Effect Of Rock Fragments And Plant Roots On Water Movement In Forest Soils

Rock fragments and plant roots have a complex influence on soil hydrological processes(e.g.,runoff generation,water infiltration and solute transport)in forests,which pose important impacts on related processes in the soil–plant–atmosphere continuum.Due to the topography and hydrothermal conditions,the soil structure in the Gongga Mountain of western Sichuan is fragmented,the land surface is dominated by coarse-grained slope deposits,the root system is well developed within the highly vegetated soil,and the root-soil-rock structure is complex.Although there is some knowledge about the hydrological connectivity and flow production processes on the slope surface in Gongga Mountain,the understanding of the subsurface structural characteristics of the slope surface and the effects of plant root and rock fragments on soil water movement are still lacking,which greatly limits the development and application of the theory and modeling methods for flow generation and confluence on slopes in this region.In order to better understand the relationship between water flow patterns and the presence of plant roots/rock fragments in natural forest systems in mountainous regions,this study investigated the effect of rock fragments and plant roots on water movement in topsoil and subsoil of broadleaved and coniferous forest soils collected from Gongga Mountain,which is located in Eastern Margin of Qinghai Tibetan Plateau,China.Firstly,depth distribution of soil characteristics,soil hydraulic properties and plant root and rock fragments in both broad-leaved and coniferous forest were studied.Results showed that no significant correlations between rock fragment content and soil hydro-physical properties(including soil texture,organic matter content,soil water content and saturated hydraulic conductivity),however bulk density of mineral soil from broadleaved forest was found to be significantly,positively correlated with rock fragment content.Secondly,the effect of plant root and rock fragment on soil water movement in soil column scale was studied.Using repacked soil columns(20 cm in height and 10 cm in diameter)containing different rock fragment contents(0,5,and 15%in v/v)or root quantity(1 or 2)with different sizes(1 or 3 mm in diameter),breakthrough curves(BTCs)of bromide(as non-reactive tracer)were obtained under saturated condition.Results showed that for the broadleaved forest soil Br~-was first detected in the effluent of the soil columns with rock fragments after a smaller number of pore volumes of injection solution than in that of the soil columns without rock fragments.This showed that the presence of rock fragments in the soil could accelerate water movement and solute transport.For the coniferous forest soil,there were no significant changes of BTCs with increasing rock fragment content.The rocks represent immobile regions,where,due to the relatively low permeability,the pores of the rock may be regarded as dead-end pores.5 or 15%of rock fragment content is not enough for creating rock-to-rock contact.Smooth breakthrough curves were observed,which followed similar trends to those typical for a homogeneous soil without plant roots.In most of the column experiments,bromide was detected earlier for soil columns containing 2 plant roots of 3 mm in diameter and the time that bromide take to breakthrough was slightly shorter compared to soil columns containing 1 root of the same diameter or soil without any root.It can be inferred that the number of flow paths at root-soil interface increases with increasing roots quantity and size.Exceptionally,Br~-was detected earlier in the effluent of control column(without plant roots)of the subsoil from the coniferous forest that for the subsoil column containing rock fragments,which could be attributed to a rather high hydraulic conductivity of subsoil matrix.Overall,these results suggest that plant roots of broadleaved and coniferous forest can increase vertical water movement in forest soils via root-soil interface to deeper soil depths,and this effect becomes more pronounced as size and quantity of plant root become larger.Thirdly,the water movement through soil columns was simulated with models.A convection dispersion equation(CDE)and two-region model were applied and the parameters were estimated by inverse modeling,in which a nonlinear least-squares program was used to fit the models to the experimental data.Results showed that with increasing rock fragment content,the dispersivity(?)generally increased while the mobile-immobile partition coefficient(?)and the mass transfer coefficient(?)decreased.The presence of rock fragments led to an increase in the fraction of immobile domain as well as soil tortuosity.A plausible explanation is that the soil beneath the rock fragments behaved as immobile domain and soil-rock interfaces could serve as preferential flow paths.In the soil columns with plant roots,higher dispersion coefficients than the control columns were observed,probably as a result of improved soil tortuosity.It is concluded that the presence of rock fragments and plant roots could distinctively influence water flow pattern in forest soils.The two-region model provided a slightly better description of the BTCs than the CDE.