Effects Of Plant Root Systems And Rock Fragments On Soil Preferential Flow In Jiufeng Area, Beijing, China

With the development of industrialization and urbanization in our country, more and more problems are appearing from time to time, such as the underground water quality deterioration, soil heavy metal pollution, soil salinization, soil nutrient loss and so on. Conspicuously, with respect to soil water and solute transport, they have becoming the important research fields of environmental science and soil science. There is no denying the fact that soil preferential flow is a common form, as well as a widespread phenomenon, of water and solute transport in soil scales. Furthermore, researchers set out to do more studies from homogenous fields to heterogeneous fields. In general, soil preferential flow has pivotal effects on ecological and hydrological processes, especially water balance and cycle. To characterize the influence of plant roots and rock fragments on soil preferential flow, Jiufeng National Forest Park in Beijing was studied so as to explore the relationship between them. In studied area, three experimental plots including Sophora japonica Linn., Platycladus orientalis Franco and Quercus dentata Thunb respectively were installed. Moreover, field dye tracing experiments and breakthrough transport curve experiments were combined to imply preferential flow characteristics and controlling factors. The results were as follows:(1) There was an obvious soil preferential flow phenomenon in Jiufeng National Forest Park, Beijing area, and macropore flow and finger flow were the most significant preferential flow types. Meanwhile, there was difference between preferential flow pathways and soil matrix in soil vertical profiles.(2) Plant roots have important influences on soil preferential flow. With respect to three experimental plots, root length density decreased with the increasing soil depth from soil surface gradually. As for different root diameters (d<1mm,1<d<3mm and3<d<5mm), the contribution of plant roots on soil preferential flow was different:94.8%for root diameter d<1mm,4.3%for root diameter1<d<3mm and0.9%for root diameter3<d<5mm. Meanwhile, there was also difference for root length density between preferential flow pathways and soil matrix. The contribution of plant roots to preferential flow was66.7%,88.9%and83.3%for d<1mm,1<d<3mm and3<d<5mm respectively. Furthermore, root biomass between preferential flow pathways and soil matrix was also different for those plots. The proportion (root biomass in preferential flow pathways bigger than soil matrix) was66.7%. (3) Rock fragments were also the main controlling factors which affect preferential flow processes. There was a complex relationship between rock fragments and preferential flow because the distribution patterns of rock fragments in Jiufeng area was accumulated or fragmented. The content of rock fragments was under11.5%. And there was a negative correlation between rock fragments and soil water flow rate (R2=0.536, n=12).(4) Based on the experiments, root-rock structure was analyzed. The relation of root-rock structure to soil water flow rate was complicated:there was a positive correlation between plant roots and flow rate and a negative correlation between rock fragments and flow rate. Root volume and root dry weight as pivotal plant roots index had various effects on flow rate:for root volume, z=f(x, y)=1.235+0.4959x-0.05052y, R2=0.7837; for root dry weight, z=f (x, y)=1.391+1.487x-0.08065y, R2=0.7605, z represented soil water flow rate (ml·min-1), x represented root volume (cm3) or root dry weight (g), y represented gravel content (%).