Soil Water Potential Gradient Research Articles

Mechanistically derived macroscopic root water uptake functions: The α and ω of root water uptake functions

AbstractWater uptake by plant roots is an important component of the soil water balance. Predicting to what extent potential transpiration from the canopy, that is, transpiration demand, can be met by supply of water from the soil through the root system is crucial to simulate the actual transpiration and assess vegetation water stress. In models that simulate the dynamics of vertical soil water content profiles as a function of water fluxes and soil water potential gradients, the root water uptake (RWU) distribution is represented by macroscopic sink terms. We present RWU functions that calculate sink terms based on a mechanistic model of water flow in the soil–root system. Based on soil–root hydraulics, we define α‐supply functions representing the maximal uptake by the root system from a certain soil depth when the root collar water potential equals the wilting point, ω‐supply factors representing the maximal supply from the entire root system, and a critical ωc factor representing the potential transpiration demand. These functions and factors are subsequently used to calculate RWU distributions directly from potential transpiration or demand and the soil water potentials. Unlike currently used approaches, which define α‐stress functions and ω factors representing ratios of actual uptake to uptake demand, the supply‐based formulations can be derived directly from soil and root hydraulic properties and can represent processes like root hydraulic redistribution and hydraulic lift.

Study on Spatiotemporal Transport Characteristics of Soil Moisture in Layered Heterogeneous Vadose Zone Based on HYDRAS-3D

The heterogeneity of layered soils affects the transport processes of water in the vadose zone. However, the mechanism of soil moisture transport in the vadose zone under heterogeneous media conditions, especially the three-dimensional transport mechanism, is a frontier problem to be studied. In order to reveal the law of soil water transport in the vadose zone under heterogeneous media, this paper constructs a three-dimensional model of water transport using HYDRAS-3D (V 2.x) software through in situ tests of water transport in 3 × 3 × 4 m sample and verifies and analyzes the model. The vertical distribution and variability of soil water content and the temporal and spatial rules of soil water transversal transport at the soil–layer interface were analyzed. The results indicate that after adjusting the parameters in the HYDRAS-3D model using measured values, the simulation results are reasonable, and the model has high reliability. To represent the water content variability characteristics of the test tube profile, water movement in the vadose zone was classified as follows: (I) steady period (April–September, less affected by rainfall), (II) slow change period (January–March), and (III) rainfall rapid change period (June, when the impact of rainfall on the test cylinder was greater). The two largest values of the soil water potential variation gradient, 19.9 and 17.8 cm/d, were observed in the silty clay layer of the test cylinder, and the influence of evaporation and infiltration in the silty clay layer was most notable at the interface. The lateral transport of soil water at the interface was influenced to a certain degree by the layered heterogeneity, wherein the fine sand layer exhibited the most pronounced impact. This was followed by silt and silty clay layers, with the silt layer showing the lowest degree of influence. The research results can provide scientific reference for the rational planning of water resources in the Yinchuan Plain.

Evaporation-driven internal hydraulic redistribution alleviates root drought stress: Mechanisms and modeling.

Many tree species have developed extensive root systems that allow them to survive in arid environments by obtaining water from a large soil volume. These root systems can transport and redistribute soil water during drought by hydraulic redistribution (HR). A recent study revealed the phenomenon of evaporation-driven hydraulic redistribution (EDHR), which is driven by evaporative demand (transpiration). In this study, we confirmed the occurrence of EDHR in Chinese white poplar (Populus tomentosa) through root sap flow measurements. We utilized microcomputed tomography technology to reconstruct the xylem network of woody lateral roots and proposed conceptual models to verify EDHR from a physical perspective. Our results indicated that EDHR is driven by the internal water potential gradient within the plant xylem network, which requires 3 conditions: high evaporative demand, soil water potential gradient, and special xylem structure of the root junction. The simulations demonstrated that during periods of extreme drought, EDHR could replenish water to dry roots and improve root water potential up to 38.9% to 41.6%. This highlights the crucial eco-physiological importance of EDHR in drought tolerance. Our proposed models provide insights into the complex structure of root junctions and their impact on water movement, thus enhancing our understanding of the relationship between xylem structure and plant hydraulics.

Hillslope-scale variability of soil water potential over humid alpine forests: Unexpected high contribution of time-invariant component

Soil water potential (SWP) gradients control water and energy fluxes in the Soil – Plant – Atmosphere Continuum. Exploring the spatial – temporal variability of SWP at the hillslope scale is an essential step towards developing ecohydrological models. In this study, SWP monitoring was carried out on two contrasting, alpine, humid forested hillslopes (i.e., coniferous and broadleaf forests) at three depths (10, 50, and 100 cm). The effects of static (soil properties and topography) and dynamic (precipitation dynamics and soil wetness conditions) factors on SWP dynamics and variability were investigated based on statistical methods, temporal stability, and variance decomposition analysis. The results showed that the SWP in the soil – root zone (10 and 50 cm depths) in the broadleaf forest exhibited higher variability and lower temporal stability than in the coniferous forest, likely because of the stronger evapotranspiration and more variable soil hydraulic conductivity. The relatively high SWP variability in the deeper soil layer (100 cm depth) of both forested hillslopes was due to the frequent lateral/preferential flow. Furthermore, topographic factors (i.e., distance of hillslope to stream and slope) and soil properties (clay and rock fragment contents) regulated the spatial patterns in SWP and influenced representative site selection in the coniferous and broadleaf forests, respectively. Decomposition of the spatial variance showed that the time-invariant component was the dominant contributor to the total spatial variance. An exponentially increasing relationship between the time-invariant component and mean SWP and a concave-up parabolic shape between the contribution of the temporal anomalies and mean SWP were observed. Our results provide an improved understanding of the temporal and spatial variability in terms of SWP in forested hillslopes.

Competition rather than facilitation affects plant performance across an abiotic stress gradient in a restored California salt marsh

The Stress Gradient Hypothesis predicts facilitation will become more important than competition where abiotic stress is high, and the framework successfully predicts positive interactions between species in many systems. Fewer studies have focused on intraspecific facilitation, and to our knowledge none examine intraspecific interactions in Pacific Coast salt marshes of North America. We used two species that tend to occur in large, monospecific patches to test for intraspecific facilitation in a restored California marsh, where tides and evaporation during summer create moisture and salinity gradients across elevation. We tested performance of Frankenia salina or Jaumea carnosa in restoration plots using two treatments: clustered plantings to promote facilitation, and widely spaced plantings to limit interaction between individuals. We characterized the soil water potential gradient and measured plant survival, cover, physiology, and susceptibility to herbivory across elevation and planting treatments. Soil water potential declined sharply with elevation, suggesting plant stress should be higher upslope. However, tissue water potential was unaffected by elevation, and survival was high—suggesting growing conditions remained benign. A seawater addition treatment did not alter the response of plants or plant interactions to the abiotic gradient. At 19 months, intraspecific competition prevailed across the entire elevation gradient for both species, with less transplant cover in clustered plantings. However, clustered Frankenia fared better during a transient period of heavy rabbit herbivory. Aside from this temporary benefit, clustering did not reduce stress and strongly suppressed growth—suggesting that restoration designs for the high marsh should minimize competition.

Root System Scale Models Significantly Overestimate Root Water Uptake at Drying Soil Conditions.

Soil hydraulic conductivity (ksoil) drops significantly in dry soils, resulting in steep soil water potential gradients (ψs) near plant roots during water uptake. Coarse soil grid resolutions in root system scale (RSS) models of root water uptake (RWU) generally do not spatially resolve this gradient in drying soils which can lead to a large overestimation of RWU. To quantify this, we consider a benchmark scenario of RWU from drying soil for which a numerical reference solution is available. We analyze this problem using a finite volume scheme and investigate the impact of grid size on the RSS model results. At dry conditions, the cumulative RWU was overestimated by up to 300% for the coarsest soil grid of 4.0 cm and by 30% for the finest soil grid of 0.2 cm, while the computational demand increased from 19 s to 21 h. As an accurate and computationally efficient alternative to the RSS model, we implemented a continuum multi-scale model where we keep a coarse grid resolution for the bulk soil, but in addition, we solve a 1-dimensional radially symmetric soil model at rhizosphere scale around individual root segments. The models at the two scales are coupled in a mass-conservative way. The multi-scale model compares best to the reference solution (−20%) at much lower computational costs of 4 min. Our results demonstrate the need to shift to improved RWU models when simulating dry soil conditions and highlight that results for dry conditions obtained with RSS models of RWU should be interpreted with caution.

Water use efficiency of castor bean under semi-arid conditions of Brazil

Castor bean is one of the main and promising agricultural crops for the production of biodiesel, castor oil and castor bean cake, which have high added value, especially the castor oil. The objective of the present study was to evaluate the water use efficiency (WUE) of castor bean cultivars by assessing the soil water balance components (storage and water storage variation; internal drainage; capillary rise; irrigation depth; rainfall and actual evapotranspiration), according to the phenological stages. The experimental design was in randomized blocks, in 8 × 6 factorial scheme (eight cultivars: BRS Paraguaçu; EBDA MPA11; EBDA MPB01; IAC 2028; IAC 226; BRS Nordestina; IAC Guarani, AG IMA 110–204 and BRS Energia and six periods of evaluation), with three repetitions and 15 plants in each experimental plot. Daily monitoring of soil water content was performed with a capacitance probe (Frequency Domain Reflectometry - FDR, PR2/6 model), at depths of 0.1, 0.2, 0.3, 0.4 and 0.6 m, with 0.6 m being the lower limit of the soil control volume for castor bean. The total soil water potential gradient was determined with tensiometers installed at depths of 0.5 and 0.7 m, and the flux densities were estimated using the Darcy-Buckingham equation. Supplemental irrigation was efficient in maintaining water availability during the crop cycle. Soil water storage was lower for the most productive castor bean cultivar (EBDA MPA 11), with an inverse relationship between yield and water storage, due to its water requirement. The highest and lowest values of internal drainage and capillary rise were found for BRS Nordestina and IAC 226, respectively. EBDA MPA 11 had the best water use efficiency among the castor bean cultivars, differing only from IAC Guarani and EBDA MPB 01.

Physical mechanisms for soil moisture effects on microbial carbon-use efficiency in a sandy loam soil in the western United States

Microbial carbon-use efficiency (CUE) is defined as the portion of carbon (C) incorporated into biomass relative to the total carbon consumed and plays a pivotal role in regulating microbially-mediated C and nutrient transformations in soil. However, little is understood about how CUE is impacted by edaphic properties, like soil moisture. Soil moisture physically regulates microbial activity through its effects on both water potential and water content. Low water potential can result in high, compensatory intracellular solute concentrations that may inhibit biochemical functions through cytoplasmic desiccation, whereas low soil water content results in thin water films that can limit substrate diffusion, reducing microbial access to dissolved substrates. Because these two aspects of soil moisture may affect microbial respiration differently than C assimilation, they may have different effects on CUE. The purpose of this research was to evaluate the relative importance of water potential and water content in regulating CUE of soil microbial communities. Moist soil incubations of a sandy loam soil were used to determine the impact of both aspects of soil moisture on CUE, and soil slurries were used to determine the impact of water potential alone. Both 13C-acetate and 15N-ammonium were added to moist soils and slurries to quantify gross rates of C and N transformations. In moist soils, acetate assimilation and respiration rates and gross N mineralization and immobilization rates increased exponentially with increasing soil moisture (−3.0 to −0.03 MPa). In contrast, acetate assimilation and respiration and gross N transformation rates remained constant in soil slurries across a similar water potential gradient, created by modifying solute concentrations. Similarly, values of CUE in moist soils increased exponentially with increasing soil moisture, whereas slurry values of CUE remained constant across the soil water potential gradient. Because no changes in rates and CUE were observed in slurries, changes observed in moist soils were attributed to limited substrate diffusion associated with low water contents rather than to adverse physiological effects associated with low water potentials. Results of this study demonstrate that limited substrate diffusion is the primary physical mechanism through which soil moisture regulates microbially-mediated C and N transformation rates and CUE in this sandy loam soil.

Linear relation between leaf xylem water potential and transpiration in pearl millet during soil drying

As soil dries, the loss of soil hydraulic conductivity limits water supply to the leaves, which is expected to generate a nonlinear relationship between leaf water potential (ψleaf) and transpiration (E). The effect of soil drying and root properties on ψleaf and E remains elusive. We measured E and ψleaf of pearl millet for varying E and soil moisture using a root pressure chamber. A model of water flow in soil and plant was used to fit the ψleaf (E) relationship. The relation between ψleaf and E was linear at all soil moistures. The slope of ψleaf (E) increased with decreasing soil moisture due to the decreasing soil-root conductance. The fact that the relation remained linear also in dry soils and high E is surprising. Indeed, it indicates that the gradients in soil water potential (ψsoil) were small, probably because of the large root surface (13.5 cm cm−3) active in water uptake. ψleaf at E = 0 was less negative than ψsoil, indicating a more negative osmotic potential in the xylem than in the soil. We propose that the linearity between ψleaf and E and the high ψleaf (E = 0) compared to ψsoil support transpiration in drying soils.

Influence of Slope Angle on Water Flow in a Three‐Layer Capillary Barrier Soil Cover under Heavy Rainfall

A three‐layer capillary barrier soil cover has been recently proposed for minimizing rainwater percolation in humid climates. Its performance has been preliminarily investigated by both physical and numerical modeling at a fixed slope angle of 10°. However, the slope angle of soil cover is varied in engineering practice and hence it is an essential design parameter. In this study, the influence of slope angle on water flow in the three‐layer soil cover was investigated via flume model tests subjected to heavy rainfall. A 3‐m long flume model of soil cover was instrumented to monitor matric potential and volumetric moisture content as well as components of water balance at slope angles of 3°, 10° and 18°. Furthermore, numerical simulations of transient seepage were conducted to back‐analyze the experiments. Both experimental and numerical results showed that the infiltration ratio increased as the slope angle increased because of the increase in the soil water potential gradient normal to the slope gradient. Meanwhile, the lateral water flow in the upper two layers was enhanced by the increase of slope angle due to water backfill effect at the top of clay layer. Correspondingly, the steeper the slope, the lower the matric potential, volumetric moisture content and soil water potential in the upper two layers were. Due to low hydraulic conductivity of the bottom clay layer, the percolation through the cover remained virtually nil at each slope angle. The above findings could have significant implications for the design of sloping three‐layer capillary barrier soil cover.

Potential Reemergence of Seasonal Soil Moisture Anomalies in North America

AbstractSoil moisture anomalies within the root zone (roughly, soil depths down to ~0.4 m) typically persist only a few months. Consequently, land surface–related climate predictability research has often focused on subseasonal to seasonal time scales. However, in this study of multidecadal in situ datasets and land data assimilation products, we find that root zone soil moisture anomalies can recur several or more seasons after they were initiated, indicating potential interannual predictability. Lead–lag correlations show that this recurrence often happens during one fixed season and also seems related to the greater memory of soil moisture anomalies within the layer beneath the root zone, with memory on the order of several months to over a year. That is, in some seasons, notably spring and summer when the vertical soil water potential gradient reverses sign throughout much of North America, deeper soil moisture anomalies appear to return to the surface, thereby restoring an earlier root zone anomaly that had decayed. We call this process “reemergence,” in analogy with a similar seasonally varying process (with different underlying physics) providing winter-to-winter memory to the extratropical ocean surface layer. Pronounced spatial and seasonal dependence of soil moisture reemergence is found that is frequently, but not always, robust across datasets. Also, some of its aspects appear sensitive to spatial and temporal sampling, especially within the shorter available in situ datasets, and to precipitation variability. Like its namesake, soil moisture reemergence may enhance interannual-to-decadal variability, notably of droughts. Its detailed physics and role within the climate system, however, remain to be understood.

Hydraulic redistribution affects modeled carbon cycling via soil microbial activity and suppressed fire.

Hydraulic redistribution (HR) of water from moist to drier soils, through plant roots, occurs world-wide in seasonally dry ecosystems. Although the influence of HR on landscape hydrology and plant water use has been amply demonstrated, HR's effects on microbe-controlled processes sensitive to soil moisture, including carbon and nutrient cycling at ecosystem scales, remain difficult to observe in the field and have not been integrated into a predictive framework. We incorporated a representation of HR into the Community Land Model (CLM4.5) and found the new model improved predictions of water, energy, and system-scale carbon fluxes observed by eddy covariance at four seasonally dry yet ecologically diverse temperate and tropical AmeriFlux sites. Modeled plant productivity and microbial activities were differentially stimulated by upward HR, resulting at times in increased plant demand outstripping increased nutrient supply. Modeled plant productivity and microbial activities were diminished by downward HR. Overall, inclusion of HR tended to increase modeled annual ecosystem uptake of CO2 (or reduce annual CO2 release to the atmosphere). Moreover, engagement of CLM4.5's ground-truthed fire module indicated that though HR increased modeled fuel load at all four sites, upward HR also moistened surface soil and hydrated vegetation sufficiently to limit the modeled spread of dry season fire and concomitant very large CO2 emissions to the atmosphere. Historically, fire has been a dominant ecological force in many seasonally dry ecosystems, and intensification of soil drought and altered precipitation regimes are expected for seasonally dry ecosystems in the future. HR may play an increasingly important role mitigating development of extreme soil water potential gradients and associated limitations on plant and soil microbial activities, and may inhibit the spread of fire in seasonally dry ecosystems.

Hydraulic redistribution: limitations for plants in saline soils.

Hydraulic redistribution (HR), the movement of water from wet to dry patches in the soil via roots, occurs in different ecosystems and plant species. By extension of the principle that HR is driven by gradients in soil water potential, HR has been proposed to occur for plants in saline soils. Despite the inherent spatial patchiness and salinity gradients in these soils, the lack of direct evidence of HR in response to osmotic gradients prompted us to ask the question: are there physical or physiological constraints to HR for plants in saline environments? We propose that build-up of ions in the root xylem sap and in the leaf apoplast, with the latter resulting in a large predawn disequilibrium of water potential in shoots compared with roots and soil, would both impede HR. We present a conceptual model that illustrates how processes in root systems in heterogeneous salinity with water potential gradients, even if equal to those in non-saline soils, will experience a dampened magnitude of water potential gradients in the soil-plant continuum, minimizing or preventing HR. Finally, we provide an outlook for understanding the relevance of HR for plants in saline environments by addressing key research questions on plant salinity tolerance.

An Alternative Tensiometer Design for Deep Vadose Zone Monitoring

The conventional tensiometer is among the most accurate devices for soil water matric potential measurements, as well as for estimations of soil water flux from soil water potential gradients. Uncertainties associated with conventional tensiometers such as caused by ambient temperature effects and the draining of the tensiometer tube, as well as their limitation for deep soil monitoring, has prevented their widespread use for vadose zone monitoring, despite their superior accuracy in general. We introduce an alternative tensiometer design that offers the accuracy of the conventional tensiometer, while minimizing the aforementioned uncertainties and limitations. The proposed alternative tensiometer largely eliminates temperature-induced diurnal fluctuations and uncertainties associated with draining of the tensiometer tube and removes the limitation in installation depth. In addition, the manufacturing costs of this alternative tensiometer design are close to those of the conventional tensiometer, while it is especially suited for monitoring of soil water potential gradients as required for soil water flux measurements.

Hydrological processes and eco-hydrological effects of farmland–forest–desert transition zone in the middle reaches of Heihe River Basin, Gansu, China

Summary The study of the hydrological processes in the transition zone is important, but more complex compared with the homogenous land use units. A typical farmland–forest–desert transition zone in the Heihe River Basin was selected to study the hydrological processes and eco-hydrological effects among these land use units by monitoring the soil water content (SWC), groundwater level (GWL), and vegetation dynamics. Results showed that the sharp fluctuations of daily SWC and GWL in the farmland and the forest were primarily attributed to the irrigation events (7 and 6 times for the farmland and forest, respectively). The hydrological links among the three land use units were exhibited in three patterns. First, the soil water of the upper soil layer near the interface of two land use units moved from the irrigated land use unit to the non-irrigated one under soil water potential gradients through physical diffusion (the lateral water flow rate was less than 1 cm d −1 ). Second, the water flowed from the irrigated land use unit to the non-irrigated one under GWL gradients through groundwater flow (the lateral groundwater flow rate was less than 10 cm d −1 ). Third, a portion of the soil water in the farmland was utilized by the extended root system of the trees. The water exchange between the farmland and the forest resulting from one irrigation event was 5–30 mm, which caused increased GWLs for 1 week. At the forest–farmland boundary, the impacts of the extended tree roots reduced maize growth and extended 10–15 m into the farmland. By contrast, no obvious impacts were observed at the forest–desert boundary. Irrigating the farmland and the forest separately and reducing the width of the forest by 15–20 m would be more beneficial for irrigation water efficiency. These results would be useful for soil water management in terms of water balance impact on ecological construction and implementation of water-saving agriculture, as well as for optimal design of land-use patterns and efficient protection of oasis ecosystems to preserve limited water resources.

A thermodynamic approach to link self-organization, preferential flow and rainfall–runoff behaviour

Abstract. This study investigates whether a thermodynamically optimal hillslope structure can, if existent, serve as a first guess for uncalibrated predictions of rainfall–runoff. To this end we propose a thermodynamic framework to link rainfall–runoff processes and dynamics of potential energy, kinetic energy and capillary binding energy in catchments and hillslopes. The starting point is that hydraulic equilibrium in soil corresponds to local thermodynamic equilibrium (LTE), characterized by a local maximum entropy/minimum of free energy of soil water. Deviations from LTE occur either due to evaporative losses, which increase absolute values of negative capillary binding energy of soil water and reduce its potential energy, or due to infiltration of rainfall, which increases potential energy of soil water and reduces the strength of capillary binding energy. The amplitude and relaxation time of these deviations depend on climate, vegetation, soil hydraulic functions, topography and density of macropores. Based on this framework we analysed the free energy balance of hillslopes within numerical experiments that perturbed model structures with respect to the surface density of macropores. These model structures have been previously shown to allow successful long-term simulations of the water balances of the Weiherbach and the Malalcahuello catchments, which are located in distinctly different pedological and climatic settings. Our findings offer a new perspective on different functions of preferential flow paths depending on the pedological setting. Free energy dynamics of soil water in the cohesive soils of the Weiherbach is dominated by dynamics of capillary binding energy. Macropores act as dissipative wetting structures by enlarging water flows against steep gradients in soil water potential after long dry spells. This implies accelerated depletion of these gradients and faster relaxation back towards LTE. We found two local optima in macropore density that maximize reduction rates of free energy of soil water during rainfall-driven conditions. These two optima exist because reduction rates of free energy are, in this case, a second-order polynomial of the wetting rate, which implicitly depends on macroporosity. An uncalibrated long-term simulation of the water balance of the Weiherbach catchment based on the first optimum macroporosity performed almost as well as the best fit when macroporosity was calibrated to match rainfall–runoff. In the Malalcahuello catchment we did not find an apparent optimum density of macropores, because free energy dynamics of soil water during rainfall-driven conditions is dominated by increases of potential energy. Macropores act as dissipative drainage structures by enhancing export of potential energy. No optimum macropore density exists in this case because potential energy change rates scale linearly with the wetting rate. We found, however, a distinguished macroporosity that assures steady-state conditions of the potential energy balance of the soil, in the sense that average storage of potential energy is compensated by average potential energy export. This distinguished macroporosity was close to the value that yielded the best fit of rainfall–runoff behaviour during a calibration exercise and allowed a robust estimate of the annual runoff coefficient. Our findings are promising for predictions in ungauged catchments (PUB) as the optimal/distinguished model structures can serve as a first guess for uncalibrated predictions of rainfall–runoff. They also offer an alternative for classifying catchments according to their similarity of the free energy balance components.

Patterns, magnitude, and controlling factors of hydraulic redistribution of soil water by Tamarix ramosissima roots

Tamarix spp. (Saltcedar) is a facultative phreatophyte that can tolerate drought when groundwater is not accessed. In addition to deep water uptake, hydraulic redistribution (HR) is another factor contributing to the drought tolerance of Tamarix spp. In this study, data on soil volumetric moisture content (θ), lateral root sap flow, and relevant climate variables were used to investigate the patterns, magnitude, and controlling factors of HR of soil water by roots of Tamarix ramosissima Ledeb. in an extremely arid land in Northwest China. Results showed evident diurnal fluctuations in θ at the depths of 30 and 50 cm, indicating “hydraulic lift” (HL). θ increased remarkably at 10 and 140 cm but decreased at 30 and 50 cm and slightly changed at 80 cm after rainfall, suggesting a possible “hydraulic descent” (HD). However, no direct evidence was observed in the negative flow of lateral roots, supporting HR (including HL and HD) of T. ramosissima. The HR pathway unlikely occurred via lateral roots; instead, HR possibly occurred through adventitious roots with a diameter of 2–5 mm and a length of 60–100 cm. HR at depths of 20–60 cm ranged from 0.01–1.77 mm/d with an average of 0.43 mm/d, which accounted for an average of 22% of the estimated seasonal total water depletion at 0–160 cm during the growing season. The climate factors, particularly vapor pressure deficit and soil water potential gradient, accounted for at least 33% and 45% of HR variations with depths and years, respectively. In summary, T. ramosissima can be added to the wide list of existing species involved in HR. High levels of HR may represent a considerable fraction of daily soil water depletion and substantially improve plant water status. HR could vary tremendously in terms of years and depths, and this variation could be attributed to climate factors and soil water potential gradient.

Nitrogen and phosphorus leaching in a tropical Brazilian soil cropped with sugarcane and irrigated with treated sewage effluent

There are concerns about groundwater contamination with N and P from fertilizers and other anthropogenic wastes. Use of treated sewage effluent (TSE) for crop irrigation can reduce the use of mineral fertilizers; however, it may add more nutrients into the soil than are necessary for crops, increasing the possibility of leaching. Thus, knowledge of nutrient dynamics in TSE irrigated soils is important for the safe use of this resource. However, the reliability of studies regarding ion leaching is limited due the high propagated variance, as these studies involve independent measurements of variables related to soil and soil solution. The objective of this research was to quantify P and N leaching in a TSE-irrigated Brazilian soil and identify the main causes of variance of this quantification. The experiment consisted of a treatment without irrigation and treatments with TSE irrigation to meet 100% and 150% of the crop water demand (CWD). Soil physical properties and soil water potential gradient were used to calculate internal drainage, and nutrient concentration was measured in soil solution samples taken with ceramic suction cups at a depth of 1m. Variance propagation was calculated by linearization, and the contribution of each variable to the total variance was isolated and quantified. Irrigation with TSE increased N leaching; however, when applied in dosages that met 100% of the CWD, it did not threaten the groundwater quality. P leaching was as low as 100gha−1 and was therefore not an environmental concern. N leaching can be estimated considering the total N input and the rainfall; however, long-term data are needed to improve the accuracy of this estimation. The variance propagation of the soil water potential measurements represented up to 70% of the nitrogen leaching variance.

Changes in soil hyphal abundance and viability can alter the patterns of hydraulic redistribution by plant roots

We conducted a mesocosm study to investigate the extent to which the process of hydraulic redistribution of soil water by plant roots is affected by mycorrhizosphere disturbance. We used deuterium-labeled water to track the transfer of hydraulically lifted water (HLW) from well-hydrated donor oaks (Quercus agrifolia Nee.) to drought-stressed receiver seedlings growing together in mycorrhizal or fungicide-treated mesocosms. We hypothesized that the transfer of HLW from donor to receiver plants would be enhanced in undisturbed (non-fungicide-treated) mesocosms where an intact mycorrhizal hyphal network was present. Contrary to expectations, both upper soil and receiver seedlings contained significantly greater proportions of HLW in mesocosms where the abundance of mycorrhizal hyphal links between donor and receiver roots had been sharply reduced by fungicide application. Reduced soil hyphal density and viability likely hampered soil moisture retention properties in fungicide-treated mesocosms, thus leading to faster soil water depletion in upper compartments. The resulting steeper soil water potential gradient between taproot and upper compartments enhanced hydraulic redistribution in fungicide-treated mesocosms. Belowground disturbances that reduce soil hyphal density and viability in the mycorrhizosphere can alter the patterns of hydraulic redistribution by roots through effects on soil hydraulic properties.

Hydraulic redistribution in Eucalyptus kochii subsp. borealis with variable access to fresh groundwater

Salinity caused by land clearing is an important cause of land degradation in the Western Australian wheatbelt. Returning a proportion of the cleared land to higher water use perennial vegetation is one option for reducing or slowing the salinisation of land. Over the course of a year patterns of water use by Eucalyptus kochii subsp borealis (C. Gardner) D. Nicolle, a mallee eucalypt species, were monitored in three landscape positions with different water availability. One treatment had groundwater at 2 m, a second at 4.5 m and a third had groundwater below a silcrete hardpan thought to be impenetrable to roots. Hydraulic redistribution was observed in all landscape positions, and rates were positively correlated with the magnitude of soil water potential gradients within the soil. High rates of hydraulic redistribution, facilitated by abundant deep water may increase tree water use by wetting surface soils and reducing stomatal closure. This effect may be countered by increased soil evaporation of water moved from root to soil following hydraulic redistribution; the net volumes of redistributed water though lateral roots was calculated to be the equivalent of up to 27% of transpiration.