Effects of moss-dominated biocrusts on soil detachment by overland flow in the Three Gorges Reservoir Area of China

Biological soil crusts(biocrusts)are important landscape components that exist in various climates and habitats.The roles of biocrusts in numerous soil processes have been predominantly recognized in many dryland regions worldwide.However,little is known about their effects on soil detachment process by overland flow,especially in humid climates.This study quantified the effects of moss-dominated biocrusts on soil detachment capacity(Dc)and soil erosion resistance to flowing water in the Three Gorges Reservoir Area which holds a subtropical humid climate.Potential factors driving soil detachment variation and their influencing mechanism were analyzed and elucidated.We designed five levels of coverage treatments(1%–20%,20%–40%,40%–60%,60%–80%,and 80%–100%)and a nearby bare land as control in a mossdominated site.Undisturbed soil samples were taken and subjected to water flow scouring in a hydraulic flume under six shear stresses ranging from 4.89 to 17.99 Pa.The results indicated that mean Dc of mosscovered soil varied from 0.008 to 0.081 kg m^-2 s^-1,which was 1.9 to 21.0 times lower than that of bare soil(0.160 kg m^-2 s^-1).Rill erodibility(Kr)of mosscovered soil ranged from 0.0095 to 0.0009 s m^-1,which was 2 to 20 times lower than that of bare soil(0.0187 s m^-1).Both relative soil detachment rate and Kr showed an exponential decay with increasing moss coverage,whereas the critical shear stress(τc)for different moss coverage levels did not differ significantly.Moss coverage,soil cohesion,and sand content were key factors affecting Dc,while moss coverage and soil bulk density were key factors affecting Kr.A power function of flow shear stress,soil cohesion,and moss coverage fitted well to estimate Dc(NSE=0.947).Our findings implied that biocrusts prevented soil detachment directly by their physical cover and indirectly by soil properties modification.Biocrusts could be rehabilitated as a promising soil conservation measure during ecological recovery to enhance soil erosion resistance in the Three Gorges Reservoir Area.

Effects of shrub-grass patterns on soil detachment and hydraulic parameters of slope in the Pisha sandstone area of Inner Mongolia,China

The characteristics of soil holding capacity for different shrub-grass patterns are important to research the mechanisms regulating vegetation on slopes.The objective of this study was to describe the characteristics and mecha-nisms of soil erosion and hydraulic parameters under differ-ent vegetation patterns in the Pisha sandstone area of Inner Mongolia on lands of 8°slope gradient.We carried out field scouring experiments on five different shrub-grass patterns as treatments,viz no shrubs(GL),shrubs on the upper part of the slope(SU),middle part of the slope(SM)and lower part of the slope(SL).We designated bare slope(BL)as the control.We employed three different water flow rates(15,20,30 L·min^(−1)).Our results showed that the contribution of plant root systems to slope sediment reduction ranged from 64 to 84%.The root systems proved to be the main contributing factor to reduction of erosion by vegetation.The relationship between soil detachment rate,stream flow power,and flow unit stream power under different scouring discharge rates showed that soil detachment declined in rank order as:BL>GL>SU>SM>SL.The SL pat-tern had the lowest soil detachment rate(0.098 g·m^(−2)·s^(−1)),flow stream power(2.371 W·m^(−2)),flow unit stream power(0.165 m·s^(−1))and flow shear stress(16.986 Pa),and proved to be the best erosion combating pattern.The results of decision coefficient and path analysis showed that stream power was the most important hydraulic parameter for describing soil detachment rate.The combination of stream power and shear stress,namely Dr=0.1ω−0.03τ−0.56(R^(2)=0.924),most accurately simulated the soil detachment characteristics on slopes.Our study suggests that the risk of soil ero-sion can be reduced by planting shrub-grass mixes on these slopes.Under the conditions of limited water resources and economy,the benefit of sediment reduction can be maxi-mized by planting shrubbery on the lower parts of slopes.

Effects of nitrogen addition on the soil detachment in the typical grasslands of the Loess Plateau

Nitrogen deposition will alleviate the nitrogen limitation in terrestrial ecosystems and greatly affect vegetation growth,thereby soil erosion.It is important to clarify the effects of nitrogen addition to the plant roots and soil properties on the soil erosion process.A nitrogen addition experiment was conducted in the grassland dominated by Bothriochloa ischaemum(Linn.)Keng(BI),which has received 0,2.5,5,and 10 g N m^(-2) yr^(-1)(N_(0),N_(2.5),N_(5)and N_(10),respectively)for three consecutive years.Then,a total of 150 undisturbed soil samples were collected(including bare soil control)and subjected to flowing water to test their soil detachment capacities under six shear stress levels(10.2 Pa to 29.9 Pa).Three-year nitrogen addition increased the soil bulk density,soil cohesion and nitrate nitrogen while decreasing the saturated hydraulic conductivity,soil water-stable aggregates,soil organic carbon,total nitrogen and ammonium nitrogen.The root mass density and root diameter decreased with nitrogen addition.And the root length,surface area and volume density of the N_(0) and N_(5) treatments were larger than those of the other treatments,while the plant roots were significantly inhibited by N_(10).Additionally,the soil detachment capacity(D_(c))and rill erodibility(K_(r))of the N_(0) and N_(5) treatments were significantly less than those of the N_(2.5) and N_(10)treatments,of which the Dc(0.020 kg m^(-2) s^(-1))of the N_(0) treatment was 69.0%,24.3%and 66.8%less than that of the N_(2.5),N_(5) and N_(10) treatments,respectively.The Kr of the N_(0) treatment was 0.0012 s m^(-1),which was 72.1%,25.0%and 70.0%less than that of the others.This study implies that an increase in nitrogen addition likely exacerbates soil erosion in the early(approximately 2.5 g N m^(-2) yr^(-1))and late phases(more than 10 g N m^(-2) yr^(-1)).However,when the nitrogen addition rate is approximately 5 g m^(-2) yr^(-1),soil erosion may be inhibited because of the responses of the plant roots and soil to nitrogen addition.

Apportioning above-and below-ground effects of moss biocrusts on soil detachment by overland flow in a subtropical climate

Biocrusts affect soil detachment through above-ground(top crust’s surface covering)and below-ground(sub-crust’s binding and bonding,B&B)effects,which might vary with biocrust development or coverage.However,these effects in humid climates are still unclear.This study was conducted to apportion and quantify the surface covering and B&B effects of moss biocrusts with five coverage levels(1%–20%,20%–40%,40%–60%,60%–80%,and 80%–100%)on soil detachment by overland flow in a subtropical humid climate.Two treatments with one being intact moss crusts and one removing the aboveground moss tissues were designed for each coverage level,and bare soil was used as the baseline.The results indicated that soil detachment capacity(Dc)and rill erodibility(Kr)decreased with biocrust coverage.After removing the above-ground moss tissues,the impeding effect of biocrusts on soil detachment weakened,but still increasing soil erosion resistance relative to bare soil.For intact crust,Dc was reduced by 50%–95%compared with bare soil,wherein 36%–55%and 14%–40%were attributed to the surface covering and B&B,respectively.The top crust contributed more than sub-crust to the soil detachment reduction,which were related to but not linear with biocrust coverage.When biocrust coverage reached mid-to-higher level(40%–100%),both top crust and sub-crust steadily contributed to soil detachment reduction with 60%and 40%,respectively.The findings advance a better understanding of the influencing mechanism of biocrusts on soil erosion in humid climates and highlight the importance of saving biocrusts as ecosystem functions.

Effect of gravel content on the detachment of colluvial deposits in Benggang

Accurately calculating detachment capacity is the most fundamental issue when establishing a soil erosion process model.Colluvial deposits of Benggang are typical soil-gravel mixtures,whereas the understanding of the soil detachment of colluvial deposits is limited.This work investigated the effects of the gravel contents on the soil detachment capacity of colluvial deposits and its hydrodynamic mechanism.The colluvial sample was collected in Anxi County,Fujian Province,Southeast China,and a small-sample scouring test was used.The slope steepness ranged from 18%to 84%,unit discharge ranged from 0.56×10^(-3)to 2.22×10^(-3)m^(2)s^(-1),and gravel content ranged from 0%to 70%.The results indicated that the gravel content is the primary factor that influences the detachment capacity,followed by the discharge and then the slope.The detachment capacity trend with the gravel content varied over different slopes and discharges.Stream power represents the best hydrodynamic parameter for modelling the detachment capacity of colluvial deposits and can be used to establish a fitting equation for the colluvium together with the mean weight diameter(MWD)(Nash-Sutcliffe efficiency(NSE)=0.96).As the gravel content increased,the soil erodibility parameters increased several folds,in some cases more than 10 folds,mainly because the soil shear strength decreased gradually.Meanwhile,as the gravel content increased,the gravel specific surface area increased,the obstruction of gravel to runoff increased,and the energy needed for runoff to overcome gravel obstruction increased,leading to 2-3 folds higher critical shear stress of runoff for soilgravel mixtures compared with pure soil.In summary,gravel can influence the detachment capacity by changing the soil properties,and the gravel content also affects the relationship between soil detachment capacity and the hydrodynamic parameters.These findings deepen the understanding of the influence of gravel on soil erosion and provide a basis for establishing a soil erosion process model in colluvial deposits.

Seasonal changes of soil erosion and its spatial distribution on a long gentle hillslope in the Chinese Mollisol region

Understanding seasonal soil erosion and deposition rates and their spatial distribution along sloping farmlands are necessary for erosion prediction technology and implementing effective soil conservation practices.To date seasonal change of soil erosion and soil redistribution on long gentle hillsiopes are not fully quantified due to the variable erosive forces in different seasons.A multi-tracer method using rare earth elements(REE)was employed to discriminate seasonal changes of soil erosion and its spatial distribution on a sloping farmland driven by snowmelt runoff,wind force and rainfall-runoff.A long-slope runoff plot with 5 m wide and 320 m long located in the typical Mollisol region of China was divided into eight segments,each of which was 40 m long and tagged with one of eight REE oxides.The spot method of a partial-area tagging scheme was employed and a grid-based layout was used for REE application.Results showed that annual soil erosion rate was 3251.01 km^(-2)for the whole runoff plot,in which snowmelt runoff erosion contributed 537.3 t km^(-2),wind erosion 363.11 km-2 and rainfall-runoff erosion 2350.6 t km^(-2).Surface runoff is the main external erosive force of hillslope soil erosion,accounting for 88.8%of the total annual soil loss.Furthermore,for the eight slope segments of the 320-m long hillslope,the sediment transport ratios of each slope segment caused by snowmelt runoff and rainfall-runoff erosion were more than 23.5%and 34.7%,respectively.The results will enrich the understanding of seasonal soil erosion on long hillsiopes.