Except for the vertical water infiltration in the inner field.(2)The CT scan of macopore pore results showed that the different sites soil macropores were obviously different.The topsoil layer had the highest ratio,while the subsoil layer and substratum was lower.The porosity of each sites is shown that irrigation ditch>field>ridge between fields>field-ridge intermediate zone.The macropore porosity under topsoil is ridge between fields>field-ridge intermediate zone>irrigation ditch>field.In addition,the number of macropore in the transition zone is quite different,the irrigation ditch had largest number of macropore,the field was the least.The macopore perimeter was decrease with the increase of depth in the transition zone.The topsoil pore perimeter is larger and with an average of 8.88 mm.There is a great difference in the pore circumference of each sites.Among them,the circumference is the lowest in the surface,but with depth increase,the variable coefficient was large.In the irrigation ditch and ridges,the macopore perimeter is uniformly fluctuated.And at least.the variation coefficient of other points in field is increased with the soil depth incerased.The distribution of different macopore diameter in the transition zone showed that the 0.6?2 mm had much large percentage,while the diameter of>2 mm is relatively low.At all sites,surface soil macropore proportion is higher(more than 70%),and each frequency of diameter are mainly distributed in 0?10 cm soil layer,while 10 cm and below is lower.(3)Dye tracer experiment results show that stained soil area ratio(SAR)fluctuated downwards with incerasing depth of the soil profile and a peak was observed in the 0?20 cm soil layer,accounting for 53.85%?88.55%of the total SAR of the soil profile;that the field was the highest in SAR in this soil layer,and followed by the transition zone,and the ridge;that although SAR was quite low in soil below 20 cm in depth at all the monitoring sites,the ridge was still 3.15 times as high as the field in mean SAR;that staining of the soil appeared quite consistent horizontally and vertically,that with increasing soil depth,the area of staining at horizontal levels gradually decreased from a plane to a dot.The ridge,impact in soil texture,was higher than the field in staining path number(SPN)that in the 0?29 cm soil layer.staining path width(SPW)varied in the range 10?80 mm and beyond,while in the soil layers down below,only a few paths<10 mm were observed;that the water flow in the field was mainly of the type of heterogeneous finger flow-highly-interacted macropore flow,while in the transition zone and ridge,it was mainly of the type of mixing macropore flow-highly-interacted macropore flow,for the SPW therein varied in the range of 10-80 mm and below.When water flowed from the field into the transition zone,it lost through seepage vertically and laterally,the two typical pathways,(?)via earthworm pores,root pores and fissures,which were more plentiful in the ridge,facilitating vertical and horizontal seepage of the water,while the plough pan in the field made it hard for the water to leach downwards,thus forcing the water to flow into the ridge and then percolate rapidly into deep soil layers;and(?)further into ditches through the intermediate between ridge and ditch,of which the two were relatively different in elevation,so the water infiltrating into the ridge continued flowing vertically through the ridge and eventually into the ditch,besides percolating downwards.(4)The breakthrough curve of KNO3 at paddy ridge results showed that with the increasing of infiltration time,the cumulative infiltration capacity of the transition zone increases gradually,and the total infiltration amount reaches 100 cm after the 72 h test.However,the infiltration rate was influenced by bulk density,organic matter and clay content and so on,and the stabilization infiltration rate was 40 cm/d.The outflow of concentration about ionic in the transition zone is increased with the test time.At layers of 10?20 cm and 30?40 cm has more lateral flow in ridge.Between 40?60 cm,there is less leakage,and the relative concentration is between 0.60?0.80.The rate of soil infiltration rate was decreased gradually after the rapid increasing at different depth of ridge site.At the 30?40 cm in site A and 10?20 cm in site B,The soil infiltration rate is the highest and the fluctuation is most obvious.While,the lateral seepage of other soil layer was lower,which between 0.005?0.02 cm/d.The types of water flow through the ridge can be divided into 3 types,such as "quickly response","delayed response" and "rise slowly".Due to the factors like the macropore and root system in ridges,the water and nitrogen in the field will be lost rapidly through ridges zone.Moreover,especially the field-ridge intermediate zone,when its' length ratio of other ridge between field is smaller,the rate of lateral flow would be declioned.At this paper,through the soil samples analysis,CT scan,dye tracing experiment and breakthrough curve of KNO3 at paddy ridge in the Jianghan Plain,affected by the distribution of macropores in the field,transition zone and ridge,only a small amount of water in the field infiltrated through the plough pan and a large one lost through seeping laterally into the ridge and then percolating quickly and vertically into deep soil layers through macropores,thus leading to low water use efficiency and nitrogen pollution of water bodies.It is,therefore,essential to pay high attention to the influence of water flow in the ridges on leakage of soil moisture and solutes from paddy fields. |
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