Soil Water Relations Research Articles

Soil water relations, crop production and root pruning of a belt of trees

In southwestern Australia extensive revegetation with perennial plants is required to reverse hydrological imbalance and associated secondary salinity. The effect of a Pinus radiata/Schinus areira belt on soil water relations and crop production on a duplex soil in a medium rainfall area (480 mm) was studied. Root pruning was used to manage tree–crop competition for resources. The tree belt altered the spatial distribution of rainfall and a rain shadow was observed on the leeward side. P. radiata and S. areira reduced soil moisture to a depth of a least 5 m and laterally to a distance of 15 m. This reduced soil moisture buffered against the formation/duration of perched water-tables and groundwater recharge. Buffer strength declined with increasing distance from the trees. P. radiata and S. areira responded differently to root pruning. Sap flux of P. radiata was unaffected by root pruning, whereas, sap flux in S. areira essentially ceased after root pruning. It is suggested that the contrasting response to root pruning was due to the genetic predisposition of each species and environmental conditions. Tree–crop competition for resources reduced wheat growth and grain yield to a distance of 20–30 m from the trees over several years. Partial root pruning sections of the tree belt prior to the break of season reduced, but did not eliminate, tree–crop competition. Data suggested that the crop was moisture limited in pruned sections of the tree belt and that moisture limitation reduced crop growth and grain yield in pruned sections of the tree belt. Increased soil moisture content was not observed in pruned bays until late spring and by the following autumn soil in pruned bays at 10 m from the tree belt was 100 mm wetter than soil in unpruned bays. This increased soil moisture may have implications for crop growth, formation of perched water-tables and groundwater recharge in the second year after root pruning.

Irrigated cotton lint yields as affected by phosphorus fertilizer and landscape position

Phosphorus (P) is the second most limiting nutrient in cotton (Gossypium hirsutum L.) production after nitrogen. Response to P fertilizer, however, is often hard to predict in the Southern High Plains, even with soil test-based applications. Landscape position has a strong influence on yields and perhaps on fertilizer response as well. The objective of this 5-year study (1994–1998) was to determine P fertilizer response in irrigated cotton in different landscape positions. We used an 825-m transect of end to end 15-m plots across a broad swale in an Amarillo fine sandy loam in Lamesa, TX that included three landscape positions, sideslope, bottomslope and drainageway. A randomized complete block design was used with 11 replicates and 5 P rates (0, 22.4, 33.6, 44.8, and 56 kg P ha−1). Analysis of variance showed a linear or quadratic response to P fertilizer in 3 of 5 years, and an effect of landscape position in 4 of 5 years. Four-year lint yield averages, excluding 1997, were 1355 kg ha−1 in the bottomslope position in the landscape, and 1210 and 1226 kg ha−1 on the sideslopes and in the narrow drainageway, respectively. Cross-correlation using the 55 plots as a transect revealed few effects of soil properties on lint yield, but negative correlation between yield and elevation. In 1997, the one year without a landscape effect on yield, more rain fell during the growing season than in the other 4 years. This suggests that yield potential is higher in lower landscape positions because of more favorable soil water relations. Cotton lint response to P fertilizer was evident only in the bottomslopes and in the drainageway, and was absent in the sideslopes, meaning that variable rate fertilizer applications could be linked to management zones based on landscape position.

Managing For Mulch

vegetation that is allowed to naturally decompose isn’t realized by many ranchers and some range mangers. Mulch plays a critical role in soil-water relationships that is often overlooked. This, coupled with current drought conditions in the southwestern United States justifies a consideration of the importance of mulch on rangelands. Several early classic studies (reviewed in this article) reveal the benefits of managing for mulch. Functions Of Mulch Mulch generally refers to the dead vegetative material that covers the surface of the earth. It is also sometimes referred to as plant litter. Mulch can be divided into three classes that include ungrazed mature vegetation residues stil l attached to the plant (cured herbage), vegetation residues detached from plants covering the soil surface (ground litter), and decomposing residues partially or completely incorporated into the soil (humus). Each of the three mulch components contributes in a vital way to soil, plant and watershed health. Standing cured herbage retards the flow of rainfall and dislodged soil from the site. Ground litter provides a cushion between rain drops and the soil preventing “splash effect” or dislodgement of soil particles from rain drops. It also insulates the soil from the sun and reduces evaporation from the soil surface. Humus provides nutrients for plant growth and binds soil particles together. It plays a critical role in deAugust 2001 3

On the use of thermodynamic methods to describe water relations in plants and soil

Both the existing theory as well as the measurements that have traditionally been used to describe the water relations of plants in thermodynamic terms are not consistent with standard thermodynamic theory. In this article, three main errors are identified: (1) the assumption that the internal energy of a system includes the gravitational potential energy, when it does not; (2) the assumption that water spontaneously moves along gradients of chemical potential, when the Gibbs theory predicts that it would spontaneously move across phase boundaries, so that the chemical potential would be equal in adjacent phases; and (3) the assumption that plants and soil are single-phase systems, when they are in fact multi-phase systems. Examples are used to demonstrate the above errors. Measurements which can be used to separate plant and soil systems into bulk phases (i.e. solid, liquid and gas), which have long been used by wood and soil scientists, are shown to be ideal for the purpose of describing plant and soil systems using thermodynamic methods.

Moisture retention properties of a mycorrhizal soil

The water relations of arbuscular mycorrhizal plants have been compared often, but virtually nothing is known about the comparative water relations of mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis typically affects soil structure, and soil structure affects water retention properties; therefore, it seems likely that mycorrhizal symbiosis may affect soil water relations. We examined the water retention properties of a Sequatchie fine sandy loam subjected to three treatments: seven months of root growth by (1) nonmycorrhizal Vigna unguiculata given low phosphorus fertilization, (2) nonmycorrhizal Vigna unguiculata given high phosphorus fertilization, (3) Vigna unguiculata colonized by Glomus intraradices and given low phosphorus fertilization. Mycorrhization of soil had a slight but significant effect on the soil moisture characteristic curve. Once soil matric potential (Ψm) began to decline, changes in Ψm per unit change in soil water content were smaller in mycorrhizal than in the two nonmycorrhizal soils. Within the range of about −1 to −5 MPa, the mycorrhizal soil had to dry more than the nonmycorrhizal soils to reach the same Ψm. Soil characteristic curves of nonmycorrhizal soils were similar, whether they contained roots of plants fed high or low phosphorus. The mycorrhizal soil had significantly more water stable aggregates and substantially higher extraradical hyphal densities than the nonmycorrhizal soils. Importantly, we were able to factor out the possibly confounding influence of differential root growth among mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis affected the soil moisture characteristic and soil structure, even though root mass, root length, root surface area and root volume densities were similar in mycorrhizal and nonmycorrhizal soils.

Modeling water content of a vineyard soil in the Douro Region, Portugal

The soil moisture of a vineyard soil under two different plantation systems, bench terrace and straight down the slope (German system), was measured weekly from 1991 to 1997 in the Demarcated Region of Douro – the Port wine producing region in Portugal. The soils of the region are extremely stony, located on steep slopes along narrow valleys. These conditions complicate the study of soil water relations with deterministic functions. We fitted a large data set to stochastic models, with the objectives of (a) determining which stochastic model best forecasts soil moisture in various planting systems, (b) based on chosen models, objectively evaluate variation of soil moisture in each planting system, and (c) finding an objective factor on which to base management decisions. Soil moisture variation was best represented by a time frequency model (Fourier series). Soil under straight down the slope plantation had significantly higher water reserves for most of the year than bench terrace plantation but the difference was not high enough to drive a significant shift on production and fundamentally affect the choice of which system to choose for new plantations. The model-generated data suggested a superficial and subsuperficial downflow of water from upper slopes in the straight plantation relative to bench terraces. However, the model failed to shed additional information on the mechanisms related to soil water balance. Data generated by the time frequency model might prove useful for other management decisions such as schedule and intensity of topping and pesticide application.

Soil Water, Fertility and Sustainable Agricultural Production in Arid and Semiarid Regions on the Loess Plateau*

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Vertical rooting patterns of mature Quercus trees growing on different soil types in northern Germany

At two different sites in northern Germany, the vertical distribution patterns of roots with diameters of up to 10 mm were investigated with the trench profile wall technique beneath mature trees of Quercus petraea (Matt.) Liebl. growing on clayey (cambi-stagnic gleysol) or sandy soil (dystric cambisol). The rooting patterns were related to soil bulk density, maximum plant-available soil water and soil chemistry, and were compared with the rooting pattern of Q. petraea growing on a silty soil on limestone (luvisol) and with that of Q. robur L. growing on clayey loam (stagnic cambisol). All three Q. petraea sites differed in their vertical rooting pattern, as was indicated by significant differences in the parameter β which was calculated from the cumulative root fraction (Y) for the cumulative rooting depth (d; Y=1−βd). At the clayey site, a relatively large fraction of roots was found in deeper soil layers (highest β value); whereas, at the silty site, the rooting pattern was most superficial (lowest β value). No significant difference in the vertical rooting pattern was observed between Q. petraea and Q. robur growing on clayey soil. The soil-area related biomasses of living roots did not differ between the clayey and the sandy site of Q. petraea.Comparisons of the rooting patterns with soil water relations and soil chemistry lead to the hypothesis that under the climatic conditions of Central Europe, the vertical root distribution of Q. petraea is more influenced by the availability of nutrients, especially that of nitrogen, than by the amounts of plant-available soil water.

Interactions between root restriction, irrigation and rootstock treatments on ‘Queen Cox’ apple trees: Effects of soil and plant water relations

SummaryThe purpose of this research was to determine if established, orchard-grown apple trees that were con®ned in root-restricting membranes received adequate water for growth when irrigated. Previous data had shown positive effects of root restriction on reducing shoot extension growth in apple. Soil matric potential (Ψsm), leaf stomatal conductance (g) and leaf water potentials (Ψl) were measured over daily cycles during the period of maximum tree water use (July and August). Measurements were also made of the Ψsm and fruit size throughout the growing season. Daily and seasonal Ψsm showed that the soil within the root restricting membranes (+R) when irrigated (+I) remained closed to field capacity (<100 HPa), which was not the case for unirrigated soil within the membranes (-I +R). Ψl measured before dawn, showed that similar levels of drought stress were evident between irrigated and restricted (+I +R) and unrestricted (+I -R and -I -R) trees. The Ψsm, and Ψl for trees with roots within restricting membranes were significantly more negative, in the absence of irrigation (-I +R). Measurement of g showed that root restricted trees were transpiring at similar rates in the presence of irrigation (+I +R) as unrestricted trees (+I -R) with or without irrigation. Stomatal closure could not explain the increased Ψl observed for the restricted irrigated (+I +R) treatment compared with unrestricted (-R) trees. A reduced stomatal aperture was the most likely explanation for the reduction in growth previously observed with the restricted unirrigated trees. Fruit size was also affected by root restriction and the effect became greater as the roots became more restricted with tree age. Similarly, there was also a negative effect, in one year, of root restriction on fruit size at harvest, even in the presence of irrigation. Data show that reductions in soil water availability, Ψl and g, for the root-restricted trees (+I +R), were unlikely to be the causes for the previously observed reductions in shoot growth (tree size). These results imply that other factors were in operation, among which root-synthesized chemical regulators of shoot growth are the most likely candidates.

Impact of fly ash amendment and incorporation method on hydraulic properties of a sandy soil.

Coal fly ash has physical and chemical characteristics that makeit useful as a soil amendment, one of the more important beingthe potential to permanently improve the soil water relations ofsandy, drought-prone soils. We axemined changes in theinfiltration rate and water holding capacity of a sandy soilafter application of high rates (up to 950 Mg ha-1) of aClass F fly ash. Fly ash was applied to large field plots byeither conventional tillage (CT; moldboard plow-disk) orintensive tillage (IT; chisel plow-rotovate-disk), and tomicroplots using a rototiller. Infiltration rate (i) wasmeasured in both studies with a disk permeameter on threeoccasions over a 12-month period. Ash effects on gravimetric water content (θg) at the 0–40 cm soil depth were measuredduring a 168 hr period following a 2.5 cm rainfall event andwater release curves (33 to 500 kPa) were constructed in thelaboratory using soils from the large plots. In both studiesi was decreased by ∼80% one year after additionof fly ash and θgin ash-amended soil was higher than unamended soil throughoutthe 168 hr monitoring period. Soil water distribution variedwith tillage; the IT treatment had the highest θg increasesin the 0–20 cm depth while the CT treatment had θgincreases throughout the 0–40 cm depth. Soil water content anddistribution in ash-amended microplots were similar to ITtreatments. Fly ash amendment not only increased water holdingcapacity but also increased plant available water by 7–13% inthe 100–300 kPa range. These results suggest fly ash amendmentmay have the potential to improve crop production in excessivelydrained soils by decreasing i and increasing the amountof plant available water in the root zone.

The Effect of Hysteresis on Microbial Activity in Computer Simulation Models

Microbial activity in soils depends on the status of the soil water, which is expressed by pressure head (h) or water content (θ). There is no unique relationship between θ and h because moisture relations exhibit hysteresis. For convenience microbial activity has usually been related to the main drying curve but in general this will lead to bias in computer models that aim to simulate microbial activity. This article aims to evaluate the magnitude of the bias resulting from the use of computer simulation models of mineralization coupled to a model that follows the hysteresis of water relations in soil. Simulations of mineralization were found to accumulate a bias of more than 15% of the mean annual mineralization by harvest. This bias was found to be most serious where soil is continually subject to severe wetting and drying cycles such as can be found in irrigated and tropical agriculture.

Soil Water Use by Diploid and Tetraploid Russian Wildrye

Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski] is an important cool‐season forage grass for the Northern Great Plains, but it exhibits poor seedling vigor. Tetraploid (2n=4x=28) plants have improved seedling vigor over diploid (2n=2x=14) plants, but data is lacking on plant‐soil‐water relations. Objectives of this research were to compare water‐use efficiency (WUE), evapotranspiration (ET), roofing activity, and water extraction in relation to Haun stage for diploid and tetraploid Russian wildrye. A diploid and tetraploid entry were evaluated for 3 yr under two water treatments (50 and 150% of monthly average precipitation for 1 April to 1 November) and two fertilizer rates (10 and 134 kg N ha−1) in a rain shelter. The soil was a Parshall fine sandy loam (Coarse‐loamy, mixed, superactive, frigid Pachic Haplustoll). Cumulative ET averaged across the 50 and 150% water treatments was 138 and 141 mm for the diploid and tetraploid, respectively. Rate of root activity progression was linear with Haun to the 1.2‐m depth and ranged from 0.148 to 0.163 m per Haun stage for the diploid and tetraploid, respectively, at 50% water. Forage yield of the tetraploid was greater than the diploid at the 50%, but not at the 150% water treatment. Water‐use efficiency was higher for the tetraploid (3.8 g kg−1) than the diploid (3.1 g kg−1) at the 50% water treatment, but at 150% water the entries were not different. Overall, the tetraploid entry exhibited a more favorable WUE and other soil water relations than did the diploid entry.

CO2 enrichment in a maturing pine forest: are CO2 exchange and water status in the canopy affected?

A net, leaf net CO2 assimilation ca, CO2 concentration of air surrounding a leaf ci, leaf intercellular CO2 concentration Δ, 13C isotope discrimination δ13C, relative stable carbon isotope content ɛ, ratio of Anet at ca = 560μmol mol–1 to Anet at ca = 360 μmol mol–1 FACE, free‐air CO2 enrichment gw, stomatal conductance to water vapour Πi, initial leaf osmotic potential Rt, relative water content at incipient turgor loss Ψl, xylem water potential of leaves Ψm, soil matric potential Elevated CO2 is expected to reduce forest water use as a result of CO2‐induced stomatal closure, which has implications for ecosystem‐scale phenomena controlled by water availability. Leaf‐level CO2 and H2O exchange responses and plant and soil water relations were examined in a maturing loblolly pine (Pinus taeda L.) stand in a free‐air CO2 enrichment (FACE) experiment in North Carolina, USA to test if these parameters were affected by elevated CO2. Current‐year foliage in the canopy was continuously exposed to elevated CO2 (ambient CO2+200μmol mol–1) in free‐air during needle growth and development for up to 400 d. Photosynthesis in upper canopy foliage was stimulated by 50–60% by elevated CO2 compared with ambient controls. This enhancement was similar in current‐year, ambient‐grown foliage temporarily measured at elevated CO2 compared with long‐term elevated CO2 grown foliage. Significant photosynthetic enhancement by CO2 was maintained over a range of conditions except during peak drought.There was no evidence of water savings in elevated CO2 plots in FACE compared to ambient plots under drought and non‐drought conditions. This was supported by evidence from three independent measures. First, stomatal conductance was not significantly different in elevated CO2 versus ambient trees of P. taeda. Calculations of time‐integrated ci/ca ratios from analysis of foliar δ13C showed that these ratios were maintained in foliage under elevated CO2. Second, soil moisture was not significantly different between ambient and elevated CO2 plots during drought. Third, pre‐dawn and mid‐day leaf water potentials were also unaffected by the seasonal CO2 exposure, as were tissue osmotic potentials and turgor loss points. Together the results strongly support the hypothesis that maturing P. taeda trees have low stomatal responsiveness to elevated CO2. Elevated CO2 effects on water relations in loblolly pine‐dominated forest ecosystems may be absent or small apart from those mediated by leaf area. Large photosynthetic enhancements in the upper canopy of P. taeda by elevated CO2 indicate that this maturing forest may have a large carbon sequestration capacity with limiting water supply.

Soil water relations of a Nigerian typic haplustult amended with fresh and burnt rice-mill wastes

The soils in the Abakaliki agro-ecological region of southeastern Nigeria are plagued with characteristics that impede optimal soil water conditions. Four rates (12.5, 25.0, 37.5 and 50.0 Mg ha −1) of fresh (FW) and burnt (BW) rice-mill wastes were incorporated in a typic haplustult planted with maize ( Zea mays L.) to improve the immediate and long-term water conditions of the soil. Results showed that significant changes in bulk density and total porosity occurred at the application rates of 25.0 and 50.0 Mg ha −1 in the FW and BW amendments, respectively. At an application rate of 50.0 Mg ha −1, the permanent wilting point water content (18.0%) in the FW amendment was significantly higher than the BW amendment. Similarly, the FW amendment at the same application rate resulted in the infiltration rates (i) of 2250 and 2181 mm h −1 in the first and second seasons, respectively. These figures are significant improvements relative to the BW amendments and the control. Percent organic carbon (OC), dry aggregate size >0.5 mm and water stable aggregates (WSA) > 0.5 mm accounted for 43% and 77%, respectively of the improved infiltration. Sorptivity and A parameter of Philip's equation increased significantly with increasing rate of the FW amendment. Appropriate moisture conservation techniques are required to accelerate the rate of decomposition of the FW amendment in order to enhance its contribution to soil water retention.

Comparison between mechanistic and functional models for estimating soil water balance: deterministic and stochastic approaches

In many models used to simulate soil-water relationships, representations of the transport mechanisms in the soil-plant-atmosphere continuum, range from mechanistic to functional. The objective of this paper is to compare two functional models, FAO ( Doorenbos and Pruitt, 1977) and Ritchie (1985)models, with a mechanistic model ( Maraux and Lafolie, 1998) to simulate the soil water balance of maize and sorghum grown in sequence in Nicaragua. In the FAO model, the soil is described as a single reservoir which is characterized by its amount of water varying on a daily time scale, depending on the rain, drainage, and actual evapotranspiration. In the Ritchie model, the soil is regarded as a multilayered soil profile. The maximum evapotranspiration is divided between soil evaporation and plant transpiration, and drainage occurs if the amount of water arriving in the last layer corresponds to a water content greater than the field capacity. The mechanistic model is based on the Richards' equation. Comparison of the three models was first made according to a deterministic approach with parameters coming from the same database. We then considered a stochastic approach for which 800 hydraulic characteristics of the soil were generated, according to the spatial variability observed at the field scale and to the scaling theory applied to similar porous media. A distribution of the stochastic parameters used in the three models was thus derived. Results showed that the order of magnitude of the evapotranspiration was similar for the three models (902, 874, 842 mm cumulative evapotranspiration for a 203 day period for the MM, Ritchie, and FAO models, respectively). Adding a capillary rise mechanism in the functional models improved moderately the soil-water balance. Evapotranspiration and drainage showed moderate sensitivity to spatial variability in soil hydraulic properties (coefficients of variation less than 1.6%), whereas final water storage (after 203 days) showed a greater sensitivity (coefficients of variation from 7.9–15.7%, depending on the model).

Tree rooting patterns and soil water relations of healthy and damaged stands of mature oak (Quercus robur L. and Quercus petraea (Matt.) Liebl.)

At three sites in northwestern Germany, which represent the centres of the present oak damage, root distribution and biomass beneath healthy and damaged trees of mature pedunculate oak (Quercus robur L.; Neuenburg site) and sessile oak (Q. petraea [Matt.] Liebl.; Lappwald and Sprakensehl sites) were investigated, and soil texture, bulk density, duration of waterlogging periods and the water available in the mineral soil were determined. For Neuenburg and Sprakensehl, the available soil water was related to leaf water parameters determined in a separate investigation. At the clayey and hydromorphic sites of Neuenburg and Lappwald, the measurements were performed in each one healthy and one damaged part of the site, which differed in the number of oaks with crown damage. In the damaged stand of Neuenburg, the clay content of the subsoil was higher than in the healthy stand, and the soil water availability was reduced especially in dry periods. Compared to healthy oaks of the healthy stand, the density of finest plus fine roots as well as the biomasses of finest roots were lower beneath damaged oaks of the damaged stand. With decreasing relative available soil water (actually available water in relation to water available at the saturation state), the relative leaf water content decreased in damaged, but not in healthy oaks. At Lappwald, similar differences in soil water availability between the healthy and the damaged stand were found, but had no effect on the distribution or biomass of the roots. At the sandy site (Sprakensehl), the available soil water decreased drastically during a dry period, and predawn leaf water potentials of both healthy and damaged oaks declined with decreasing relative available soil water. However, the damaged oaks were not inferior to the healthy ones with respect to root density and biomass. It is concluded that, in the damaged stand of Neuenburg, the high clay content of the subsoil, which results in prolonged periods of waterlogging, in sharp changes from waterlogging to drought and decreased water availability in dry periods, is the reason for the reduced biomass and density of roots of the pedunculate oak. Thus, in northwestern Germany, unfavourable soil water relations are considered as a factor contributing to crown damage of pedunculate oak at hydromorphic sites, but not to damage of sessile oak.

Patchiness in semi‐arid dwarf shrublands: Evidence from satellite‐derived indices for elevated CO2 assimilation rates on a geochemical mound in the Karoo National Park, South Africa

Satellite‐derived vegetation indices were used to identify a geochemical mound of higher active greenness in the Karoo National Park, Beaufort West, South Africa. We determined whether this mound was occupied by plants with higher CO2 assimilation rate. Plant cover on and off the mound was determined. Three woody species with high cover were selected for further investigation. Two deep‐rooted species, Rhigozum obovatum Burch. and Eriocephalus ericoides (L.f.) Druce, showed greater net CO2 assimilation rates on the mound. Net CO2 assimilation rates for the third species, Pentzia incana (Thunb.) Kuntze were similar both on and off the mound. In an attempt to find a mechanistic basis for the elevated CO2 assimilation rates, the relationships between soil factors, foliar nutrients and CO2 assimilation capacity were also examined. Our results suggest that the elevated net CO2 assimilation was not mediated via improved soil or plant water relations on the actively greening mound, nor through a difference in nitrogen levels in the soil or plant material, but possibly by way of the higher sub‐soil phosphorus levels measured from the geochemical mound. Genotype and cover cannot alone be used for rangeland condition assessment since localized elevated soil nutrient status (patchiness) contributes to greater photosynthetic carbon gain which may confer superior browsing responses to plants occurring on these mounds.

Book Reviews: Water Relations of Plants and Soils

Book Reviews: Water Relations of Plants and Soils

Soil-water and soil physical properties under contour hedgerow systems on sloping oxisols

Hedgerows planted along the contour on steep lands in the humid tropics reduce soil erosion and build terraces over time. The objectives of this study in two Hapludoxes in the Philippines were to evaluate changes after 4 years in soil properties and soil water relations on transects perpendicular to the cropped alleys between four grass and tree hedgerow systems and a control. Hedgerow plants included Gliricidia sepium, Paspalum conjugatum, and Penisetum purpureum. Soil properties evaluated as a function of position in the alley (upper, middle, or lower elevation in an alley) included bulk density, mechanical impedance, soil water transmissivity, water retention, soil water pressure, and soil water content. In general, soil properties were not affected by hedgerow system, but were affected by position in the alley. Nearness to the hedgerow, but not hedgerow species, affected soil water distribution ( P = 0.05). Plant available water at the 10–15 cm depth was 0.16 m 3 m −3, 0.13 m 3 m −3, and 0.08 m 3 m −3 for the lower, middle, and upper alley position, respectively. Water transmissivity decreased from 0.49 mm s −1 in the lower alley to 0.12 mm s −1 in the upper alley. The lower soil water contents and soil water pressures in and near the hedgerows confirmed competition for water between the hedgerow species and the food crop in the alley, a condition that is expected to suppress food crop production.

Importance of soil-water relations in assessing the endpoint of bioremediated soils

This study was conducted to investigate water movement in hydrocarbon contaminated soils. Three soils were studied, a hydrocarbon contaminated soil, the same soil after 3 years of bioremediation, and a control soil from the same site. There was a critical soil water content around 18% (bioremediated soil) and 20% (contaminated soil), above which the sorptivity of the contaminated soil was near that of the control soil. For soils with water contents below this value, there was a strong divergence in sorptivity between contaminated and control or bioremediated soils. Results suggest that water availability in contaminated soils will be highly dependent on soil water properties as water potential approaches the permanent wilting point (-1.5 MPa matrix potential). Infiltration of water into air dry (2% m.c. w/w) hydrocarbon contaminated soils was up to three orders of magnitude slower than for the control soil. For air dried soils, the infiltration rate of the contaminated and bioremediated soils was constant with time. This was in contrast to the control soil where infiltration rate was a function of the reciprocal of the square root of time.