Unit Volume Of Soil Research Articles

Quantitative importance of subsoil nitrogen cycling processes in Andosols and Cambisols under temperate forests

Nitrogen availability often limits plant growth in forest ecosystems. Plants, including trees, utilize soil inorganic nitrogen transformed via nitrogen cycling processes, including mineralization and nitrification. While most plant fine roots are distributed in surface soil layers, plants take up nitrogen even from subsurface soil layers in addition to the surface soil layers, especially under high nutrient competition. However, there is still limited knowledge about nitrogen cycles within deeper soil layers. In this study, we investigated the vertical profiles (0–60 cm) of the net nitrogen mineralization and nitrification rates at four Japanese forest sites with two different soil types (Andosols and Cambisols). The partial least square path modeling (PLS-PM) was used to determine factors affecting nitrogen cycling processes. The net nitrogen mineralization and nitrification rates per unit soil weight were considerably higher in the surface soil layer than in the deeper soil layers in Andosols but not in Cambisols. PLS-PM analysis showed that microbial biomass and soil organic matter quantities were the important factors influencing the net nitrogen mineralization and nitrification rates, indicating that a similar mechanism, which drives the spatial variations of nitrogen cycling processes in the surface soil layer, predominantly regulates the vertical variations of the processes. Moreover, it was estimated that the net nitrogen mineralization rate could be comparable at all soil types and depths when the rate was expressed per unit soil volume. Therefore, subsoil layers could be a quantitatively important source of plant-available nitrogen in Andosols and Cambisols.

Changes in preferential flow caused by root effects in black locust plantations of different stand ages in the semi-arid region of the Loess Plateau

Preferential flow significantly influences regional water cycles, and affecting surface and subsurface runoff processes. By comparing soil infiltration characteristics across various forest ages, it can be evaluated how artificial black locust forests on the Loess Plateau contribute to the resilience of soil and water conservation. In this study, we analyzed the spatial distribution characteristics of roots of black locust plants using field investigations and statistical analyses and determined the effects of the root characteristics on the preferential flow in the semi-arid of the Loess Plateau, China. The results showed that the root biomass, root length density, and root volume ratio of young forests were significantly lower than those of middle-aged and mature forests (P < 0.05), and root characteristic parameters varied significantly among different root diameters and soil layers.. The average volume of decayed root channels in mature forests, accounting for approximately 0.16 % in a unit volume of soil, was 3.1 times that of young forests and 4.9 times that of middle-aged forests. Preferential flow increased with forest age in artificial black locust forests. 2–5 mm, 30–40 mm and > 50 mm roots promoted preferential flow to varying degrees, while roots of 5–15 mm inhibited the preferential flow. < 2 mm, 10–30 mm and 40–50 mm roots has no effect on preferential flow (P > 0.05). The size and density of decayed root channels significantly improve soil preferential flow, and the differences among forest ages are linked to the network created by rotten roots. These findings indicate that living roots of different diameters and decayed roots contribute to soil preferential flow to varying extents. The results provided a conceptual basis for understanding the potential impact of vegetation roots on surface runoff during the vegetation restoration ecosystems process in the Loess Plateau.

Numerical evaluations on the effects of different factors on thermo- mechanical behaviour of an energy pile group

Energy pile is a kind of economical and efficient underground energy structure. Obtaining the effects of different factors on the performance of energy pile help to improve the working efficiency of the energy pile. However, the thermo-mechanical behaviour of energy pile groups are more complex than that in single energy pile due to the mutual interference of energy pile groups. In this paper, the energy pile group with the layout of 3 × 3 was simulated by a 3-D numerical model. The influences of soil type, pile spacing, pile material performance and pile arrangement form on the thermo-mechanical behaviour of the energy pile group were investigated. The results show that increasing pile spacing can not only improve the thermal performance of energy pile group but also reduce the pile displacement, but it will lead to the increase of pile axial force because of the weakening of pile group effect. The thermal performance of energy pile group is the highest under the rock-soil condition, followed by sand and clay. However, the pile displacement is the largest when the soil type is sand, followed by clay, and the rock-soil is the smallest, and the influence of soil type on pile axial force is just opposite to pile displacement. At the same time, the increase of thermal conductivity of pile material benefits to heat transfer performance of energy pile group, but it also aggravates the heat accumulation and results in a larger top displacement and axial force of pile. As for concrete compression modulus, increasing the concrete compression modulus can increase the axial force of pile, but the change of concrete compression modulus has no effect on the pile displacement. In terms of layout of energy pile group, the per unit soil volume heat exchange rate of energy pile group in the cross arrangement form is larger than that in sequential arrangement form. However, the variation range of soil temperature in the cross arrangement form is larger than that in the sequential arrangement form, thus increasing pile displacement and reducing pile axial force.

Soil erosion effectively alleviated by Miscanthus sacchariflorus, a potential candidate for land deterioration improvement

Plants can reduce runoff and improve soil properties in erosion-prone areas, thereby enhancing resistance to erosion. The main goal of this research was to identify the effects of planting density on the growth and soil fixation ability of Miscanthus sacchariflorus, which is a candidate for application in soil conservation. In this study, four planting densities (low density (I, 70 × 70 cm, row spacing × plant spacing), medium density (II, 50 × 50 cm), high density (III, 30 × 30 cm), and extremely high density (IV, 15 × 15 cm)) were set and plant growth, root architecture and soil erosion resistance were measured. The results showed that tiller number, leaf length, and biomass decreased with increased planting density. Root length and surface area per plant of fibrous roots and rhizomes were significantly (P < 0.05) decreased under density IV relative to density I, indicating that higher densities inhibited the per plant growth of M. sacchariflorus. However, the vegetation coverage and the density of root mass, length (RLD), surface area (RSAD), and volume per unit soil volume of M. sacchariflorus showed opposite trends. Additionally, the water stability and soil anti-erodibility from density III were significantly (P < 0.05) higher than those of other planting densities. Furthermore, correlation analysis showed that the RLD and RSAD of fibrous roots were the most important factors for erosion resistance of the soil. Overall, the density III may be optimal for M. sacchariflorus seedlings planted to alleviate soil erosion and this species may be a suitable and ecological candidate for soil conservation.

Combining UAV remote sensing and pedological analyses to better understand soil piping erosion

The study and mapping of evidences of soil piping using non-destructive techniques is a key issue in quantitative geomorphology. This paper attempts to combine Aerial mapping systems (Unmanned Aerial Vehicles (UAV)), soil physical and chemical attributes, and near-surface geophysical survey tools (ground penetrating radar and electrical resistivity tomography) to provide a comprehensive understanding of geometric features of soil piping in arid and semi-arid regions. The UAV Mapping System was applied to prepare ortho-photos, topography, analytical hill shading, drainage density, and land use maps of two different piping sites (site 1: rangelands, and site 2: agricultural lands) in Sarakhs plain, Razavi Khorasan Province, northeastern Iran. The physical and chemical soil attributes were analyzed in six soil profiles to check the hypothesis that these soil attributes control the occurrence of piping-related features (i.e., sinkhole, blind gully, gully), and to test if there are any differences in soil properties between the two land use types. The near surface geophysical tools were used to determine the approximate size of soil pipes, and to simulate their internal structure. The results of the derived UAV’s independent variables confirmed that typical soil erosion features related to piping (i.e., sinkhole, blind gully, gully) developed exactly in adjustment with subsurface processes (i.e., drainage density). The quantitative algorithms of pedology revealed significant differences of Na+ and SAR for soil profiles with and without piping erosion, but these soil properties themselves are not enough to explain piping development at the two study sites. The volume of GPR line surveys was 1701.5 m3 in site 1 and 1203 m3 in site 2. The potential distribution of subsurface pipes revealed by applying GPR were more extensive than those deduced from soil surface observations: three-dimensional pipe number density (number of soil pipes per unit soil volume; # m−3) of potential pipes and collapsed cavities simulated with a mean pipe length and pipe depth of 143.4 cm and 88 cm in rangelands and 175.75 cm and 79.14 cm in agricultural lands, respectively. The maximum density of pipes or pipe roof collapses occurred within soil depth boundaries of 0–50 cm at site 1 and 30–100 cm at site 2. The ratio of surface erosion features to subsurface potential piping in rangeland and agricultural lands was ca. 26.1 % and 10.4 %, respectively. The electrical resistivity tomography (ERT) measurements indicated higher resistivity values in piping-prone areas, where pipes were initiated at the bedrock interface. Integrating all data obtained by different techniques in this study allowed to better understand the extent of piping erosion in both surface and subsurface soil horizons.

ROOT DISTRIBUTION OF MATURE OIL PALMS IN MINERAL AND PEAT SOILS AND ITS IMPLICATION ON FERTILISER PLACEMENT

The correct placement of fertilisers is critical, as losses can be very significant especially in regions of high and frequent rainfall, non-terraced slopes, sandy textured soils and peat. To maximise uptake efficiency, fertilisers should be evenly broadcast over the soil surface area that contains the highest density of feeder roots. In order to determine the latter, an investigation was undertaken to ascertain the biomass and distribution of roots of various ages of mature oil palms planted in both mineral and second generation peat soils. Results of the study indicate that if site factors are not limiting, palm age and past fertiliser placement history are two major factors influencing oil palm root development and distribution. In mature well decomposed peat, although roots could be found growing 4 m from palm bole, actual root biomass per unit volume of soil was low. Only 21 per cent of the feeder roots of 8-year-old palms were found growing outside the weeded palm circles (WPC). Even within the latter, approximately 50 per cent were concentrated within a radius of 1 m from the palm bole. The low bulk density and high porosity of peat appears to discourage roots from growing beyond this distance. In mineral soils, there was a consistent and gradual increase in root spread beyond the WPC, with palm age. Feeder root distribution beyond a 2 m radius ranged from as low as 26 per cent in 6-year-old palms planted on terraces to 41 per cent for 8-year-old palms established in non-terraced soils. Only palms older than 10 years of age had root biomass greater than 50 per cent beyond this radius. In all sites, there was an increase in primary and secondary root biomass with soil depth and a linear decrease in feeder root biomass down the soil profile. Soil chemical analysis indicated that apart from palm age, the horizontal and vertical distribution of feeder roots was strongly influenced by soil fertility gradients created by past fertiliser placement history. As fertilisers were previously applied entirely in the WPC for younger palms (&lt;8 years), there was a significant decline in soil fertility with increasing distance from the palm bole and increasing soil depth. The majority of feeder roots were concentrated within the 2 m radius from the palm bole and in the top 20 cm of soil, where nutrient levels were the highest. In older palms (&gt;10 years) where fertilisers had been broadcast over frond heaps in the interpalm spaces and interrows, feeder root biomass was higher outside the WPC, as soil fertility gradients were less apparent. Taking into account the root distribution patterns of oil palm in relation to palm age, specific recommendations on fertiliser placement for oil palm grown on mineral and peat soils are made, so as to improve fertiliser uptake and utilisation.

Getting to the root of the problem: improving measurements of mangrove belowground production and carbon sequestration.

This article is a Commentary on Arnaud et al. (2021), 232: 1591–1602.

Water Infiltration into Sand, Silt, and Clay at Field Capacity

Field capacity (FC), permanent wilting (PWP), and plant available water (PAW) are essential parameters to estimate for soils because they are essential for water irrigation management. However, these parameters were reported in water volume per unit soil volume. Knowing the soil required water volume does not imply immediate water availability, that is, the speed at which the water could be supplied to the soil. This is because there is a lag time between water irrigation initiation and the water increment in the soil depth. This study uses the field capacity’s soil water content to simulate the water infiltration using Richards’ equation. The studied soil medium was silt, sand, and clay. The study allows an estimate of water infiltration time and infiltrated water to relate to the soil depth of interest. The clayey has the highest FC, and the silty soil has the highest PAW. The results revealed silty soil could contain more readily water for plant growth than sand and clay. This study also revealed silty soil to be a better soil medium than sand and clay. It has the best tradeoff between water infiltration time and the infiltrated amount of water for plant absorption. This study’s coupling technique will be a useful tool for farmers and field practitioners to assess any site based on the soil texture at an early stage of water irrigation investigation.

High nutrient supply and interspecific belowground competition enhance the relative performance of Picea mariana (Mill). B.S.P seedlings over Picea glauca [Moench] Voss. under elevated CO2

Key message We tested how nutrient supply and interspecific belowground competition affect ecophysiological and morphological responses to elevated CO 2 in black ( Picea mariana (Mill). B.S.P) and white spruce ( Picea glauca [Moench] Voss.). It is found that belowground competition and high nutrient greatly enhanced the relative performance of black spruce over white spruce at elevated CO 2 . ContextWe have previously found that interspecific belowground competition reduce growth, whole seedling photosynthesis, and biomass allocation to leaf and that belowground competition and nutrient supply affect responses to elevated CO2 in the above two species, but we did not examine the physiological and morphological mechanisms of the responses.AimsTo examine the interactive effects of belowground competition, nutrient supply and elevated CO2 on root morphology, photosynthetic rate, and biochemical and photochemical capacity of photosynthesis in black spruce (Sb, Picea mariana [Mill.] B.S.P.) and white spruce (Sw, Picea glauca [Moench] Voss.).MethodsSeedlings were grown in individual containers (no belowground competition) or in a common container (belowground competition) under 380 vs. 720 µmol mol−1 CO2 and high vs. low nutrient supply in the greenhouse for one growing season.ResultsElevated CO2 stimulated photosynthesis and nutrient use efficiency to a much greater degree in black than white spruce when they were grown in the same container, particularly under high nutrient supply. The ability to produce a greater length of roots per unit volume of soil was associated with the response of black spruce.ConclusionThe synergistic effects of elevated CO2 and belowground competition on the physiology and root morphology of black spruce suggest that elevated CO2 will likely increase the relative competitiveness of black spruce over white spruce.

Improving Inferences from Hydrological Isotope Techniques.

Plant water isotopic compositions are widely used to describe patterns of soils water uptake. Although valuable, the technique only provides relative uptake distributions, which can be misleading. Without information on total transpiration, the technique cannot address central questions on drought response, competition, and species coexistence.

Study of the thermal field of a mixture of soil and PCM materials with simulation of the warming effect during a phase change

Phase-change materials (PCM) possess an ability to accumulate and generate a lot of heat during the substance change from one phase to another. Such materials can be used with soil as an alternative type of thermal regulation of the subgrade during construction in cold regions. This paper reviews the impact of two types of PCM (PCMf-hybrid liquid and PCMm- microencapsulated paraffin) on the soil temperature. An experiment was conducted with three groups of samples: (1) samples with PCMm or PCMf added at a ratio of 2% and 4% of the dry weight of the soil; (2) samples reinforced with 4% of cement with an addition of PCMm or PCMf; and (3) samples with PCMf added after drying and re-adding water up to the optimal amount. PCMf and PCMm have been tested in DSC at various scanning speed levels. The results showed that, compared to that of the PCMm sample, PCMf has a greater enthalpy both during freezing and thawing, as well as the temperature peak during freezing is closer to zero. The study of temperature changes in soil samples with PCMm, PCMf, and cement added at the change of the ambient temperature T0=10°C→(-10°C)→(10°C) showed that the soil samples with various PCM additions have a lower temperature change rate than the soil samples without PCM additions. The results of the Scanning Electron Microscope (SEM) microfilming confirmed satisfactory components concentration in the soil mixture. There is also a noticeable number of craters and less porosity in the samples containing liquid PCMf and cement. A two-dimensional model of PCMm microcapsule per unit volume of soil was analyzed with the help of Comsol Multiphysics software (a powerful software product for modeling heat transfer and phase change in porous medium). It was determined that, during the phase change, the PCMm microcapsule has a thawing effect on the soil. During the examination of a three-dimensional model of eight PCMm capsules per unit volume of soil, it was noted that the thawing effect extends to 18.2% of the calculated area.

Suppression of Cheatgrass by Perennial Bunchgrasses

Long-term control of the invasive annual grass cheatgrass is predicated on its biological suppression. Perennial grasses vary in their suppressive ability. We compared the ability of a non-native grass (“Hycrest” crested wheatgrass) and two native grasses (Snake River wheatgrass and bluebunch wheatgrass) to suppress cheatgrass. In a greenhouse in separate tubs, 5 replicates of each perennial grass were established for 96 d, on which two seeds of cheatgrass, 15 cm apart, were then sown in a semicircular pattern at distances of 10 cm, 30 cm, and 80 cm from the established perennial bunchgrasses. Water was not limiting. After 60 d growth, cheatgrass plants were harvested, dried, weight recorded, and tissue C and N quantified. Soil N availability was quantified at each location where cheatgrass was sown, both before sowing and after harvest. Relative to cheatgrass grown at 80 cm, all perennial grasses significantly reduced aboveground biomass at 30 cm (68% average reduction) and at 10 cm (98% average reduction). Sown at 10 cm from established perennial grasses, cheatgrass aboveground biomass was inversely related with perennial grass root mass per unit volume of soil. All cheatgrass sown at 10 cm from “Hycrest” crested wheatgrass died within 38 d. Before sowing of cheatgrass, soil 10 cm from established perennial grasses had significantly less mineral N than soil taken at 30 cm and 80 cm. Relative to cheatgrass tissue N for plants grown at 80 cm, cheatgrass nearest to the established perennial grasses contained significantly less tissue N. All perennial grasses inhibited the NO2− to NO3− nitrification step; for “Hycrest” crested wheatgrass, soil taken at 10 cm from the plant had a molar proportion of NO2− in the NO2− + NO3− pool of > 90%. In summary, a combination of reduced nitrogen availability, occupation of soil space by perennial roots, and attenuation of the nitrogen cycle all contributed to suppression of cheatgrass.

An improved method for quantifying total fine root decomposition in plantation forests combining measurements of soil coring and minirhizotrons with a mass balance model.

Accurate measurement of total fine root decomposition (the amount of dead fine roots decomposed per unit soil volume) is essential for constructing a soil carbon budget. However, the ingrowth/soil core-based models are dependent on the assumptions that fine roots in litterbags/intact cores have the same relative decomposition rate as those in intact soils and that fine root growth and death rates remain constant over time, while minirhizotrons cannot quantify the total fine root decomposition. To improve the accuracy of estimates for total fine root decomposition, we propose a new method (balanced hybrid) with two models that integrate measurements of soil coring and minirhizotrons into a mass balance model. Model input parameters were fine root biomass, necromass and turnover rate for Model 1, and fine root biomass, necromass and death rate for Model 2. We tested the balanced hybrid method in a loblolly pine plantation forest in coastal North Carolina, USA. The total decomposition rate of absorptive fine roots (ARs) (a combination of first- and second-order fine roots) using Models 1 and 2 was 107±13gm-2year-1 and 129±12gm-2year-1, respectively. Monthly total AR decomposition was highest from August to November, which corresponded with the highest monthly total ARs mortality. The ARs imaged by minirhizotrons well represent those growing in intact soils, evident by a significant and positive relationship between the standing biomass and the standing length. The total decomposition estimate in both models was sensitive to changes in fine root biomass, turnover rate and death rate but not to change in necromass. Compared with Model 2, Model 1 can avoid the technical difficulty of deciding dead time of individual fine roots but requires greater time and effort to accurately measure fine root biomass dynamics. The balanced hybrid method is an improved technique for measuring total fine root decomposition in plantation forests in which the estimates are based on empirical data from soil coring and minirhizotrons, moving beyond assumptions of traditional approaches.

A study on effects of leaf and root characteristics on plant root water uptake

Water uptake intensity (WUI), which depicts the relationship between the rate of plant root water uptake per unit soil volume and soil matric suction, has been commonly used to analyse the transpiration-induced soil hydrological changes in civil infrastructure. However, the effects of plant characteristics on WUI are not clear, causing difficulties when estimating the effects of vegetation on soil moisture/suction changes. The aim of this study is to examine any correlations between plant characteristics and WUIs. Changes in suction and volumetric water content due to transpiration of eight tree individuals (Schefflera heptaphylla) vegetated in compacted silty sand were monitored to estimate the distribution of WUI within the root zone. Results show that the WUI remains a maximum and constant when suction is less than a threshold value of suction, beyond which the WUI decreases approximately linearly. The threshold suction is similar within the root zone of each tree individual, but it has a negative correlation with the ratio of leaf area (LA) and root length (RL). The reduction rate of WUI is positively correlated with root length density (RLD). This suggests that plants with a lower LA/RL ratio and lower RLD may be more capable of maintaining their water uptake ability, as high suction is induced.

Hydromechanics and Kinematics in Preferential Flow

Preferential flow covers macropore flow, nonequilibrium flow, and finger flow that are here exclusively approached with gravity-driven viscous flow. The basic unit is a water content wave whose two parameters are the waveʼs film thickness and its mainly vertical contact area per unit soil volume. The spatiotemporal wave properties depend on soil structure and on the intensity and duration of water input to the surface. Kinematic wave theory provides the mathematical tool for solving the analytical expressions. Three cases are the basic building blocks for approaching preferential flow: (a) single pulse, (b) a faster pulse trails a slower pulse, and (c) a faster pulse overtakes a slower pulse. Analytical procedures are presented for each case, and the potential of their applications is discussed. The analytical expressions and the superfluous representative elementary volume greatly facilitate preferential flow modeling.

Quantifying the contribution of mass flow to nitrogen acquisition by an individual plant root.

The classic model of nitrogen (N) flux into roots is as a Michaelis-Menten (MM) function of soil-N concentration at root surfaces. Furthermore, soil-N transport processes that determine soil-N concentration at root surfaces are seen as a bottleneck for plant nutrition. Yet, neither the MM relationship nor soil-N transport mechanisms are represented in current terrestrial biosphere models. Processes governing N supply to roots - diffusion, mass flow, N immobilization by soil microbes - are incorporated in a model of root-N uptake. We highlight a seldom considered interaction between these processes: nutrient traverses the rhizosphere more quickly in the presence of mass flow, reducing the probability of its immobilization before reaching the root surface. Root-N uptake is sensitive to the rate of mass flow for widely spaced roots with high N uptake capacity, but not for closely spaced roots or roots with low uptake capacity. The results point to a benefit of root switching from high- to low-affinity N transport systems in the presence of mass flow. Simulations indicate a strong impact of soil water uptake on N delivery to widely spaced roots through transpirationally driven mass flow. Furthermore, a given rate of N uptake per unit soil volume may be achieved by lower root biomass in the presence of mass flow.

Quantitative losses vs. qualitative stability of ectomycorrhizal community responses to 3years of experimental summer drought in a beech-spruce forest.

Forest ecosystems in central Europe are predicted to face an increasing frequency and severity of summer droughts because of global climate change. European beech and Norway spruce often coexist in these forests with mostly positive effects on their growth. However, their different below-ground responses to drought may lead to differences in ectomycorrhizal (ECM) fungal community composition and functions which we examined at the individual root and ecosystem levels. We installed retractable roofs over plots in Kranzberg Forest (11°39'42″E, 48°25'12″N; 490m a.s.l.) to impose repeated summer drought conditions and assigned zones within each plot where trees neighboured the same or different species to study mixed species effects. We found that ECM fungal community composition changed and the numbers of vital mycorrhizae decreased for both tree species over 3 drought years (2014-2016), with the ECM fungal community diversity of beech exhibiting a faster and of spruce a stronger decline. Mixed stands had a positive effect on the ECM fungal community diversity of both tree species after the third drought year. Ectomycorrhizae with long rhizomorphs increased in both species under drought, indicating long-distance water transport. However, there was a progressive decline in the number of vital fine roots during the experiment, resulting in a strong reduction in enzyme activity per unit volume of soil. Hydrolytic enzyme activities of the surviving ectomycorrhizae were stable or stimulated upon drought, but there was a large decline in ECM fungal species with laccase activity, indicating a decreased potential to exploit nutrients bound to phenolic compounds. Thus, the ectomycorrhizae responded to repeated drought by maintaining or increasing their functionality at the individual root level, but were unable to compensate for quantitative losses at the ecosystem level. These findings demonstrate a strong below-ground impact of recurrent drought events in forests.

Calculation procedures to estimate fine root production rates in forests using two-dimensional fine root data obtained by the net sheet method.

Several recent studies have used the net sheet method to estimate fine root production rates in forest ecosystems, wherein net sheets are inserted into the soil and fine roots growing through them are observed. Although this method has advantages in terms of its easy handling and low cost, there are uncertainties in the estimates per unit soil volume or unit stand area, because the net sheet is a two-dimensional material. Therefore, this study aimed to establish calculation procedures for estimating fine root production rates from two-dimensional fine root data on net sheets. This study was conducted in a hinoki cypress (Chamaecyparis obtusa (Sieb. & Zucc.) Endl.) stand in western Japan. We estimated fine root production rates in length and volume from the number (RN) and cross-sectional area (RCSA) densities, respectively, for fine roots crossing the net sheets, which were then converted to dry mass values. For these calculations, we used empirical regression equations or theoretical equations between the RN or RCSA densities on the vertical walls of soil pits and fine root densities in length or volume, respectively, in the soil, wherein the theoretical equations assumed random orientation of the growing fine roots. The estimates of mean fine root (diameter <1 mm) production rates were ∼80-100 g m-2 year-1 using the empirically obtained regression equations, whereas those from the theoretical equations were ∼40-50 g m-2 year-1. The difference in the estimates was attributed to larger slope values of the empirical regression equations than those of the theoretical equations, suggesting that fine root orientation was not random in our study site. In light of these results, we concluded that fine root production rates were successfully estimated from two-dimensional fine root data on the net sheets using these calculation procedures, with the empirical regression equations reflecting fine root orientation in the study site.

Viscous‐Flow Approach to In Situ Infiltration and In Vitro Saturated Hydraulic Conductivity Determination

Core Ideas Viscous film flow can explain non‐equilibrium in the Richards equation. Parameters of viscous flow were determined in situ and in vitro. If macropore‐flow restriction applies, then kinematic wave theory is applicable. Infiltration is dominantly gravity driven, and a viscous‐flow approach was developed. Laminar film flow equilibrates gravity with the viscous force and a constant flow velocity evolves during a period lasting 3/2 times the duration of a constant input rate, qS. Film thickness F and the specific contact area L of the film per unit soil volume are the key parameters. Sprinkler irrigation produced in situ time series of volumetric water contents, θ(z,t), as determined with TDR probes. The wetting front velocity v and the time series of the mobile water content, w(z,t) were deduced from θ(z,t). In vitro steady flow in a core of saturated soil provided volume flux density, q(z,t), and flow velocity, v, as determined from a heat front velocity. The F and L parameters of the in situ and the in vitro experiments were compared. The macropore‐flow restriction states that, for a particular permeable medium, the specific contact area L must be independent from qS i.e., dL/dqS = 0. If true, then the relationship of qS ∝ v3/2 could scale a wide range of input rates 0 ≤ qS ≤ saturated hydraulic conductivity, Ksat, into a permeable medium, and kinematic‐wave theory would become a versatile tool to deal with non‐equilibrium flow. The viscous‐flow approach is based on hydromechanical principles similar to Darcy's law, but currently it is not suited to deduce flow properties from specified individual spatial structures of permeable media.

Use of water extraction variability to screen for sunflower genotypes well adapted to soil water limitation.

In conditions of water deficit, plant yield depends mostly on the ability of the plant to explore soil profile and its water uptake capacity per unit volume of soil. In this study, the value of soil water extraction properties for use in sunflower breeding was evaluated. Five experiments were carried out in pots, in greenhouses, from 2005 to 2009, in Montpellier, France. Elite sunflower cultivars and experimental hybrids obtained from a factorial cross between five female and five male inbred lines were grown. The soil water extraction performance of the plants was characterised by the soil water content at minimal stomatal conductance (SWCgs=0) and the index of water extraction (IEgen), which was calculated as the relative value of SWCgs=0 to the performance of the cultivar NKMelody. Heritability (H2) was estimated for the experimental hybrids. Phenotypic variability of the SWCgs=0 was observed with a significant effect of the environment and the genotype. The latest released cultivars were observed as the best performing one in water extraction with an IEgen under 0.85. This trait was found to be suitable for use in comparisons of the soil water extraction performances of different genotypes. The high H2 value for SWCgs=0 (0.77 and 0.81) and the significant correlation (r2=0.70, P<0.001) between the values obtained for the experimental hybrids and the mean values of the general combining ability (GCA) for the parental lines showed that this trait is heritable and could be used in plant breeding programs. Phenotyping methods and the usefulness of this trait in crop modelling are discussed.