Effects And The Mechanism Of Biological Soil Crusts On Water Erosion Prevention On The Loess Plateau

Biological soil crusts(biocrusts)that constituted by pioneer organisms such as cyanobacteria,mosses and lichens extensively developed in the Loess Plateau region due to the “Grain for Green” ecological project.They have significant roles in improving soil physical and chemical properties,protecting soil from erosion and promoting ecosystem restoration.Stabilizing soil and preventing soil loss is the most important ecological role of biocrusts in many ecoregions.Accordingly,to clarify the mechanism of biocrusts on erosion prevention is an urgent problem to be solved.The development and distribution characteristics of biocrusts were analysed after a profound field survey,and soil physical and chemical properties were measured.Then soil erodibility(K value)of biocrusts was calculated through EPIC model.In addition,simulated rainfall and runoff scouring under control conditions were used to determine the effects of biocrusts on soil anti-erodibility and anti-scourability,respectively.The study revealed the mechanism of biocrusts on resisting water erosion on Loess Plateau.Simultaneously,the results of the study elucidated the pattern and degree of biocrusts preveting soil loss.The dominated effect factors of biocrusts on water erosion were then analyzed.Finally,the relationship between biocrust development and soil loss was determined.The results will promote the establishment of water erosion predicting model,considering biocrusts.The main conclusions were as follows:(1)Biocrusts established extensively in the Loess Plateau region,and covered up from 55% to 80% of the land area.In this region,the initial successional stage of biocrusts was cyanobacterial crust,and then biocrusts were dominated by mosses.The growth of mosses could express the development of biocrusts.The distributions of biocrusts were significantly affected by precipitation,soil texture,slope aspect,slope gradient,slope postion and vascular plants,which had the greatest influence on the distribution of moss crusts.The coverage of biocrusts increased significantly along a precipitation gradient decreasing from 550 mm to 250 mm per annum.Moss crust coverage increased with the decrease of precipitation,while the coverage of cyanobacterial crust decreased.The lichen coverage was only 10% in the study area.The coverage of mosses on sandy soil was twice as much as on loam soil,while cyanobacteria on loam soil were 27%higher than that on sandy soil.Aspect effects expressed on slopes with mosses becoming more common on north slopes,while cyanobacterial being common on south slopes.The lichen distribution on north and south slopes had no differences.Mosses and cyanobacteria were more distributed on the slopes of small gradient,while lichen preferred steep slopes.Mosses were more distributed on the top and bottom of slopes.Cyanobacteria distribution had no differences on different slope position.Vascular plants profoundly influenced moss distribution,which declined significantly with increase of vascular plants.The coverage of cyanobacterial crust decreased logarithmically,while the coverage of moss crust increased exponentially with the increase of rehabilitation years.Biocrusts were dominated by mosses after restoration for 5 years.(2)Soil organic matter(SOM),total nitrogen(TN),fine soil particles and soil cohesion increased,while soil bulk density was reduced significantly due to the estabalishment of biocrusts.The contents of SOM and TN of biocrusts in the later successional stage(moss biomass was 8.84±0.27 g/dm2)increased by 1.6 and 1.3 times,respectively,compared to cyanobactieal crust in the earlier successional stage(cyanobacterial bimass was 3.67±1.10 mg/g).Fine soil particles(<0.01 mm)content increased with the development of biocrusts,while coarse soil particles(0.05-0.25 mm)content decreased.Soil bulk density dereased by 14% in the later successional stage of biocrusts compared with that in the earlier stage.Soil cohesion of biocrusts was about 6-7 times as much as the soils,biocrusts removed in the successional stages of biocrusts.(3)Soil erodibility decreased siganificantly due to the development of biocrusts,which was attributed to the very strong effect of moss crust on SOM.Soil erodibility reduced observably with the increase of moss biomass,and it dcreased by 21% in the later successional stage of biocrustscompared with that in the earlier stage.Consequently,the development of moss crust could express the variation of soil erodibility of biocrusts in the Loess Plateau region.(4)The establishment of biocrusts can enhance the soil surface resistance capacity to raindrop through reducing raindrop kinetic energy by their cover.Soil loss was thus decreased.The effect of biocrusts on soil anti-erodibiltiy was reflected in the composition and biomass of the biocrust community.The cumulate raindrop kinetic energy of biocrusts enduring increased remarkably with the increase of moss cover and biomass,soil loss thus decreased notably.Biocrusts,including more than 0.67 g/dm2 and 33% of moss biomass and cover and 17.9 mg/g of cyanobacterial biomass,could protect soil from raindrop under extreme rainfall condition.Simultaneously,cyanobacterial curst protected soil from raindrop in company with moss crust,especially in the early developmental stage of biocrusts.When the cover ratio of moss crust to cyanobacterial crust was more than 0.6,soil loss tended to zero.Moss cover could express the variation of soil anti-erodibiltiy of biocrusts in the Loess Plateau region.(5)Soil anti-scourability increased dramatically due to the establishment of biocrusts,which was reflected in the composition and biomass of the biocrust community.Biocrusts enhanced soil anti-scourability directly by their cover and indirectly through modification of soil properties that affect soil anti-scourability.Moss cover could express the variation of soil anti-scourability.Soil anti-scourability increased clearly along with the development of biocrusts.Biocrusts,including more than 1.32 g/dm2 and 36% of moss biomass and cover and 24.8 mg/g of cyanobacterial biomass,could resist the runoff scouring under extreme rainfall conditions.Simultaneously,cyanobacterial curst protected soil from runoff in company with moss crust,especially in the early developmental stage of biocrusts.When the cover ratio of moss crust to cyanobacterial crust was more than 0.6,biocrusts could prevent the runoff erosion under extreme rainfall conditions.(6)The effect patterns on water erosion were distinct in different erosion stages and biocrust development stages.The key factors of biocrusts on water erosion were the composition and biomass of the biocrust community.And moss cover could express the capacity of soil erosion resistance better than biomass of mosses and cyanobacteria.In the stage of raindrop erosion,biocrust cover(C factor)contributed 60% of the soil raindrop erosion prevention.And in the runoff scouring stage,biocrust cover and modification of soil properties(K factor)by biocrusts contributed 50% of soil resisting runoff scouring.Biocrusts in the earlier development stage reduced water erosion through modification of soil properties,which could reduce 65.2% of soil loss by water.Cyanobacterial crust reduced water erosion through its cover.Moss crust in earlier stage prevented water erosion through the modification of soil properties,while in the later stage their cover was the dominated factor.