Effects of planting density on tree growth and induced soil suction

Plant evapotranspiration (ET) is recognised to affect the soil suction of slopes and landfill covers. Previous work has focused on ET-induced suction by a single plant, with little attention paid to the effects of planting density. This study aims to quantify any changes in tree growth and tree-induced suction during ET and rainfall under different planting densities. A tree species, Schefflera heptaphylla, which is commonly found in Asia, was planted in silty sand at spacings of 60, 120 and 180 mm representing three different planting densities. For each case, three replicates were tested to consider any effects of tree variability. In total, the responses of suction for 297 seedlings subjected to drying and a rainfall event with a 10-year return period were measured. The test results show that reducing the tree spacing from 180 to 60 mm induced greater tree-tree competition for water, as indicated by a 364% increase in peak suction upon ET. Such tree-tree interaction led to (i) a 19-35% reduction in the leaf area index; (ii) a 17-36% decrease in root length; and (iii) an obvious decay of roots. Upon the rainfall event, the infiltration rate for vegetated soil with trees planted at a spacing of 60 mm was up to 247% higher than those for soil with a wider tree spacing, where mainly fresh roots were found. Although most suction within the root zone (i.e., top 100 mm) was lost due to increased infiltration at 60 mm spacing, suctions in deeper depths below root zone were largely preserved

Effects of biochar on water retention and matric suction of vegetated soil

Biochar is the pyrolysed biomass which has been used as a soil amendment to improve plant performance. This study aims to investigate the effects of biochar-plant interaction on soil water retention and matric suction in silty sand (SM). In total, four soil conditions, namely soil with and without tree (Schefflera heptaphylla), biochar-amended soil (BAS) with and without tree, were subjected to drying. Three replicates were considered for each vegetated soil. By adding 10% (v/v) of biochar, the optimum water content of soil increased from 12 to 17%, while the maximum dry density decreased from 1890 to 1740 kg/m(3). For both bare and vegetated soils with biochar, desorption rates of soil water-retention curves were smaller than those without biochar, while there were almost no changes in air-entry value (AEV). Biochar reduced matric suction in bare soil by 35-70% and in vegetated soil by 12-36%

Influence of soil nutrients on plant characteristics and soil hydrological responses

The effects of nutrients on plant growth have been widely studied. In agricultural research, some studies have showed significant root growth in high-nutrient-supplied soil, however, some other studies, reported better lateral root growth in low-nutrient-supplied conditions. However, the effects of NPK (nitrogen, phosphorous, potassium) water-soluble fertilisers on plant roots and shoot growth in bioengineered soil slopes and their influence on soil suction and water-retention ability are still unclear. This paper quantifies the growth of a tree in NPK nutrient-supplied soil and its effects on tree-induced soil suction and water-retention ability during evapotranspiration. Three replicates of Schefflera heptaphylla (Ivy tree) were grown for 6 months in nutrient-poor and nutrient-supplied compacted soils typical of bioengineered slopes and embankments. Plant characteristics, induced suction and volumetric water content (VWC) were monitored throughout the growth period. After 6 months of growth, leaf area index (LAI) and peak root area index (RAI) increased by 350 and 133%, respectively, in nutrient-supplied soil compared with the nutrient-poor soil. This is because nitrogen stimulated chlorophyll synthesis enabling plants to produce larger leaves. Additionally, nitrogen mediated phosphorous to be utilised by roots in soil to induce growth of fine roots and thus increase the root surface area and root volume in soil pores. After 3 days of drying, the vegetated nutrient-supplied soil induced 15–50 kPa higher suction than the vegetated nutrient-poor soil due to the higher RAI, LAI and root volume of the plants grown in nutrient-supplied soil, which enable plants to absorb and transpire more water. In contrast, the water-retention ability reduced in the nutrient-rich vegetated soil because more clustered fine roots in soil pores decreased the pore diameter and increased suction

Seasonal movement and groundwater flow mechanism in an unsaturated saprolitic hillslope

Numerous field monitoring programs have been conducted to investigate the performance of an unsaturated soil slope subjected to rainfalls in wet seasons. Most case histories focus on the response of matric suction, which is one of the two stress-state variables governing unsaturated soil behaviour. However, effects due to another variable, net normal stress, are often ignored. Also, slope performance under alternative wet and dry seasons is rarely reported and analysed. In this study, a saprolitic hillslope situated in Hong Kong was instrumented heavily to investigate its seasonal movement due to changes of the two variables and also groundwater flow mechanism. Two-year seasonal variations of matric suction and net normal stress were monitored by tensiometers together with heat dissipation matric water potential sensors and earth pressure cells, respectively. During heavy rainstorms in wet season, there was a substantial recharge of the main groundwater table, causing a significant increase of positive pore-water pressure in deeper depths. Rupture surface likely developed at depths between 5.5 and 6 m, hence resulting in a "deep-seated" mode of downslope movement. The downslope movement resulted in a peak increase of horizontal stress. In dry seasons, matric suction of up to 190 kPa was recorded, and the associated soil shrinkage led to substantial upslope rebounds. The stress built up in wet seasons hence reduced. After monitoring period of 2 years, downslope ratcheting is identified. Up to 40 % of the downslope displacements were recovered by the upslope rebounds