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Aims: Gradients in water potential of soil and plant system drives the water movement in soil-plant-atmospheric continuum. Here, we demonstrate how root water potential measured directly from the roots upon changes in soil water potential would contribute to the understanding of the drought response in Chrysopogon zizanoides. Methods: Plants of Chrysopogon zizanoides L. were sampled at different soil water status (inducing drought) and growth periods (3-, 4- and 5- months). The roots and leaves of the plants were dissected to measure the root water potential and specific leaf area, respectively. The root water potential was measured in a WP4C dew-point potentiometer. Root diameter corresponding to the roots measured for root water potential was also measured. Results: Our findings showed a logarithmic increase in gradient between soil and root water potential under the induced drought stress, similar to the existing findings of root hydraulic conductance. Specific leaf area significantly decreased with root water potential, indicating the hydraulic continuity between roots and leaves. A new power law correlation between root diameter and root water potential established a trait-based understanding of root water uptake. Conclusion: The aggregation of such root water potential measurements using potentiometer would offer strategies to explore the implications of below-ground plant behaviour in applications such as slope stability and irrigation. [ABSTRACT FROM AUTHOR]

Biochar produced from the pyrolysis of plant-based feedstock has been advocated as an alternative soil amendment for landfill cover. Previous literature indicated that the pyrolysis temperature influences the intra-pore distribution and surface functional groups (especially hydroxyl groups), resulting in "love-hate relationship" of the biochar-amended soil (BAS) with water. From the purview of geotechnical engineering, the effects of pyrolysis temperature on geotechnical properties are rarely investigated. In total, three biochar rates (0, 5, and 10%) were considered for a set of geotechnical experiments in sand clay mixture soil with biochar produced at 350°C and 550°C. Test results show that biochar addition in soil, in general regardless of pyrolysis temperature, increased the optimum moisture content (OMC), plasticity index, and soil water retention characteristics (SWRC) and decreased the maximum dry density (MDD), shear strength parameters (cohesion, friction), and erosion rates. Whilst comparing the pyrolysis temperature effects on two biochar-amended soils, only marginal effects (in terms of magnitude) on SWRC were observed. The most significant decrease of MDD (or increase of OMC) for 5% (w/w) and 10% (w/w) biochar additions occurred at pyrolysis temperatures of 550 °C and 350°C, respectively. In addition, biochar produced at lower pyrolysis temperature (350 °C) was more effective in reducing cracks and enhancing shrinkage area ratio. Ten percent of biochar addition with pyrolysis temperature of 350 °C was the optimum combination in resisting soil erosion. The study provides evidence that the geotechnical properties of biochar-amended soils for landfill cover soil applications could be tailor made by controlling the pyrolysis temperature. [ABSTRACT FROM AUTHOR]

Critical suction ranges for plants upon drought stress are essential for devising precision irrigation schemes in de-desertification measures in arid urban regions. The objective of the current study is to develop a novel framework for drought stress evaluation, considering measured changes in photosynthetic leaf parameters with root zone soil suction. A native grass species Axonopus compressus was grown in three different growth media—bare sand, sand with periodic organic nutrient treatment and sand amended with 5% biochar for 56 days, followed by continued drought. The corresponding grass leaf response of stomatal conductance (SC), quantum photosynthetic efficiency (QPE), effective photochemical quantum yield, i.e. Y(II), and maximum photochemical quantum yield, i.e. (Fv/Fm), were measured. The threshold suction at around 10 kPa for all treatment types indicates the state at which an initial drop in transpiration occurred, marked by significant change in SC, QPE and Y(II). A new terminology termed as "tipping suction" was coined to describe the condition wherein the grass reaches a "life or death state", a stage at which the grass could progress to permanent wilting or return to sustenance. This stage is marked by a rapid decrease of Y(II), and the tipping suction ranging between 55 and 200 kPa is dependent on soil water retention capacity and the plant root biomass. The tipping suction differs from the permanent wilting point wherein complete suppression of Fv/Fm occurred. This study suggests that it is ideal to calculate plant-available water content by considering threshold suction and tipping point for maintaining optimum growth and resist plant mortality, respectively. [ABSTRACT FROM AUTHOR]

Permanent wilting point (PWP) is generally used to ascertain plant resistance against abiotic drought stress and designated as the soil water content (θ) corresponding to soil suction (ψ) at 1500 kPa obtained from the soil water retention curve. Determination of PWP based on only pre-assumed ψ may not represent true wilting condition for soils with contrasting water retention abilities. In addition to ψ, there is a need to explore significance of additional plant parameters (i.e., stomatal conductance and photosynthetic status) in determining PWP. This study introduces a new framework for determining PWP by integrating plant leaf response and ψ during drought. Axonopus compressus were grown in two distinct textured soils (clayey loam and silty sand), after which drought was initiated till wilting. Thereafter, ψ and θ within the root zone were measured along with corresponding leaf stomatal conductance and photosynthetic status. It was found that coarse textured silty sand causes wilting at much lower ψ (≈ 300 kPa) than clayey loam (≈ 1600 kPa). Plant response to drought was dependent on the relative porosity and mineralogy of the soil, which governs the ease at which roots can grow, assimilate soil O2, and uptake water. For clay loam, the held water within the soil matrix does not facilitate easy root water uptake by relatively coarse root morphology. Contrastingly, fine root hair formation in silty sand facilitated higher plant water uptake and doubled the plant survival time. [ABSTRACT FROM AUTHOR]

Purpose: Biochars produced from different feedstocks (such as wood, pig manure) possess varying physical and chemical properties, which have influence on crack and evaporation rate of biochar-amended soil (BAS). Furthermore, influence of compaction state and drying-wetting cycles on evaporation rate and cracking of BAS has not been investigated comprehensively. The objective of this study was to investigate the effects of biochar types, compaction state of BAS, and drying-wetting cycles on crack propagation and retained water (or evaporation rate). Material and methods: An animal and plant feedstock-based biochars were produced in-house from pig manure (PM) and wood (W), respectively. In addition, nano structured chalk and wheat biochar (CWB) were also produced. Soil amended with individual biochars was compacted in petri-glass discs at two densities. Disc specimens were subjected to multiple drying-wetting cycles, and evaporation rate of specimens and crack area were monitored throughout the experimental period (70 days). Images were captured after every 24 h and processed using image processing technique to obtain the crack intensity factor (CIF). Results and discussion: The results show that plant-based W BAS showed the high water retention, i.e., low evaporation rate and low CIF. Furthermore, the crack potential of CW BAS was seen to be higher. In dense compacted soil, maximum CIF% can be reduced from 3.9 to 0.4% for W BAS, from 3.9 to 1.7% for PM BAS, and from 3.9 to 1.6% for CW BAS. Conclusion: WB was able to resist cracking more efficiently than other types of biochar. Evaporation was found to be minimal for plant-based W BAS at 10% biochar percentage. Higher biochar content in soil was seen to increase the water retention of BAS significantly. Dense state of BAS at high biochar content (i.e., 10%) was effective in reducing evaporation rate and crack progression. [ABSTRACT FROM AUTHOR]