Dynamic Mechanism Of Soil Erodibility And Soil Erodibility Calculation Model

Soil erodibility is a key indicator to evaluate soil susceptibility to erosion and crucial forpredicting soil loss and evaluating its environmental effects. Mechanism study on soilerodibility, which plays an important role in the domain of soil erosion, provides theoreticalfoundation for soil loss quantification and prediction. To meet needs of implementing soilconservation practices in China, especially for one of the serious eroded area--the black soilregion in NE China, a systematic study on the dynamic mechanism of soil erodibility and itscalculation was conducted. The experimental investigations on dynamic mechanism of soilerodibility and its indicators were studied by field investigating, in-situ monitoring, rainfallsimulation experiments, combining with laboratory physical-chemical analysis and statisticalanalysis. The key soil erodibility factors and their suitable indicators were proposed; theintrinsic mechanisms and variation characteristics of soil erodibility key indicators (e.g., soiltexture, soil organic matter content and soil aggregate stability), which caused the variationsof soil erodibility, were exposited. Moreover, the applicability of EPIC (Erosion ProductivityImpact Calculator), USLE (Universal Soil Loss Equation), RUSLE2(Revised Universal SoilLoss Equation), and Dg models (soil erodibility estimator based on geometric mean diameterof the soil particles) were assessed, at the watershed scale and regional scale; revised modelsfor the aforementioned models were also given. A soil erodibility estimator with highpredicting precision for China was also established and validated. Main results of this studywere as follows:1) A set of soil erodibility key factors and corresponding indicators were proposed byusing a combined analysis of sensitivity analysis, correlation analysis, path analysis, andfactor dimension reduction. The key factors reflecting soil erodibility behaviors included: soiltexture key factor, soil structure key factor, soil shear-strength key factor, and soil organicmatter key factor. Among them, soil texture key factor was the fundamental index for soilerodibility, and was decisive for quantifying the K value. While, it’s also found that soilorganic matter key factor played an assisting role for the other three soil erodibility factors,which was the same path for the other key factors to affecting soil erodibility. Moreover, thecharacterized indicators for the soil erodibility key factors were indicated, which containedCLA (clay content), Dg (geometric mean diameter of the soil particles), WSA>0.25(>0.25mm water-stable aggregate content), MWDWS(mean weight diameter of soil aggregate by shakingtreatment in LB method), soil shear-strength (τ), soil bulk density (ρ), MWDSW(mean weightdiameter of soil aggregate by slow wetting treatment in LB method), and SOM (soil organicmatter content). An adjustable combination of the aforementioned characterized indicatorswas suggested, by considering with the specific research purpose and the ease of indicatorobtaining.2) Dynamic characteristics of soil texture key factor were analyzed, and a more efficientsoil texture expression method was suggested. Based on Shirazi’s method, a revised soiltexture expression method containing Dg and δg was proposed, which has the feature tonormalizing and conversing soil particle distribution information. The revised Shirazi’smethod was improved by extending its extensionality for different soil texture taxonomies,which make sure that the revised method can be applied for Chinese texture taxonomy andother optional classification. The related error was reduced to-4.83%and19.34%for siltcontent and clay content respectively, when converted from the optional classification to theUSDA soil texture taxonomy. Results also showed that clay content represent a significantfluctuations increase trend along the increase of soil thickness (A horizon) at a watershedscale; while, for the slope scale, it showed that the upper slope and slope shoulder area areeroded area, and the slope toe is found as a materials imported area (i.e., depositional area).Moreover, for the time scale, clay content showed a significant decrease trend along the longreclamation time and it did not reach the level to change soil texture.3) Spatial and temporal variations of soil organic matter key factor were clarified. Asignificant spatial varies of A horizon thickness was found for both watershed scale and slopescale. At watershed scale, it showed that A horizon thickness increased from the upper reachto the down reach of Binzhouhe Basin, and the thickness changed from23to51cm; at aslope scale, it indicated that the thickness of A horizon was thicker at slope toe than the toparea of a slope, and a few soil profiles with berried soil phenomenon were also been found atthe slope toe area. We also exposited temporal variations of soil organic matter at differenttime scale. Under a single rainstorm or few years time scale (less than five years in thisresearch), soil organic matter content represent a slight decrease trend, and for a long timeseries (30to100years) a significant decrease was found. Moreover, we found that <0.25mmmicro-aggregate was the main transport particle size under a short slope-length and straightblack soil slope. Soil organic matter was eroded and transport combined with themicro-aggregate, and the loss rate was13.2to35.6g/m2for a single rainfall event.4) The breakdown mechanism of soil aggregate was indicated by applying the threetreatments (i.e., slow wetting treatment, fast wetting treatment, and stirring after pre-wetting treatment) of LB method and Yoder method. We found that clay content has a significanteffect on soil aggregate stability, aggregate stability increased with the increase of claycontent. Impact of initial soil moisture on aggregate stability was also found; it showed thatthe slaking effect was weakened along the increase of soil initial moisture. The decreaseprocess of slacking effect showed obviously periodic behaviors, three stages were divided to I)high-intensity slaking stage, with the initial soil moisture less than10%, II) sharplydecreasing stage, with initial soil water content between10%and20%, and III) stable stage,whose slacking effect nearly die out, with initial moisture larger than20%. The dominant soilaggregate breakdown mechanism were slaking and swelling for the research area, and theorder of their affected degree on breaking soil aggregates was as follow: slaking> swelling>mechanical breakdown effect.5) We investigated the mechanism of soil aggregate stability’s variation by exploringfreeze-thaw cycles’ impact on soil aggregate stability and micro-aggregate distribution.Results showed that the role of freeze-thaw cycles on aggregate stability was realized by theinfluences of initial aggregate water content and numbers of freeze-thaw cycles. It indicatedthat stabilities for3-5mm and1-2mm soil aggregates decreased with the increase of initialwater content. Moreover, freeze-thaw serious destroyed soil aggregate stability at verybeginning (i.e., freeze-thaw cycles less than3times); on the contrary, freeze-thaw cyclesturned to increasing soil aggregate stability along the increase times of freeze-thaw cycles.Different frozen temperature (-10℃and-25℃) did not show significant difference tofreeze-thaw cycles impact.6) We explored the dry-wetting cycles’ impact on soil aggregate stability and itsmicro-aggregate distribution, and found threshold value for the dry-wetting effect. Resultsshowed that serious damage on aggregate stability happened at the beginning of dry-wettingcycles, and the damage degree was more obvious on the large soil aggregate (i.e.,3-5mmaggregate). It’s also found that dry-wetting cycles promoted <0.2mm micro-aggregateconvert to0.2-1mm aggregate for all initial soil aggregate size. And the promotion wascomplicated by the accumulation of dry-wetting effect; there was a threshold as3timesdry-wetting cycles, when it reached the threshold the degree of dry-wetting impact onaggregate stability reducing sharply.7) A Chinese soil erodibility estimator was established, based on a Chinese soilerodibility database. A comprehensive assessment for USLE, RUSLE2, EPIC, and Dg models’applicability was conducted at watershed scale and regional scale. Results showed thatRUSLE2was suitable for the research watershed, and the USLE and Dg models can beapplied directly for the black soil region and the Loess Plateau, respectively, without calibration. Based on the Chinese soil erodibility database, a multiple regression, obtained bythe nonlinear best fitting techniques, yielded a significant relationship (DG-OM model),explaining K values with a combination of Dg (geometric mean diameter) and OM (soilorganic matter). Moreover, soil erodibility values for the four main water erosion areas inChina were in the order as: the Loess Plateau> the black soil region (NE China)> the purplesoil region (SE China)> the red soil region (Quaternary red clay, South China).