The Dynamics Of Soil Bulk Density,Water Retention Curve And Thermal Properties Under Wetting And Drying Cycles

Soil is a complex system of three-phase,and its structure behaves strongly temporal and spatial variability.Soil structure and its stability play an important role in a variety of processes in the soil,such as erosion,infiltration,root penetration,aeration,or mechanical strength.Soil bulk density(?b),water retention curve(SWRC),pore size distribution(PSD)and other physical indicators could describe soil structural properties.Wetting and drying(W/D)cycles is one of important factors impact soil structure.Fields soils are usually subjected to periodical W/D cycles,which can significantly alter soil moisture distribution and hydro-mechanical behavior.In the process of measuring soil physical properties by traditional methods,soil samples experienced several W/D cycles.In this study,the time domain reflectometry(TDR)technique,thermo-TDR technique and soil matric potential sensors were used to measure soil volumetric water content(?v),thermal properties and matric potential,to determine the dynamics of soil ?b,SWRCs and thermal properties during several W/D cycles.By determining the dynamics of soil ?b,SWRC,parameters of van Genuchten model,PSD and thermal properties during several W/D cycles under laboratory and natural conditions,we studied and discussed the dynamic variation of soil structure under W/D cycles.The main results are as following:First,the influence of W/D on soil Pb of tilled layer.During the process of W/D cycles,under the influence of water flow,adsorption of water and capillary force,soil particles will rearrange and recombine,and form a new three-phase structural system.The tilled layer suffered nine W/D cycles.During the first to third cycle,soil ?b increased with time and depth.After the fourth W/D cycle,soil ?b increased by about 0.2 cm3 cm-3,and reached a relative stable stage.The largest change of soil ?b occurred during the fourth cycle,and the soil was almost saturated leading the most significant changes in soil porosity.In the stage of Pb increased,5-10 cm and 10-15 cm had the similar ?b value which larger than 0-5 cm.Under the W/D cycles,soil ?b increased with depth,and the difference of three layers increased gradually.For the tilled layer of maize filed with plough tillage(PT)and compaction tillage(CT),soil ?b of two treatments increased and reached stable stage after 40 days at 0-5 cm.At 5-10 cm and 10-15 cm,?b of CT kept stable,but PT treatment decreased and after 40 days reached stable stage.Unlike the bare soil,soil ?b of three layers tended to be uniform,and the difference of tilled layers decreased under W/D cycles.Thermo-TDR technique and thermal conductivity model were utilized to calculate soil ?b.Despite the ?-based method underestimated soil ?b,this method could reflect the dynamics of soil ?b.Compared with the core method,soil ? model could give reliable soil Pb values with root mean square error(RMSE)within 0.10 g cm-3,and relative error was less than 10%.Second,the influence of W/D on SWRC and PSD of tilled layer.Soil ?b increased gradually with time and depth,and became relatively stable after four W/D cycles.The SWRCs shifted towards higher 0 values at a specific suction.The magnitude of SWRC change was relatively small during the first to third W/D cycles when the degrees of saturation were relatively low,reached the maximum after the fourth W/D cycles during which the soil was fully saturated,and became less significant thereafter.Soil water holding capacity was improved during the W/D processed mainly due to the reduction of effective porosity and development of residual porosity.Third,the influence of W/D on SWRC and PSD of sand,loam and silt loam.During five W/D cycles,three textures soil showed that W/D cycles decreased soil saturated water content(?s)and the parameters a of van Genuchten model,and increased air-entry value and shape factor(n)increased.The pore size density of pore which larger than median pore diameter(MPD)decreased with W/D cycles.Compared with 3 cm,the larger change of SWRC and PSD occurred at 6 cm under the influence of W/D cycles.During the process of W/D cycles,soil residual water content(?r)of loam and silt loam increased,but at 3 cm ?r of sand did not change and decreased with W/D cycles at 6 cm.In terms of PSD,the MPD density of sand and loam increased gradually,despite the results of silt loam to the contrary.The MPD of sand and silt loam decreased,but this value of loam kept stable.The density of pore diameter which less than MPD in sand,loam and silt loam showed increased,decreased and kept unchanged with five W/D cycles,respectively.Fourth,the influence of W/D on soil thermal properties of sand,loam and silt loam.During the processes of W/D cycles,the relationship between heat capacity(C)and ?v of sand and loam was linear function.Its slope and intercept related to the specific heat of soil water and ?b.Under W/D cycles,the slope and intercept of sand and 3 cm of loam remained unchanged.But at 6 cm of loam,the slope decreased and intercept increased,which indicated that W/D cycles might increase soil ?b and absorbed water content which reduced the specific heat of water.The C vs.?v curve of silty loam was not linear relation,and the falling rate of C with ?v decreased at 0.15-0.20 cm3 cm-3.The main influence of W/D cycles on soil thermal conductivity(?)was on sand and 6 cm of loam:when ?v was larger than 0.2 cm3 cm-3,? increased with W/D cycles and depth at specific ?v.This tendency did not occur on silt loam and 3 cm of loam,which ?-?v,curves remained stable during five W/D cycles.The influence of W/D cycles on soil thermal diffuusivity(?)of sand and loam occurred at high ?v range:soil ? increased with W/D cycles and depth at specific ?v,and the larger change was at 6 cm layer.Instead,? of silt loam decreased with W/D cycles and depth.Soil thermal properties,especially soil C,were not sensitive to the change of soil structure under W/D cycles,could not reflect the dynamic of soil porosity.This study helps intense understanding the influence of W/D cycles on soil total porosity,SWRC and PSD,and has a great pontential in applications for research on coupled movement of soil water,heat and solute transport processes.