Mechanism Of Soil Water-solute Transport And Preferential Flow Characteristic In A Semi-arid Grassland

To explore the mechanism of soil water-solute transport and preferential flow in a semi-arid grassland can further understand grassland eco-hydrological process.In this study,we investigated the soil and plant properties in a grazing exclusion（19 years,GE）,cold-season grazing（19 years,CG）,and free-grazing grassland（19 years,FG）in a semi-arid grassland in Inner Mongolia.Dye tracer infiltration was adopted to obtain the water infiltration patterns from different types of grazing management.Finally,root biomass and root morphological traits were measured in a field experiment.The influences of grazing management on soil properties,vegetation characteristics,root morphology traits,solute-water migration and preferential flow behavior were established to determine the mechanism of soil water-solute transport and preferential flow characteristic in a semi-arid grassland.The results could provide a scientific basis for the restoration and management of grassland vegetation in semi-arid grassland.The main conclusions of our investigations are as follows:（1）GE directly inhibited hoof trampling and excessive herbivory and enhanced plant density and root biomass.Plant height,coverage,aboveground biomass,root diameter and diversity index in the GE were significantly higher than those in the CG and FG grassland.The total soil porosity and capillary porosity of grassland under different grazing management were in the order of GE>CG>FG.MWD was significantly positively correlated with>0.25mm water-stable aggregate content and total porosity,and negatively correlated with soil bulk density.Environmental factors such as soil water content,soil bulk density,porosity,organic carbon,soil mechanical composition and aggregate stability were significantly correlated with vegetation community characteristics.（2）The restoration of soil pore system can promote soil water infiltration,and the saturated water conductivity ranked a descending sequence as GE（0.35mm/min）>CG（0.25mm/min）>FG（0.05mm/min）.In GE site,>0.5mm macropores determined water transport,<0.25mm and 0.33～0.5mm pores determined water transport in CG site,<0.33mm pores determined water transport in CG site.Differences in infiltration capacity lead to different temporal and spatial variation patterns of soil water under different grazing management site,grazing management significantly affected the response rate and replenishment amount of soil water content during rainfall.The soil water content of CG and GE remained at a high level in early April,while the soil surface water content showed a downward trend with the progress of plant growing season,and soil water was obviously replenished when rainfall events occurred during the period.The intensity and depth of soil water replenishment were closely related to the frequency and intensity of rainfall events.In FG grassland,the soil water content in the deep layer（25～40cm）was less affected by rainfall.The response rate of different grazing management grassland to rainfall ranked a descending sequence as GE>CG>FG.In light rain events,soil water response is related to many factors such as soil water content and rainfall duration.（3）Preferential flow occurred in all the grasslands in the study area,but the degree and type of preferential flow were different.Homogeneous matrix flow was mostly observed in the topsoil layers,which accounted for 15.01%,19.20%,and 22.11%at the GE,CG,and FG sites,respectively,for the 15-mm rainfall amount.Inversely,macropore flow,especially with mixed interaction and low interaction,dominated the preferential flow in the subsoil（>40mm depth）,accounting for 61.88%,59.96%,and 49.83%at the GE,CG,and FG sites,respectively.With the increase in the rainfall amount,the interaction between soil macropore flow and soil matrix gradually increased,the proportion of soil preferential flow in the soil profile gradually decreased,and the proportion of soil matrix flow gradually increased.At a rainfall amount of 60 mm,the mean P_FI of the GE,CG,and FG sites was 1.01,0.49,and0.07,respectively.Three-dimensional visualizations of soil infiltration at the GE,CG,and FG sites showed that spatial distributions of water infiltration were distinctly different among the three grazing management practices.Soil from the GE site had greater preferential flow compared with the CG and FG sites.（4）The variation coefficient of soil water content and solute concentration was the highest in GE grassland,followed by CG grassland,and the lowest in FG grassland.The variation law of soil solute concentration was consistent with water content.Under the 60mm rainfall,the maximum preferential infiltration（PIV）of GE,CG and FG grassland was9.83mm,10.4mm and 1.22mm,respectively.The maximum contribution of preferential infiltration（Con）of GE,CG and FG grassland was 67.74%,70.18%and 13.3%,respectively.There was a significant correlation between soil solute concentration and preferential flow development index in 10～20cm soil depth in GE grassland and CG grassland,indicating that preferential flow development could promote solute transport in deep soil（10～20cm）in GE and CG grassland,while preferential flow development had little effect on solute transport in deep soil in FG grassland.