Exploring The Mechanism Of Forming Soil Aggregate In The Loess Plateau With A New Index By Apparent Energy (E_A)

Soil aggregate stability is considered a quality indicator of soil healthand soil structure.The Loess plateau（LPC）was regarded as the severe erosion-affected area in China;however,after thevigorous revegetation campaign under the‘Grain for Green’Project’（GGP）,there was a notable positive effect on the soil aggregate stability of this area.Previously,various soil stability methods and indexes were used to evaluate the effect of the revegetation campaign under GGP on the soil aggregate stability of this area.Nevertheless,they were mostly limited to exploring the quantitative stability of soils based on aggregates diameter（mean weight diameter,MWD;particle size distribution,PSD;water-stable aggregate,WSA）.Hence,there was a need for an index that could better insight into the aggregate stability of soils based on their quantitative and qualitative assessment in the Loess plateau region.In this perspective,we have conducted the following studies and got some conclusions.1.Three renowned methods assessing soil aggregate stability based on different methodological foundations were strongly associated with each other.The modified Yoder,Le Bissonnais,and Ultrasonic agitation were used to evaluate the aggregate stability of eight（four forest,three shrubs,and one grass）soils of the Ziwuling area by comparing their relative results.These methods were characterizedin different methodological bases,i.e.,different initial and final aggregate fraction classes,stability parameters,the dispersion medium,and dispersion agents.However,the stability parameters of these methods showed a strong association with each other（r=0.55–0.93）.The Results of this study revealed that the vegetation type was not the dominant factor determiningthe soil aggregate stability,as the most stable soil(Qurecus wutaishansea（QW）and the most unstable soil（Robinia pseudoacacia（RP））were both developed under forest cover.Overall,the soil organic carbon（SOC）,total nitrogen（TN）,fine root biomass（FRB）,and clay contents were identified as the major characteristics responsible for the stability of Ziwuling soils（p<0.05 and 0.01）.2.Anapparent energy（E_A）was developedinconsideration of the limitations of stability indexed by previous stability methods,whichis based on the dispersion energy and resulting change in mean weight diameter（（?）MWD）.To evaluate the ability of E_Aby ultrasonic agitation（UA）in assessing aggregate stability,mean weight diameter（MWD）and geometric mean diameter（GMD）were employed for comparison in measuring the stability of four different soils from Qinling Mountains（QL,eluvial cinnamon soil）,Guanzhong Plain（GZ,lou soil）,Ansai（AS,loessial soil）and Jingbian（JB,loessial soil）area.The MWD value of surface and subsurface soils were rangedfrom 0.15 to 0.58 mm and 0.17 to 0.37 mm,respectively,while GMD was ranged from 0.14 to 0.33 mm.Contrary to MWD and GMD,E_Awas significantly positively correlated with sand content instead of organic（OM）.The JB soil was among the soils with the least MWD and GMD values by the wet sieving method;however,it was the most stable soil indicated by E_A（high E_Avalue）because it required high dispersion energy to be disintegrated.Hence,E_Awas proved better to explain the apparent stability of soils due to the quantitative assessment of（?）MWD（quantity of soil particles）by dispersion energy（soils’resistance against disruptive forces）.There was a more significant difference of apparent stability among the soils by E_Athan MWD and GMD;hence,E_Awas proved good to differentiate the apparent stability of different soils.3.The E_Aexplored that the quantity of litter input to soil took a more substantial effect on aggregate stability than its quality.Stability indexes revealed that most soils got stable aggregate-state after 200 days of incubation,with a minimum or no change in aggregate stability.In this study,a soil of Qurecus wutaishansea（QW）was amended with the fragment of Oak litter（<2.5 mm）by different qualities（leaves and branches）and quantities(0,10,20and 30 g C kg^-1 soil)with five replications.These soils were incubated for about three yearsin the Ziwuling mountain area,aiming to assess the effect of litter decomposition on soil aggregate stability and formationby stability indexes,including E_A.Stability indexes,especially,E_Aexplored the effect of different quantities and qualities of litter addition in the soils and aggregate formation mechanism.Generally,soils with 30 g C kg^-1 soil addition were more stable than soil without litter addition（CK）.Results of E_Arevealed that soil with high leave carbon(HCL,30 g leaves-C kg^-1soil)was more stable thansoil with high branchcarbon(HCB,30 g branches-C kg^-1).Results of MWD and GMD indicated that,after200 days of incubation,soils got their stable aggregate stability state.Overall,stability results of E_Aand MWD unfolded that high litter carbon amended soils（HCL,HCB）were significantly stable than control soil（CK）.Contrarily,there was a non-significant difference of aggregate stability between soils amended with leaves input than soils with branches.In conclusion,the effect of litter quantity(30 g branches-C kg^-1)on aggregate stability and aggregate formation was more evident than litter quality（leaves and branches）.4.E_Awas found to be affected significantly by soil pH,soil organic carbon（SOC）,total carbon（TC）and carbon to nitrogen ratio（C/N）during the formation of soil aggregate.Results indicated that pH,SOC,TC,and C/N were more strongly associated（p<0.01）with E_A than MWD,GMD,and K index.Among the wet sieving-based indexes,the GMD was proved less effective index due to limited association with stability-influential soil properties.Among the soil properties,SOC and C/N significantly affect apparent stability and aggregate stability of soils.5.There were strong associations of E_Awith the bacterial community at phylum and genus level than soil fungi during the formation of soil aggregates.The significantly positively correlated（p<0.01,0.05）bacterial phyla with E_Awere Chloroflexi,Myxococcota,Desulfobacterota,Verrucomicrobiot and Planctomycetota.Similarly,highly associated bacterial genera include Anaeromyxobacter,JG30-KF-CM45,Bradyrhizobium and Citrifermentans.Bacterial groups A4b and Citrifermentans were also positively correlated with MWD and GMD（p<0.01）.Phylum Ascomycota and genus Un-Hypocereals were positively associated with E_A（p<0.05）.Wet sieving-based indexes（MWD,GMD and K）were not closely associated with any phylum of the fungi and bacteria.Overall,these results indicated that the above-mentioned five bacterial and one fungal group were effective for the stability of<2 mm sized-aggregates（E_A）in this study.Furthermore,groups A4b and Citrifermentans were equally supportive for the aggregate stability of aggregates sized<2mm and<5 mm（MWD,GMD）.