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.

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.

Effects of physical crust on soil detachment by overland flow in the Loess Plateau region of China

Physical soil crust(PSC),a key component of surface soil structure,exists extensively in loess areas.PSC is considered to have a significant effect on soil detachment processes.However,the long-term effects and the corresponding mechanisms of PSC on soil detachment by overland flow are still not well understood,especially in natural environments.To investigate temporal variation in soil erosion resistance and the underlying factors during PSC formation,an 8×8-m soil plot was exposed to natural conditions in the Loess Plateau over a 524-day period spanning two rainy seasons and a winter between them.A flume test was conducted to determine soil detachment capacity(Dc)under six designed flow shear stress levels(5.66-22.11 Pa)using crusted(SC)and non-crusted(NSC)soil samples at different PSC development stages.Subsequently,two soil erosion resistance parameters,rill erodibility(K_(r))and critical shear stress(τ_(c)),were calculated.Over time,in the SC and NSC treatments,K_(r)decreased from 0.516 to 0.120 s m^(-1)and 0.521 to 0.223 s m^(-1),respectively,whileτ_(c)increased from 0.49 to 4.42 Pa and 0.26-2.46 Pa,respectively.Variation in soil erosion resistance was rapid in the first one to two months,and then slowed down,with slight fluctuations afterwards.In the SC treatment,K_(r)was 42%lower andτ_(c)was 67%greater than those in the NSC treatment.Soil properties changed greatly for both treatments.SCT increased from 0 to 7.09 mm in the SC treatment.Coh increased from 2.91 to 9.04 kPa and 3.01-4.78 kPa in SC and NSC treatments,respectively.Both soil erosion resistance parameters could be well predicted by SCT and Coh in the SC treatment(R^(2)≥0.82),while their best predictor was Coh in the NSC treatment(R^(2)≥0.90).The results demonstrate that PSC formation enhances soil erosion resistance in the soil detachment process in the loess region under natural conditions.Our study revealed the important role and complexity of PSC in the process of soil erosion,and provided theoretical and data support for accurate understanding and prediction of soil erosion.