Effects Of Near Surface Characteristics Of Typical Grassland On Slope Soil Erosion In Loess Plateau

Since 1999,in order to prevent serious soil erosion,large-scale vegetation restoration has been carried out in the Loess Plateau,and the ecological environment has been significantly improved.Vegetation is one of the effective measures to control soil erosion.During the process of vegetation restoration,near surface characteristics such as canopy,litter,biological crust and root system have changed significantly.However,the mechanism of these changes to control soil erosion is still unclear.Based on this,this paper selected typical grasslands（Bothriochloa ischaemum（L.）Keng and Artemisia gmelinii Web.）in the middle and late stage of vegetation succession in the Loess Plateau,and set up five kinds of near surface feature treatments by layer by layer elimination method.Each treatment was arranged with two runoff plots for repeated treatment.The dynamic changes of natural rainfall（a total of 58 rainfalls,including 29 effective rainfalls）,vegetation near surface features and soil physical and chemical properties were observed and recorded for two consecutive years,This paper systematically studied the near surface characteristics,soil physical and chemical properties and runoff and sediment yield characteristics of typical grassland,clarified the response mechanism of soil erosion process,quantified the relationship between canopy,litter,biological crust,plant roots and runoff and sediment,and determined the contribution rate of runoff and sediment reduction of near surface factors of grassland,It provides theoretical and data support for the systematic evaluation of soil and water conservation benefits of grassland in the Loess Plateau and the construction of soil erosion prediction model suitable for the Loess Plateau.The main conclusions are as follows:（1）The soil physical and chemical characteristics of Bothriochloa ischaemum（L.）Keng and Artemisia gmelinii Web.grassland fluctuated in a certain extent,but there was no significant change.As a whole,the bulk density and porosity of the two grasslands were basically the same.The average bulk density of Bothriochloa ischaemum（L.）Keng and Artemisia gmelinii Web.were 1.10 g/cm³ and 1.12 g/cm³,respectively,with a range of0.99～1.19 g/cm³ and 1.05～1.17 g/cm³;The average total porosity is 58.83%and 57.75%respectively,and the variation range is 54.99%～62.53%and 55.89%～60.45%respectively;The average capillary porosity is 45.40%and 46.54%respectively,and the variation range is42.29%～50.78%and 43.71%～49.46%respectively.The soil saturated hydraulic conductivity of Bothriochloa ischaemum（L.）Keng and Artemisia gmelinii Web.affected by root system was higher,the average value was 0.27 cm/min and 0.17 cm/min,the range of change was 0.18～0.39 cm/min and 0.11～0.27 cm/min respectively.Due to the differences of root morphology and distribution,the soil cohesive force of Bothriochloa ischaemum（L.）Keng（8.04 k Pa）was higher than that of Artemisia gmelinii Web.（7.93 k Pa）,and the variation range of the two was 5.51～9.96 k Pa and 4.97-9.93 k Pa,respectively.（2）The near-surface characteristics of grassland increased with the change of seasons in different degrees.Coverage,height,aboveground biomass and root biomass of Bothriochloa ischaemum（L.）Keng community were 48.44,67.11,76.55%,19.5,40.8,69.9 cm,265.0,284.0,365.0 g/m²,4.8,4.9 and 5.3 kg/m³ respectively in July,August and September The coverage,height and biomass of Artemisia gmelinii Web.community were14.97,45.22,54.5%,62.6,67.8,70.1 cm,180.3,252.2,272.1 g/m²,2.2,2.7 and 3.0 kg/m³respectively.The accumulative coverage of litter,biomass and crust cover were 14.9%～34.3%,69.4～198.6 g/m² and 77.1%～94.8%,respectively.In addition,the above-ground biomass of Bothriochloa ischaemum（L.）Keng and Artemisia gmelinii Web.were increased linearly with their coverage and plant height,and the above-ground biomass was increased linearly with litter coverage and biomass.Because of the difference of plant properties and soil microenvironment,the near-surface characteristics of the two kinds of grassland are also different.（3）The interaction of grassland near surface characteristics significantly improved the anti erosion effect of slope.With the increase of near surface elements,the runoff and sediment yield of different near surface treatments（T1～T4）decreased gradually.In terms of cumulative runoff,T1-T4 treatments decreased 81.3%,87.5%,53.2%and 17.9%compared with T0 of Bothriochloa ischaemum（L.）Keng respectively,and 49.4%,19.1%,5.7%and0.6%compared with T0 of Artemisia gmelinii Web..For the cumulative sediment yield,it decreased by 97.2%,91.9%,64.7%and 43.9%in Bothriochloa ischaemum（L.）Keng plot,and by 58.3%,17.3%,6.1%and 2.5%in Artemisia gmelinii Web.plot.In addition,the runoff and sediment characteristics of grassland slope are affected by near surface characteristics,soil properties and rainfall characteristics to varying degrees.Among them,the sediment yield per unit area and runoff depth of the two grasslands increased with the maximum 30 minute rainfall intensity(I₃₀)in a power function（P<0.01）.However,due to different rainfall intensities and no significant changes in vegetation and soil in short-term monitoring,there is no significant correlation between runoff and sediment yield and vegetation and soil as a whole.Finally,the regression analysis results showed that the slope sediment yield could be characterized by the power function of soil cohesive force,rainfall intensity and slope runoff((Sed=10^3.735coh^-4.490Q^1.076I₃₀^0.298,R²=0.710,P<0.001,NES=0.846)).（4）The current reduction of two typical grassland is different from that of the surface elements.The results showed that the biological skin flow reduction in the area of Bothriochloa ischaemum（L.）Keng was negative（-26.00 L）,which promoted the flow,while other factors were all shown to inhibit the flow.The change range of the runoff reduction in the sample land was 77.74～146.77 L,and that of Artemisia gmelinii Web.was 7.73～56.69L.The near surface elements of both grassland can significantly inhibit sediment production,and the range of sediment reduction is 0.09-0.93 kg and 0.62～7.19 kg,respectively.Further analysis shows that i30 is the key factor affecting the sediment reduction of grassland near surface elements.The flow reduction of the canopy,litter,root system,crown and litter of Artemisia annua increased with the power function of I₃₀（P<0.01 or P<0.05）,while the sediment reduction of all the near surface elements of the two grassland was increased by power function（P<0.01 or P<0.05）.In addition,regression analysis results show that the soil loss ratio is inversely proportional to vegetation coverage,crusty coverage,litter biomass and root biomass coefficient.（5）On the whole,the two typical grasslands can effectively inhibit the process of soil erosion,but the relative contribution of runoff and sediment reduction of different near surface elements is different.For reducing runoff,the total contribution of Bothriochloa ischaemum（L.）Keng plot was 82.1%,of which the contribution rate of biological crust was–5.4%,and the other factors were canopy（46.8%）>litter（27.6%）>root（13.2%）.The total contribution of runoff reduction of Artemisia gmelinii Web.was 56.1%,and the contribution of each factor was canopy（25.7%）>litter（23.8%）>crust（4.3%）>root（2.3%）.For sediment reduction,the total contribution of Bothriochloa ischaemum（L.）Keng and Artemisia gmelinii Web.were 99.4%and 97.5%respectively.Among them,the contribution of different factors to sediment reduction of Bothriochloa ischaemum（L.）Keng plot from small to large was canopy（32.6%）>litter（25.8%）>crust（22.5%）>root（18.5%）.The performance of Artemisia gmelinii Web.plot was crust（29.1%）>litter（26.8%）>canopy（24.3%）>root（17.3%）.In addition,due to the difference of vegetation community characteristics,the overall runoff reduction benefit of Bothriochloa ischaemum（L.）Keng sample plot was 46.3%higher than that of Artemisia gmelinii Web.sample plot,and the sediment reduction benefit was 1.9%higher than that of Artemisia gmelinii Web.sample plot.