Study On Characteristics Of Soil Infiltration And Nitrogen Leaching Of Eucalyptus Plantation During Short Rotation

Soil infiltration is a key process affecting soil water distribution and solute transport in plantations.Clarifying the soil infiltration capacity and its influencing factors,and the effect of fertilization on groundwater of eucalyptus plantation are of great significance to develop the scientific basic on the soil water dynamics and the ecological afforestation of eucalyptus plantation.In this study,the first-generation plantations of Eucalyptus urophylla×E.grandis in different forest age（1,2,3 and 5 years）during short rotation were used as the research objects.A series of field experiments were conducted to determine the soil infiltration capacities in short-rotation eucalyptus plantations by using the surface-positioned double-ring infiltration instrument.The root biomass of fine root（<2 mm）and medium root（2-5 mm）on near-surface soil layer（0-50 cm）and soil physical and chemical properties（0-60 cm）were measured for analyzing their effects on the soil infiltration capacities.The undisturbed and filled soil columns were used to simulate the changing processes of total dissolved nitrogen（TDN）,ammonium nitrogen（NH₄⁺-N）nitrate nitrogen（NO₃^--N）in leachate driven by total infiltration and matrix infiltration at the topsoil（0-20 cm）in short-rotation eucalyptus plantation.The coupling relationship between infiltration process and the changing process of concentration and content of different forms dissolved nitrogen and EC changing process were studied.The main results are as follows:（1）According to the change rate of infiltration rate in short-rotation eucalyptus plantations,the soil infiltration process were divided into five stages and the average infiltration rate of the corresponding stage.The average infiltration rate ranged from 762 mm/h to 1463 mm/h.initial infiltration rate ranged from 5420 mm/h to 8454 mm/h,infiltration rate of stage I ranged from3130 mm/h to 5421 mm/h,infiltration rate of stage II ranged from 1594 mm/h to 2257 mm/h,infiltration rate of stage III ranged from 484 mm/h to 934 mm/h,steady infiltration rate ranged from 318 mm/h to 510 mm/h.Above six infiltration indicators increased with forest age increasing,the soil infiltration capacity was significantly increased after the third year of planting of eucalyptus plantations.The results analyzed by the structural equation model indicated that the near-surface fine root biomass of eucalyptus plantations showed a strong plugging effect on soil infiltration in both initial and steady infiltration stages.Moreover,soil total porosity and soil organic matters were main influencing factors to promote soil infiltration capacity in short-rotation eucalyptus plantations.（2）The undisturbed and filled soil columns can well simulate the total and matrix infiltration process of topsoil.The dissolved nitrogen and EC driven by the two infiltration pathways peaked at the initial leaching stage and exhibited an initial effect.Compared with the concentrations of indicators in leachate driven by the total infiltration,the concentration of different forms of dissolved nitrogen and EC were higher in the leachate driven by the matrix infiltration.The concentrations of dissolved nitrogen and EC at initial leaching stage were the highest in 3-year-old eucalyptus plantation driven by the matrix infiltration,with the following values:TDN:31.02 mg/L,NH₄⁺-N:1.79 mg/L,NO₃^--N:9.36 mg/L,EC:400.06μs/cm.With the infiltration process continuing,the concentration and content of different forms of dissolved nitrogen,and EC in the leachate gradually decreased with the infiltration rate decreasing.Thus,the nonlinear relationships between the infiltration process and different dissolved nitrogen concentrations in the leachate were established driven by the total infiltration;the linear relationships between infiltration process and concentration and content of different dissolved nitrogen,and EC in leachate driven by the total infiltration and matrix infiltration were established.Because of the initial effect,above equations indicated that the influence of rainfall frequency on the dissolved nitrogen amounts and the EC value in leachate were higher than those of rainfall duration,when the rainfall amounts were the same.The NO₃^--N was main form of nitrogen leaching in short-rotation eucalyptus plantation,whereas the NH₄⁺-N concentrations were low.However,the NH₄⁺-N concentrations at the initial leaching stage were higher than the threshold of surface water quality driven by the two infiltration pathways,which was a potential threat causing the groundwater pollution in eucalyptus plantations.