Study On Macropore Flow In Soils Under Different Land Use Types In Changwu Tableland

Macropore flow is common in soil and plays an important role in soil water infiltrationprocess. Study on soil macropore flow under different land use types were conducted onWangdong small watershed of Changwu Tableland. Farmland, forestland and grasslandwere selected as the sample plots. The dye infiltration experiments and image processingtechnology were used in this study. Soil water content, soil bulk density, root biomass, rootlength density, root area index, average root diameter and specific root length were alsomeasured in this study. Characteristic of percentage of stained area with soil depth wascompared between different land use types. The relationship between percentage of stainedarea and soil water content, soil bulk density and parameters of root distribution wasanalyzed. In order to understand the mechanism of water infiltration well, experiment oncharacteristic of rainfall infiltration in macroporous soil was also carried out in soilcolumns on Changwu tableland. Results show as follows:(1) Soil bulk density of different land use types was small at surface soil. Soil bulkdensity of farmland was lower than forestland and grassland at0to30cm soil layer. But itwas higher than forestland and grassland under40cm soil layer. Root biomass of farmland,forestland and grassland distributed at0.84-38.50g/m~2,2.51-150.70g/m~2and0.88-217.61g/m~2respectively. Root biomass, root length density and root area index ofgrassland soil at0-20cm soil layer were the highest, the next was forestland, and then wasfarmland. But Root biomass, root length density and root area index of forestland was thehighest below20cm soil layer, the next was grassland, and then was farmland. The averageroot diameters of different land use types measured at the sample plots were all more than0.5mm, which can be the macropore flow channel.(2) Macropore flow occurred in all the soils under the three land use types. Theformation of soil macropore flow was relevant with soil heterogeneity and soil porosity.The infiltration depths of macropore flow under different land use types were significant different. Infiltration depth of forestland was the highest, which can reach to100cm. Thenext was grassland which was about95cm. Infiltration depth of farmland were less thanthe others, which can reach to80cm. The stained area in soil profile of farmland andforestland was mainly distributed at0to60cm soil layer. And the stained area of grasslandwas mainly distributed at0to40cm soil layer. Soil root biomass, root length density androot area index were the main factors which effected the distribution of soil macroporeflow. Soil water content and soil bulk density could influence the formation of macroporeflow, but the mechanism was complicated and could not be described as the linearrelationship. Root diameter and root connectivity could joint influence the transport of soilmacropore flow. Depths of interaction of100mm irrigation quotas to the soil water underdifferent land use types were different. They were200cm,100cm and140cm for farmland,forestland and grassland separately. Phenomenon of water redistribution existed in soils ofall the three land use types. Range of soil water redistribution in farmland, forestland andgrassland were0-140cm,0-60cm and0-120cm respectively.(3) Impact of rainfall on soil water content was mainly concentrated above the depth of160cm. The extent of impact decreased with depth. Down to the depth of240cm, therainfall peak information carried by soil water content almost disappeared. The wettingfront velocity was correlated positively with rain intensity, and negatively with initial soilwater content. The depth of wetting front transportation was positively correlated with therainfall and rainfall intensity. Soil water at the depth of300cm could be supplied by rainfall.Piston flow was the mainly mechanism of groundwater recharge in loess. The lag time ofrainfall infiltration recharge to the soil water of100-200cm soil layer was about15-18d,and the lag time to the soil water of300cm soil layer was about30-45d.