简介：TocomparethedevelopmentofphysicalcrustsinthreetypicalcultivatedsoilsofChina,ablacksoil(LuvicPhaeozem),aloesssoil(HaplicLuvisol),andapurplesoil(CalcaricRegosol)werepackedinsplashplateswithcoveredanduncoveredtreatments,andexposedtosimulatedrainfall.Meshescoveredabovethesurfacesofhalfofsoilsamplestosimulatetheeffectsofcropresidueoncrusting.Theresultsindicatedaprogressivebreakdownofaggregatesonthesoilsurfaceasrainfallcontinued.Thebulkdensityandshearstrengthonthesurfaceofthethreesoiltypesincreasedlogarithmicallyasrainfalldurationincreased.Duringthefirst30minofsimulatedrainfall,thepurplesoildevelopeda7-8mmthickcrustandtheloesssoildevelopeda3-4mmthickcrust.Theblacksoildevelopedadistinguishable,butstillunstable,crustafter80minofsimulatedrainfall.Soilorganicmatter(SOM)content,themeanweightdiameter(MWD)ofsoilaggregates,andsoilclaycontentwerenegativelycorrelatedwiththerateofcrustformation,whereasthepercentageofaggregatedispersion(PAD),theexchangeablesodiumpercentage(ESP),andthesiltandsandcontentswerepositivelycorrelatedwithcrusting.Mechanicalbreakdowncausedbyraindropimpactwastheprimarymechanismofcrustformationintheblacksoilwithmorestableaggregates(MWD25.0mm,PAD3.1%)andhigherSOMcontent(42.6gkg~(-1)).Slakingandmechanicaleluviationweretheprimarymechanismsofcrustformationinthepurplesoilwithlowclaycontent(103gkg~(-1)),cationexchangecapacity(CEC,228mmolkg~(-1)),ESP(0.60%),andSOM(17.2gkg~(-1)).MechanicalbreakdownandslakingwerethemostimportantintheloesssoilwithlowCEC(80.6mmolkg~(-1)),ESP(1.29%),SOM(9.82gkg~(-1)),andhighPAD(71.7%)andMWD(4.6mm).Simulatedresiduecoverreducedcrustformationinblackandloesssoils,butincreasedcrustformationinpurplesoil.

简介：AsoneofthedevelopingcountriesChinahasanarablelandpercapitafarbelowtheworld'saveragelevel.Withahigh-densitypopulationanthequickdevelopmentofeconomyandurbaniztion,theYangtzeRiverDeltashowsthetypicalcharacteristicsoflanduseindevelopedregionsofChina,whichare:highlandreclamationrateandlowarablelandpercapita;intensivelanduseandhighoutputvalue;andrapidincreasingofconstructionlandareaandfastdiminishingofarableland.TheanalysisindicatesthattheprocessofthearablelandchangesintheYangtzeRiverDeltacouldbedividedintofourdifferentchangestagesoverthepast50years.