Hydraulic properties of layered soils influence survival of Rhodes grass (Chloris gayana Kunth.) during water stress

Survival of vegetation on soil-capped mining wastes is often impaired during dry seasons due to the limited amount of water stored in the shallow soil capping. Growth and survival of Rhodes grass (Chloris gayana) during soil drying on various layered capping sequences constructed of combinations of topsoil, subsoil, seawater-neutralised residue sand and low grade bauxite was determined in a glasshouse. The aim was to describe the survival of Rhodes grass in terms of plant and soil water relationships. The soil water characteristic curve and soil texture analysis was a good predictor of plant survival. The combination of soil with a high water holding capacity and low soil water diffusivity (e.g. subsoil with high clay contents) with soil having a high water holding capacity and high diffusivity (e.g. residue sand) gave best survival during drying down (up to 88 days without water), whereas topsoil and low grade bauxite were unsuitable (plants died within 18-39 days). Clayey soil improved plant survival by triggering a water stress response during peak evaporative water demand once residue sand dried down and its diffusivity fell below a critical range. Thus, for revegetation in seasonally dry climates, soil capping should combine one soil with low diffusivity and one or more soils with high total water holding capacity and high diffusivity.

Estimation of water retention curve from mercury intrusion porosimetry and van Genuchten model

The water retention curve (WRC) is a hydraulic characteristic of concrete required for advanced modeling of water (and thus solute) transport in variably saturated, heterogeneous concrete. Unfortunately, determination by a direct experimental method (for example, measuring equilibrium moisture levels of large samples stored in constant humidity cells) is a lengthy process, taking over 2 years for large samples. A surrogate approach is presented in which the WRC is conveniently estimated from mercury intrusion porosimetry (MIP) and validated by water sorption isotherms: The well-known Barrett, Joyner and Halenda (BJH) method of estimating the pore size distribution (PSD) from the water sorption isotherm is shown to complement the PSD derived from conventional MIP. This provides a basis for predicting the complete WRC from MIP data alone. The van Genuchten equation is used to model the combined water sorption and MIP results. It is a convenient tool for describing water retention characteristics over the full moisture content range. The van Genuchten parameter estimation based solely on MIP is shown to give a satisfactory approximation to the WRC, with a simple restriction on one. of the parameters.

Artificial pumping errors in the Kool-Parker scaling model of soil moisture hysteresis

Hysteresis models that eliminate the artificial pumping errors associated with the Kool-Parker (KP) soil moisture hysteresis model, such as the Parker-Lenhard (PL) method, can be computationally demanding in unsaturated transport models since they need to retain the wetting-drying history of the system. The pumping errors in these models need to be eliminated for correct simulation of cyclical systems (e.g. transport above a tidally forced watertable, infiltration and redistribution under periodic irrigation) if the soils exhibit significant hysteresis. A modification is made here to the PL method that allows it to be more readily applied to numerical models by eliminating the need to store a large number of soil moisture reversal points. The modified-PL method largely eliminates any artificial pumping error and so essentially retains the accuracy of the original PL approach. The modified-PL method is implemented in HYDRUS-1D (version 2.0), which is then used to simulate cyclic capillary fringe dynamics to show the influence of removing artificial pumping errors and to demonstrate the ease of implementation. Artificial pumping errors are shown to be significant for the soils and system characteristics used here in numerical experiments of transport above a fluctuating watertable. (c) 2005 Elsevier B.V. All rights reserved.

Facilitating soil-and plant-water research through the use of TDR

The development of TDR for measurement of soil water content and electrical conductivity has resulted in a large shift in measurement methods for a breadth of soil and hydrological characterization efforts. TDR has also opened new possibilities for soil and plant research. Five examples show how TDR has enhanced our ability to conduct our soil- and plant-water research. (i) Oxygen is necessary for healthy root growth and plant development but quantitative evaluation of the factors controlling oxygen supply in soil depends on knowledge of the soil water content by TDR. With water content information we have modeled successfully some impact of tillage methods on oxygen supply to roots and their growth response. (ii) For field assessment of soil mechanical properties influencing crop growth, water content capability was added to two portable soil strength measuring devices; (a) A TDT (Time Domain Transmittivity)-equipped soil cone penetrometer was used to evaluate seasonal soil strengthwater content relationships. In conventional tillage systems the relationships are dynamic and achieve the more stable no-tillage relationships only relatively late in each growing season; (b) A small TDR transmission line was added to a modified sheargraph that allowed shear strength and water content to be measured simultaneously on the same sample. In addition, the conventional graphing procedure for data acquisition was converted to datalogging using strain gauges. Data acquisition rate was improved by more than a factor of three with improved data quality. (iii) How do drought tolerant plants maintain leaf water content? Non-destructive measurement of TDR water content using a flat serpentine triple wire transmission line replaces more lengthy procedures of measuring relative water content. Two challenges remain: drought-stressed leaves alter salt content, changing electrical conductivity, and drought induced changes in leaf morphology affect TDR measurements. (iv) Remote radar signals are reflected from within the first 2 cm of soil. Appropriate calibration of radar imaging for soil water content can be achieved by a parallel pair of blades separated by 8 cm, reaching 1.7 cm into soil and forming a 20 cm TDR transmission line. The correlation between apparent relative permittivity from TDR and synthetic aperture radar (SAR) backscatter coefficient was 0.57 from an airborne flyover. These five examples highlight the diversity in the application of TDR in soil and plant research.

Sugarcane water use from shallow water tables: implications for improving irrigation water use efficiency

The resource potential of shallow water tables for cropping systems has been investigated using the Australian sugar industry as a case study. Literature concerning shallow water table contributions to sugarcane crops has been summarised, and an assessment of required irrigation for water tables to depths of 2 m investigated using the SWIMv2.1 soil water balance model for three different soils. The study was undertaken because water availability is a major limitation for sugarcane and other crop production systems in Australia and knowledge on how best to incorporate upflow from water tables in irrigation scheduling is limited. Our results showed that for the three soils studied (representing a range of permeabilities as defined by near-saturated hydraulic conductivities), no irrigation would be required for static water tables within 1 m of the soil surface. Irrigation requirements when static water tables exceeded 1 m depth were dependent on the soil type and rooting characteristics (root depth and density). Our results also show that the near-saturated hydraulic conductivities are a better indicator of the ability of water tables below 1 m to supply sufficient upflow as opposed to soil textural classifications. We conclude that there is potential for reductions in irrigation and hence improvements in irrigation water use efficiency in areas where shallow water tables are a low salinity risk: either fresh, or the local hydrology results in net recharge. (C) 2003 Elsevier B.V. All rights reserved.

Potential yield and water-use effficiency benefits in sorghum from limited maximum transpiration rate

Limitations on maximum transpiration rates, which are commonly observed as midday stomatal closure, have been observed even under well-watered conditions. Such limitations may be caused by restricted hydraulic conductance in the plant or by limited supply of water to the plant from uptake by the roots. This behaviour would have the consequences of limiting photosynthetic rate, increasing transpiration efficiency, and conserving soil water. A key question is whether the conservation of water will be rewarded by sustained growth during seed fill and increased grain yield. This simulation analysis was undertaken to examine consequences on sorghum yield over several years when maximum transpiration rate was imposed in a model. Yields were simulated at four locations in the sorghum-growing area of Australia for 115 seasons at each location. Mean yield was increased slightly ( 5 - 7%) by setting maximum transpiration rate at 0.4 mm h(-1). However, the yield increase was mainly in the dry, low-yielding years in which growers may be more economically vulnerable. In years with yield less than similar to 450 g m(-2), the maximum transpiration rate trait resulted in yield increases of 9 - 13%. At higher yield levels, decreased yields were simulated. The yield responses to restricted maximum transpiration rate were associated with an increase in efficiency of water use. This arose because transpiration was reduced at times of the day when atmospheric demand was greatest. Depending on the risk attitude of growers, incorporation of a maximum transpiration rate trait in sorghum cultivars could be desirable to increase yields in dry years and improve water use efficiency and crop yield stability.

A simulation model of cereal-legume intercropping systems for semi-arid regions I. Model development

Cereal-legume intercropping plays an important role in subsistence food production in developing countries, especially in situations of limited water resources. Crop simulation can be used to assess risk for intercrop productivity over time and space. In this study, a simple model for intercropping was developed for cereal and legume growth and yield, under semi-arid conditions. The model is based on radiation interception and use, and incorporates a water stress factor. Total dry matter and yield are functions of photosynthetically active radiation (PAR), the fraction of radiation intercepted and radiation use efficiency (RUE). One of two PAR sub-models was used to estimate PAR from solar radiation; either PAR is 50% of solar radiation or the ratio of PAR to solar radiation (PAR/SR) is a function of the clearness index (K-T). The fraction of radiation intercepted was calculated either based on Beer's Law with crop extinction coefficients (K) from field experiments or from previous reports. RUE was calculated as a function of available soil water to a depth of 900 mm (ASW). Either the soil water balance method or the decay curve approach was used to determine ASW. Thus, two alternatives for each of three factors, i.e., PAR/SR, K and ASW, were considered, giving eight possible models (2 methods x 3 factors). The model calibration and validation were carried out with maize-bean intercropping systems using data collected in a semi-arid region (Bloemfontein, Free State, South Africa) during seven growing seasons (1996/1997-2002/2003). The combination of PAR estimated from the clearness index, a crop extinction coefficient from the field experiment and the decay curve model gave the most reasonable and acceptable result. The intercrop model developed in this study is simple, so this modelling approach can be employed to develop other cereal-legume intercrop models for semi-arid regions. (c) 2004 Elsevier B.V. All rights reserved.

Seasonal patterns of growth, dehydrins and water-soluble carbohydrates in genotypes of Dactylis glomerata varying in summer dormancy

Background and Aims Summer dormancy in perennial grasses has been studied inadequately, despite its potential to enhance plant survival and persistence in Mediterranean areas. The aim of the present work was to characterize summer dormancy and dehydration tolerance in two cultivars of Dactylis glomerata (dormant 'Kasbah', non-dormant 'Oasis') and their hybrid using physiological indicators associated with these traits. Methods Dehydration tolerance was assessed in a glasshouse experiment, while seasonal metabolic changes which produce putative protectants for drought, such as carbohydrates and dehydrins that might be associated with summer dormancy, were analysed in the field. Key Results The genotypes differed in their ability to survive increasing soil water deficit: lethal soil water potential (ψ(s)) was -3(.)4 MPa for 'Kasbah' (although non-dormant), -1(.)3 MPa for 'Oasis', and -1(.)6 MPa for their hybrid. In contrast, lethal water content of apices was similar for all genotypes (approx. 0(.)45 g H2O g d. wt(-1)), and hence the greater survival of 'Kasbah' can be ascribed to better drought avoidance rather than dehydration tolerance. In autumn-sown plants, 'Kasbah' had greatest dormancy, the hybrid was intermediate and 'Oasis' had none. The more dormant the genotype, the lower the metabolic activity during summer, and the earlier the activity declined in spring. Decreased monosaccharide content was an early indicator of dormancy induction. Accumulation of dehydrins did not correlate with stress tolerance, but dehydrin content was a function of the water status of the tissues, irrespective of the soil moisture. A protein of approx. 55 kDa occurred in leaf bases of the most dormant cultivar even in winter. Conclusions Drought avoidance and summer dormancy are correlated but can be independently expressed. These traits are heritable, allowing selection in breeding programmes.

Chloride as a signature indicator of soil textural and hydrologic stratigraphies in variable charge deep profiles

Soil properties that influence water movement through profiles are important for determining flow paths, reactions between soil and solute, and the ultimate destination of solutes. This is particularly important in high rainfall environments. For highly weathered deep profiles, we hypothesize that abrupt changes in the distribution of the quotient [QT = (silt + sand)/clay] reflect the boundaries between textural units or textural (TS) and hydrologic (HS) stratigraphies. As a result, QT can be used as a parameter to characterize TS and as a surrogate for HS. Secondly, we propose that if chloride distributions were correlated with QT, under non-limiting anion exchange, then chloride distributions can be used as a signature indicator of TS and HS. Soil cores to a depth of 12.5 in were taken from 16 locations in the wet tropical Johnstone River catchment of northeast Queensland, Australia. The cores belong to nine variable charge soil types and were under sugarcane (Saccharun officinarum-S) production, which included the use of potassium chloride, for several decades. The cores were segmented at I m depth increments and subsamples were analysed for chloride, pH, soil water content (theta), clay, silt and sand contents. Selected bores were capped to serve as piezometers to monitor groundwater dynamics. Depth incremented QT, theta and chloride correlated, each individually, significantly with the corresponding profile depth increments, indicating the presence of textural, hydrologic and chloride gradients in profiles. However, rapid increases in QT down the profile indicated abrupt changes in TS, suggesting that QT can be used as a parameter to characterize TS and as a surrogate for HS. Abrupt changes in chloride distributions were similar to QT, suggesting that chloride distributions can be used as a signature indicator of QT (TS) and HS. Groundwater data indicated that chloride distributions depended, at least partially, on groundwater dynamics, providing further support to our hypothesis that chloride distribution can be used as a signature indicator of HS. Copyright (c) 2005 John Wiley & Sons, Ltd.

Management of a previously eroded tropical Alfisol with herbaceous legumes: Soil loss and physical properties under mound tillage

A study was carried out on a previously eroded Oxic Paleustalf in Ibadan, southwestern Nigeria to determine the extent of soil degradation under mound tillage with some herbaceous legumes and residue management methods. A series of factorial experiments was carried out on 12 existing runoff plots. The study commenced in 1996 after a 5-year natural fallow. Mound tillage was introduced in 1997 till 1999. The legumes - Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides - were intercropped with maize in 1996 and 1998 while yam was planted alone in 1997 and 1999. This paper covers 1997-1999. At the end of each year, residues were either burned or mulched on respective plots. Soil loss, runoff, variations in mound height, bulk density, soil water retention and sorptivity were measured. Cumulative runoff was similar among interactions of legume and residue management in 1997 (57-151 mm) and 1999 (206-397 mm). However, in 1998, cumulative runoff of 95 mm observed for Mucuna-burned residue was significantly greater than the 46 mm observed for cowpea-burned residue and the 39-51 mm observed for mulched residues of cowpea, Mucuna and Pueraria. Cumulative soil loss of 7.6 Mg ha(-1) observed for Mucuna-burned residue in 1997 was significantly greater than those for Pueraria-mulched (0.9 Mg ha(-1)) and Mucuna-mulched (1.4 Mg ha(-1)) residues whereas in 1999 it was similar to soil loss from cowpea treatments and Pueraria-burned residue (2.3-5.3 Mg ha(-1)). There were no significant differences in soil loss in 1998 (1-3.2 Mg ha(-1)) whereas Mucuna-burned residue had a greater soil loss (28.6 Mg ha(-1)) than mulched cowpea (6.9 Mg ha(-1)) and Pueraria (5.4 Ms ha(-1)). Mound heights (23 cm average) decreased non-linearly with cumulative rainfall. A cumulative rainfall of 500 mm removed 0.3-2.3 cm of soil from mounds in 1997, 3.5-6.9 cm in 1998 and 2.3-4.6 cm in 1999, indicating that (detached but less transported) soil from mounds was far higher than observed soil loss in each year. Soil water retention was improved at potentials ranging from -1 to -1500 kPa by Mucuna-mulched residue compared to the various burned-residue treatments. Also, mound sorptivity at -1 cm water head (14.3 cm h(-1/2)) was higher than furrow sorptivity (8.5 cm h(-1/2)), indicating differences in hydraulic characteristics between mound and furrow. Pueraria-mulched residues for mounds had the highest sorptivity of 17.24 cm h(-1/2), whereas the least value of 6.96 cm h(-1/2) was observed in furrow of Mucuna-burned residue. Pueraria phas eoloides was considered the best option for soil conservation on the previously eroded soil, cultivated with mound tillage. (c) 2005 Elsevier B.V. All rights reserved.

Summer dormancy in Dactylis glomerata L.: the influence of season of sowing and a simulated mid-summer storm on two contrasting cultivars

A series of trials to increase understanding of the summer dormancy trait in Dactylis glomerata was conducted. Autumn-sown reproductive and younger, spring-sown plants of 2 drought-resistant cultivars, contrasting for summer dormancy, were established and then tested in summer 2002 under long drought, drought + midsummer storm, or full irrigation. The autumn-sown reproductive plants of cv. Kasbah were summer dormant under all moisture regimes and exhibited the characteristic traits including growth cessation, rapid herbage senescence, and dehydration of surviving organs (-6.7MPa). Cultivar Kasbah used 8% less soil water over the summer and also began to rehydrate its leaf bases from conserved soil water before the drought broke. The non-dormant cv. Medly grew for 10 days longer under drought and whenever moisture was applied; Medly also responded to the storm with a decline in dehydrin expression in leaf bases, whereas no decline occurred in Kasbah, presumably because it remained dormant and therefore much drier. The irrigated, younger, spring-sown swards of cv. Kasbah had restrained growth and produced only about 25% of the herbage of cv. Medly. Drought reduced activity and growth of young plants of both cultivars, but whereas Medly regrew in response to the storm, cv. Kasbah did not, indicating that dormancy, although only partially expressed after spring sowing, was reinforced by summer drought. A longer drought in 2003 caused a 22% loss of the basal cover in cv. Medly, whereas Kasbah fully maintained its sward and therefore produced a higher post-drought autumn yield. This work confirms summer dormancy as a powerful trait for improving persistence over long, dry summers.

The effect of salinity on plant available water

Salinity acts to inhibit plant access to soil water by increasing the osmotic strength of the soil solution. As the soil dries, the soil solution becomes increasingly concentrated, further limiting plant access to soil water. An experiment was conducted to examine the effect of salt on plant available water in a heavy clay soil, using a relatively salt tolerant species, wheat ‘Kennedy’, and a more salt sensitive species, chickpea ‘Jimbour’. Sodium chloride was applied to Red Ferrosol at 10 rates from 0 to 3 g/kg. Plants were initially maintained at field capacity. After 3 weeks, plants had become established and watering was ceased. The plants then grew using the water stored in the soil. Once permanent wilting point was reached plants were harvested, and soil water content was measured. The results showed that without salt stress, wheat and chickpea extracted approximately the same amount of water. However, as the salt concentration increased, the ability of chickpea to extract water was severely impaired, while wheat’s ability to extract water was not affected over the range of concentrations examined. Growth of both wheat and chickpea was reduced even from low salt concentrations. Possible explanations for this are that the effect on growth is due to Cl- toxicity and that this occurs at lower concentrations than the osmotic effect of salinity, or that the metabolic demands of maintaining plant water balance and extracting soil water under saline conditions result in reduced growth.

Cutting management of multipurpose tree legumes: effects on green herbage production, leaf retention and water-use-efficiency during the dry season in Timor, Indonesia

Edible herbage production and water-use-efficiency of three tree legumes (Leucaena leucocephala cv. Tarramba, L. pallida x L. leucocephala (KX2) and Gliricidia sepium), cut at different times of the year (February, April, June and uncut) were compared in a semi-arid area of Timor Island, Indonesia. Cutting in the early and mid dry-season (April and June) resulted in higher total leaf production (P< 0.05) and water-use-efficiency (P< 0.05), than cutting late in the wet-season (February) or being left uncut. For the leucaena treatments removing leaf in the early to mid dry-season reduced transpiration, saving soil water for subsequent regrowth as evidenced by the higher relative water contents of leaves from these treatments. This cutting strategy can be applied to local farming conditions to increase the supply of feed for livestock during the dry season.

Selection of species and provenances for low-rainfall areas: physiological responses of Eucalyptus cloeziana and Eucalyptus argophloia to seasonal conditions in subtropical Queensland

Responses of stomatal conductance (g(s)) and net photosynthesis (A) to changes in soil water availability, photosynthetic photon flux density (Q), air temperature (1) and leaf-to-air vapour pressure deficit (D) were investigated in 4-year-old trees of a dry inland provenance of Eucalyptus argophloia Blakely, and two dry inland provenances (Coominglah and Hungry Hills) and a humid coastal provenance (Wolvi) of Eucalyptus cloeziana F. Muell. between April 2001 and April 2002 in southeast Queensland, Australia. There were minimal differences in A, g, and water relations variables among the coastal and inland provenances of E. cloeziana but large differences between E. argophloia and E. cloeziana. E. argophloia and to a lesser extent the Hungry Hills (inland) provenance of E. cloeziana maintained relatively higher pre-dawn water potential (psi(pd)) during the dry season suggesting possible access to water at depth. Simple phenomenological models of stomatal conductance as a function of Q, T and D explained 60% of variation in gs in E. cloeziana and more than 75% in E. argophloia, when seasonal effect was incorporated in the model. A Ball-Berry model for net photosynthesis explained between 70 and 80% of observed variation in A in both species. These results have implications in matching the dry and humid provenances of E. cloeziana and E. argophloia to suitable sites in subtropical environments. (C) 2004 Elsevier B.V. All rights reserved.