Initial Water Potential Research Articles

Predicting Urea Hydrolysis in a Loblolly Pine Forest Floor

In a pine forest, urea hydrolysis rates can vary widely. The rate of urea hydrolysis is controlled by the amount of urease enzyme, temperature, pH, urea concentration, but at the soil surface, soil water potential is an especially critical factor because it may vary widely in short periods of time due to precipitation, dews, and rapid evaporation. The objective of this study was to describe the quantitative relationship between soil water and urea hydrolysis rates in a loblolly pine (Pinus taeda L.) forest floor. Hydrolysis rates were measured in a pine forest with time after urea was applied in seven studies performed at different times that varied widely in soil water content and temperature. Separately in a laboratory study, the urea hydrolysis rates were measured in pine needles and in the humified layer at different water potentials. Results showed that although well‐defined relationships were found between urea hydrolysis rates and forest floor water potentials, the urea hydrolysis rates measured in the field could not be predicted based on these relationships and the water potential of the forest floor pine needles at the time of urea application. However, the percent of urea hydrolyzed at 3 and 29 d could be predicted accurately from the initial water potential of new needles and an interaction between initial water potential and relative humidity days.

Shrinkage Properties of Repacked Soil at Different States of Uniaxial Compression

Compaction can severely impair soil structure, which may in turn affect shrinkage properties. The few studies that have so far investigated compaction effects on soil shrinkage behavior have produced seemingly inconsistent results, reporting an increase, decrease, or no change in shrinkage. The objective of this study was therefore to examine the changes in soil shrinkage under controlled laboratory conditions across a wide range of compression. We analyzed shrinkage curves of repacked aggregated soil recorded at increasing levels of uniaxial compression after conditioning to different initial water potentials. With increasing compression, the shrinkage curves changed from a sigmoidal shape typical for structured soil to the simple shape typical for unstructured soil such as a clay paste, indicating the loss of structural pores. The slopes of the shrinkage curves were less steep in the structural and basic shrinkage domains at moderate compression than in the uncompressed initial state, indicating a strengthening of interaggregate bonds and thus an increase in hydrostructural stability. The highly compressed soil, in contrast, showed distinctly steeper slopes, indicating decreased hydrostructural stability due to the breaking of interaggregate bonds. The compression effects on the shrinkage curves became more pronounced with increasing initial water potential during compression. This strong dependence on the degree of compression and initial water potential may be one reason for the variation of compaction effects on soil shrinkage properties reported in the literature. The results show that soil shrinkage is well suited for the assessment of soil structural stability and compaction effects on structured soil.

Compressibility of Repacked Soil as Affected by Wetting and Drying between Uniaxial Compression Tests

According to the classical concept of precompression stress, soil compaction increases precompression stress up to the maximum applied stress, while it leaves the compression index, i.e., the slope of the virgin compression line, unaffected. In field studies performed under unsaturated conditions, however, soil compaction was usually found to increase the precompression stress less than expected and to decrease the compression index. Our hypothesis was that this discrepancy may result from changes in the soil moisture status between compaction in the field and compression tests, including the common practice of preconditioning samples to a standard initial water potential for compression tests. To test this hypothesis, we performed confined uniaxial compression tests in which samples of repacked agricultural loamy topsoil were subjected to five compression cycles with increasing maximum stress. Part of the samples were subjected to a wetting–drying cycle and subsequently conditioned to water potentials of either −1, −6, or −30 kPa before each of Compression Cycles 1, 3, and 5, while the other samples were conditioned to these water potentials only before Cycle 1 and not subjected to any wetting–drying cycles. The concept of precompression stress was found to be fully valid as long as the samples were not subjected to wetting–drying cycles and not reconditioned between compression events. In contrast, the wetting–drying cycles and sample conditioning between compression events reduced both the precompression stress and the compression index. Such effects are not accounted for by the classical concept of precompression stress.

Soil and macro-pores under uniaxial compression. I. Mechanical stability of repacked soil and deformation of different types of macro-pores

Soil compaction can seriously damage the physical structure of fertile soil. Soils restored from excavation material and re-used for agriculture are under increased risk of compaction, as they are mechanically destabilized. Impacts on the macro-pores are of particular concern, as these structures play a crucial role for many soil processes such as drainage or aeration. The objective of this study was to test the hypotheses (i) that cylindrical macro-pores may enhance the mechanical stability of soil, and (ii) that macro-pores of different origin, size and orientation also differ substantially in their compressibility. Artificial cylindrical pores of different diameters (2 and 4 mm) and orientations (vertical and oblique) were created in repacked samples of a restored agricultural topsoil by pushing rods of corresponding diameters into the soil. The samples were then conditioned to different initial water potentials (− 1 kPa, − 6 kPa, − 30 kPa) and subjected to uniaxial compression tests. Macro-pore structures were analysed by means of quantitative morphometry of three-dimensional micro-computed tomography images. The mechanical stability of the samples strongly depended on the soil water content, but was not found to be influenced by the cylindrical macro-pores. The vertical cylindrical pores were more stable than the oblique ones. The initial diameter was less important. The cylindrical pores were much more stable than the interaggregate pores originating from the packing of the soil. The mean diameter of the cylindrical pores and the interaggregate pores decreased with increasing compression, whereas the mean diameter of the total pore space (i.e. the volume-based average of both pore types) at first increased under moderate compression due to the loss of the small interaggregate pores, and only decreased under higher loads. Cylindrical pores of moist samples kept their shape throughout compression, but those of samples compressed at low water potential (or water content, respectively) were deformed to ellipsoid shape. The study highlights the important role of cylindrical (bio)pores in soil. Being much less sensitive to compaction than interaggregate pores, they may still enable sufficient soil drainage and aeration, when other pores have already collapsed.

Accuracy of top soil moisture simulation using a mechanistic model with limited soil characterization

For many applications, soil water flow models require knowledge of soil properties and conditions that is difficult to obtain. In this study, we analyzed the impact of approximations on model prediction of soil water content when only limited soil property data are available. The TEC model of Chanzy and Bruckler (1993), representing soil heat and water flows, was selected since it is based on soil properties that can be estimated with the pedotransfer function, and it is driven by standard climatic data. The impact of the choices made for soil hydraulic properties, initial soil water conditions, and lower boundary conditions for a wide range of soil were studied. In this study, emphasis was placed on estimating moisture in the surface soil layer (0–30 cm) and was limited to wet and moderately dry soil conditions, which can be critical for applications in agriculture to determine trafficable days and in surface hydrology. The results from this study indicate the importance of soil hydraulic properties and moisture profile initialization, which may lead to errors larger than 0.10 m3 m−3 on soil moisture in the surface layer. The pedotransfer function proposed by Wösten (1997) led to the best simulated soil moisture in the top 30 cm, but initialization has a strong impact. Thus a “warming up” period of 70 days is needed to mitigate the initialization error. This duration can be reduced according to the amount of precipitation. We propose starting simulations 48 hours after the last significant rainfall with an initial water potential of −10 kPa. At the bottom of the soil, a gravitational flux at a depth of at least 80 cm is recommended to compute the boundary conditions. These recommendations for implementing a soil water transfer model were evaluated against independent experimental data, and a moisture accuracy (root mean square error) of 0.04 m3 m−3 was obtained.

Altered physiology in trehalose-producing transgenic tobacco plants: Enhanced tolerance to drought and salinity stresses

Transgenic tobaccoNicotiana tabacum L. var. SR1) plants that over-express theEscherichia coli trehalose-6-phosphate synthase (TPS) gene(otsA) synthesized small amounts of trehalose (<400 µg g-1 leaf) while non-transformants produced no detectable trehalose. Some transgenic plants expressing a high level ofotsA exhibited stunted growth and morphologically altered leaves. We tested F22 homozygous plants devoid of phenotypic changes to determine their physiological responses to dehydration and salinity stresses. All transgenic plants maintained better leaf turgidity under a limited water supply or after treatment with polyethylene glycol (PEG). Furthermore, fresh weight was maintained at higher levels after either treatment. The initial leaf water potential was higher in transgenic plants than non-transformants, but, in both plant types, was decreased to a comparable degree following dehydration. When grown with 250 mM NaCl, transgenic plants exhibited a significant delay in leaf withering and chlorosis, as well as more efficient seed germination. Our results suggest that either trehalose or trehalose-6-phosphate can act as an osmoprotective molecule without maintaining water potential, in contrast to other osmolytes. Furthermore, both appear to protect young embryos under unfavorable water status to ensure subsequent germination.

Production Environment and Nitrogen Fertility Affect Carrot Cracking

Carrot (Daucus carota L.) root cracking and breakage during harvest and handling operations result in serious losses. The environmental and management factors affecting carrot cracking and breakage susceptibility were investigated in a survey of fields and a series of trials conducted in California from 2000–02. Roots, leaves and soil were collected from a total of 31 commercial fields of `Sugar Snax' carrot, and soil texture and plant and soil fertility status were determined. Soil moisture was monitored in 10 fields to determine whether irrigation management was correlated with root cracking susceptibility; in 4 of these fields roots were harvested both before 0800 hr and at 1300 hr on the same day to directly compare the effects of root water status on cracking. The effect of N fertilization on cracking and breakage was investigated in 5 field trials. The relative susceptibility of 10 cultivars to cracking and breakage was also compared. Cracking susceptibility was determined with an impact test, and breakage with a loading test. Roots were selected by size (18 to 24 mm diameter) and cooled to 5 °C before testing. The percentage of roots cracked in the impact test varied from 7% to 75% among survey fields. Initial root water potential was not correlated with cracking incidence. However, after hydrating roots to minimize differences in water potential among fields, cracking incidence was correlated with turgor potential (r = 0.41). Soil sand content and mean air temperature in the 30 days preceding harvest were also correlated with cracking (r = –0.48 and 0.36, respectively), suggesting that cracking susceptibility may be minimized in cool weather and in light-textured soil. Irrigation management in the final 30 days preceding harvest had no consistent effect on root cracking. Time of day of harvest had a small but significant effect, with roots harvested before 0800 hr being more crack-susceptible. N fertilization in excess of that required to maximize root yield significantly increased cracking susceptibility. Cultivars varied widely in cracking susceptibility, with less variation in tissue strength and stiffness. Removal of the periderm dramatically decreased susceptibility to both cracking and breakage.

Passive Pan Sampler for Vadose Zone Leachate Collection

Sampling agrochemicals in the vadose zone provides an early warning system for ground water pollution. However, accurate in situ determination of agrochemicals in the vadose zone is often not feasible because of lack of appropriate instrumentation. This paper presents a new passive soil water sampler to effectively collect vadose zone leachate. The passive pan sampler (PPS), and a suction cup lysimeter (SCL) were positioned in soil columns. Two soil types were used. Vacuums applied to both PPS and SCL were adjusted to tensiometer readings in each soil column. Measurements included volumetric soil water content by time domain reflectometry (TDR), soil water potentials by tensiometers at three positions in each soil column, and the cumulative leachate collection (CLC) for the PPS, SCL, and free drainage (FD). Measurements were made at two initial soil water potentials of less than −10 kPa and −20 to −25 kPa, referred to high (HSWP) and low (LSWP) soil water potentials. Results indicated that the PPS performed well in both soil types irrespective of the initial soil water potential. Leachate collected by SCLs in the soils was lower than that collected by PPSs. The maximum CLC for silt loam soil ranged from 9.6 to 10.0 mL cm−2 at both water potentials for PPS, 1.6 to 2.1 mL cm−2 at both water potentials for SCL, and 0 to 2.0 mL cm−2 at HSWP for FD and 0 to 0.7 mL cm−2 at LSWP for FD, respectively. The corresponding values for the sandy loam soil were 8.0, 2.7, and 5.2 mL cm−2, for PPS, SCL, and FD, respectively. The leachate collection efficiencies of PPS at HSWP (98%) and LSWP (99%) were significantly (P &lt; 0.01) higher than for SCL at HSWP (59%) and LSWP (31%), indicating greater performance of the PPS.

Compaction of agricultural and forest subsoils by tracked heavy construction machinery

Precompression stress has been proposed as a criterion for subsoil compression sensitivity in regulations, limiting mechanical loads by vehicles, trafficking on agricultural and forest soils. In this study we investigated the applicability of this criterion to the field situation in the case of tracked heavy construction machinery. ‘Wet’ and ‘dry’ test plots at three different test sites (soil types: Eutric Cambisol and Haplic Luvisol under crop rotation and Dystric Cambisol under forest) along an overland gas pipeline construction site were experimentally trafficked with heavy tracked machines used for the construction work. The comparison of samples taken from beneath the tracks with samples taken from non-trafficked areas beside the tracks showed that no significant increase in precompression stress occurred in the subsoil. Comparing calculated mean and peak vertical stresses with precompression stress in the subsoil, only little compaction effects could have been expected. Precompression stress was determined by the Casagrande procedure from confined uniaxial compression tests carried out in the laboratory on undisturbed samples at −6 kPa initial soil water potential. Dye tracer experiments showed little differences between flow pattern of trafficked and non-trafficked subsoils, in agreement with the results of the precompression stress, bulk density and macroporosity measurements. The results indicate that Casagrande precompression stress may be a suitable criterion to define the maximum allowable peak stresses in the contact area of a rigid track in order to protect agricultural and forest subsoils against compaction.

Relationship of water to adventitious rooting in stem cuttings of Populus species

Populus species, characterized by fast growth and easy vegetative propagation, are widely used in agroforestry practices. The substantial water requirement of poplars make them interesting subjects for water balance studies. No information exists on soil moisture requirements for initial root and shoot growth of Populus cuttings. This study on leafless hardwood cuttings of Populus x euramericana (Dode) Guinier cv. ‘Robusta’ examined the dynamics of water use during propagation, as influenced by two initial soil water potentials (−0.006 and −0.06 MPa). Differences in the initial water potential of the cuttings was achieved by three pretreatments i.e., fresh, soaked and dried. Initial Ψshoot was −1.45, −0.10 and −2.10 MPa in fresh, soaked and dried cuttings, respectively. Soil moisture had a major effect on rooting. Water-stressed cuttings took a longer time to root and had fewer roots. Pre-soaking of cuttings stimulated rooting, particularly under the drier soil moisture conditions. Initially the water potential of cuttings decreased with time and with the formation of roots it stabilized in all the pretreatments. The reduction in water potential of cuttings after planting was related to an increase in resistance to water flow in the xylem.

Effect of soil and plant water relations on rooting of Populus × euramericana stem cuttings

Studies on Populus ×euramericana (Dode) Guinier cv. ‘Robusta’ wereconducted to examine the rooting behaviour and changes in water potentialduringrooting, as influenced by the initial water potential of the cutting at twosoilwater potentials (−0.006 and −0.06 MPa). Differencesinthe initial water potential of the cuttings was achieved by three pretreatmentsi.e., fresh, soaked and dried. Initial Ψshoot was −1.45,−0.10 and −2.10 MPa in fresh, soaked and driedcuttings, respectively. Soil moisture had a major effect on rooting. Waterstressed cuttings took a longer time to root and had fewer roots. Pre-soakingofcuttings stimulated rooting particularly under the drier soil moistureconditions. Initially the water potential of cuttings decreased with time andwith the formation of roots it stabilized in all the pretreatments.Resistance-to-flow measurements suggested the development of tyloses and theestablishment of new xylem. No relationship was evident between stomatalresistance of leaves and water potential of cuttings. The starch content of thestem cuttings decreased significantly with time irrespective of treatment.

Photosynthetic response to dehydration and high temperature in trehalose-producing transgenic tobacco

Transgenic tobacco plants harboring E. coli TPS (trehalose-6-phosphate synthase) gene (ots A) were generated by leaf disc transformation. Transgenic plants were identified by PCR, Northern hybridization, and trehalose synthesis. All lines of transgenic plants manifested stunted growth and extended generation time in varying degree. Transgenic plants typically exhibited some morphological changes in leaf shape and branching pattern. The stunted growth was not accompanied with early senescence or necrosis. Homozygous plants obtained in F2 generation were used to examine their photosynthetic responses to dehydration and high temperature. All transgenic plants showed enhanced tolerance against dehydration as shown by improved retention in fresh weight after PEG-treatment. However, decreases in water potential by dehydration through PEG-treatment were in parallel phase in both nontransformants and transgenic plants although the initial water potential was observed higher in all transgenic plants. Upon dehydration, decrease in Pmax occurred in similar phase in both nontransformants and transgenic plants, but no significant changes in Chl fluorescence parameters were observed in both nontransformants and transgenic plants. It appears that trehalose confers transgenic plants improved water retaining ability against dehydration, but without keeping their photosynthetic capacity. On the other hand, trehalose-producing plants showed improved ability of holding photosynthetic activity after heating at 40° ~ 45°C. Both Pmax and Chl fluorescence parameters (Fo and Fv/Fm) remained more favorable in transgenic plants after heat treatment than nontransformants. The results suggest that engineering trehalose-producing plants would be more effective in conferring enhanced tolerance against high temperature than dehydration in term of maintaining photosynthetic productivity.

異なる水蒸気圧下で保持したバラ切り花の水関係に影響する要因

'Bridal Pink' roses (Rosa hybrida L.), with their cut stems in deionized water, were held at 20°C in a glass tank in which the vapor pressure was maintained at 2.3 kPa (high vapor pressure : HVP) or 1.4 kPa (intermediate vapor pressure : IVP). Large, heavy cut roses with high initial water potentials retained water longer in HVP than in IVP. Transpiration and water uptake rates of cut roses placed in HVP were markedly lower than those in IVP. Transpiration and water uptake rates of individual organs, and defoliated, disbudded, and intact cut roses revealed that water losses from leaves and/or flower buds were closely correlated to water uptake by intact cut roses. The low water potential of flower buds, which occurred earlier and severer in IVP than in HVP, is attributed to : 1) the initial increase in petal volume ; 2) the following decrease in water uptake capacity of leaves and flower buds, and 3) competition for water between leaves and a flower bud.

The influence of thermal and hydric environments on embryonic use of energy and nutrients, and hatchling traits, in the wall lizards ( Podarcis muralis)

Among (different) clutch variation in egg composition, influence of thermal and hydric environments on incubating eggs, embryonic use of energy and nutrients, and hatchling traits were studied in the wall lizard Podarcis muralis from a lowland population of Northern Spain. When initial egg mass was kept constant, we found that some measured egg variables such as water, lipid-free organic material, ash, calcium and magnesium in egg contents, and ash and magnesium in eggshell, remained remarkably constant, whereas other variables differed considerably among clutches. All viable eggs increased in mass over the course of incubation due to absorption of water, and mass gain during incubation was dependent on initial egg mass, temperature and substrate water potential. Variations in the wet mass of hatchlings among treatments stemmed mainly from variations in water content. Hatching success, embryonic use of energy and nutrients, and sex, size and mass of hatchlings were unaffected over a wide range of substrate moisture. The incubation length decreased as temperature increased. However, the effect of substrate moisture on duration of incubation varied with temperature. The influence of incubation temperature on the snout-vent length of hatchlings, if present, was very weak. Incubation temperatures did not affect the sex ratio and carcass dry mass of hatchlings, but significantly affected the tail length of hatchlings, with individuals from the highest temperature having the shortest tails. The energy expenditure of embryogenesis during incubation remained remarkably constant among treatments, and energy reserves in the hatchling were largely dependent on allocation of energy materials in eggs. A high incidence of dead-in-shell embryos occurred in eggs that were laid in June and, thereafter, when ambient temperatures were high. Deformed hatchlings were distributed nearly equally among treatments.

Development of Media and Automated Liquid Fermentation Methods to Produce Desiccation-Tolerant Propagules ofTrichoderma harzianum

Abstract A suitable medium was developed from modified Richard's medium plus V8 juice (RM8) to produce high levels of desiccation-tolerant conidia of Trichoderma harzianum strain 1295-22. The addition of 9% (v/v) glycerol to RM8 improved both biomass production and desiccation tolerance of the conidia of T. harzianum. This medium was then used in a laboratory scale fermenter (1.5 liter) to determine optimal operating conditions. The optimal temperature for conidial production and desiccation tolerance improvement in the fermenter was 32°C when dissolved oxygen was maintained at 50% saturation of air, and the stirring rate was 1000 revolutions per minute. The initial water potential of the medium (with 9% glycerol) was −3.7 MPa, the pH was 6, and neither was controlled during fermentation. Changes in medium pH and dissolved oxygen were associated with the stages of morphological development and conidiation. The pH of the medium decreased concurrently with germ-tube elongation and mycelium development and then increased to 6.0–6.2 at phialide formation. Intensive conidiation occurred at pH 6.3–6.5 and reached its maximal level at 6.9–7.1. Changes in pH values could be used as indicators to monitor the morphological development and conidiation of T. harzianum during fermentation. The use of a 48-h-old culture inoculum, rather than conidial inoculum, to start fermentation reduced the time required to complete the shift from vegetative growth to phialide formation. Intensive conidiation occurred immediately after the addition of culture inoculum and reached maximum levels within 68 h of fermentation. Dry weight of biomass increased with the duration of fermentation and was greatest at 96 h. However, no improvements in conidia/gram and CFU/gram were achieved after 72 h of fermentation. The desiccation tolerance of conidia harvested at 72 or 96 h was significantly ( P = 0.05) greater than that of conidia harvested at 48 h of fermentation. Results obtained from this study could be used for further scale-up of the fermentation process.

The dynamics of soil water following single surface wettings

SummarySingle rainfall events play an important agricultural and ecological role, especially in dry regions where precipitation is erratic. Infiltration, redistribution and evaporation of single quantities of water are important in this context and have been investigated in the laboratory.Three soils of differing texture were packed at two uniform initial water potentials (– 100 MPa and –1.5 MPa) into columns, after which 12.7, 25.4 and 50.8 mm of water were applied as a single irrigation. The columns were maintained in a controlled hot and dry atmosphere (evaporativity = 16.7 mm d‐1) for up to 30 days, during which water‐content profiles were measured at intervals.Infiltration was rapid to depths ranging between 35 and 250 mm. Thereafter redistribution was small. Evaporation caused the water profile to develop three zones: dry between the soil surface and the drying front, dry below the wetting front, and an intermediate wetter zone between the drying and wetting fronts. As evaporation continued, the drying front moved deeper into the soil and the water content in the intermediate zone decreased.During the first few hours evaporation was rapid and constant, at the evaporativity of the atmosphere. Subsequently, evaporation was slower. Total evaporation (E) increased with time (t) as Eα tn for t &gt; 1 d, where n = 0.24 for a loamy sand, 0.33 for a clay loam and 0.31 for a silty clay loam.Weighted‐mean soil‐water diffusivities, averaged over the profile above the wetting front, ranged between 1000–2000 mm2 d−1 at the start of the falling‐rate stage and 200–400 mm2 d−1 near air‐dryness, in reasonable agreement with the few results in the literature.

Water and heat fluxes in desert soils: 2. Numerical simulations

Transient one‐dimensional fluxes of soil water (liquid and vapor) and heat in response to 1 year of atmospheric forcing were simulated numerically for a site in the Chihuahuan Desert of Texas. The model was initialized and evaluated using the monitoring data presented in a companion paper (Scanlon, this issue). Soil hydraulic and thermal properties were estimated a priori from a combination of laboratory measurements, models, and other published information. In the first simulation, the main drying curves were used to describe soil water retention, and hysteresis was ignored. Remarkable consistency was found between computed and measured water potentials and temperatures. Attenuation and phase shift of the seasonal cycle of water potentials below the shallow subsurface active zone (0.0‐ to 0.3‐m depth) were similar to those of temperatures, suggesting that water potential fluctuations were driven primarily by temperature changes. Water fluxes in the upper 0.3 m of soil were dominated by downward and upward liquid fluxes that resulted from infiltration of rain and subsequent evaporation from the surface. Upward flux was vapor dominated only in the top several millimeters of the soil during periods of evaporation. Below a depth of 0.3 m, water fluxes varied slowly and were dominated by downward thermal vapor flux that decreased with depth, causing a net accumulation of water. In a second simulation, nonhysteretic water retention was instead described by the estimated main wetting curves; the resulting differences in fluxes were attributed to lower initial water contents (given fixed initial water potential) and unsaturated hydraulic conductivities that were lower than they were in the first simulation. Below a depth of 0.3 m, the thermal vapor fluxes dominated and were similar to those in the first simulation. Two other simulations were performed, differing from the first only in the prescription of different (wetter) initial water potentials. These three simulations yielded identical solutions in the upper 0.2 m of soil after infiltration of summer rain; however, the various initial water potentials were preserved throughout the year at depths greater than 0.2 m. Comparison of all four simulations showed that the predominantly upward liquid fluxes below a depth of 0.2 m were very sensitive to the differences in water retention functions and initial water potentials among simulations, because these factors strongly affected hydraulic conductivities. Comparison of numerical modeling results with chemical tracer data showed that values of downward vapor flux below the surface evaporation zone were of the same order of magnitude as those previously estimated by analysis of depth distributions of bomb 3H (volatile) and bomb 36Cl (nonvolatile).

HYDRAULIC CONDUCTIVITY &amp; EMBOLISMS IN SWEET CHERRY

Percent loss of conductivity of sweet cherry (Prunus avium `Emperor Frances') under dry conditions was determined by measuring hydraulic conductivity before and after high pressure perfusion removal of xylem embolisms. Cut stems were allowed to dry for 0 to 8 hours, recut underwater and the rate of flow of solution through the stems measured under positive pressure (∼8.0 kPa). Hydraulic conductivity (Kh)was then calculated, and typical values for well hydrated stems were 6 × 10-9 m4MPa-1s-1. Embolisms were then dissolved by high pressure perfusion (125kPa) and the subsequent flow rate measured. A second Kh was then calculated and the difference in Kh values before and after the high pressure treatment used as a measure of % loss of conductivity (or % xylem embolism). A curve of the `vulnerability' to xylem embolism was generated by plotting % loss of conductivity against initial stem water potential. The curve shows the stems undergo xylem embolism as soon as stem water potential reaches -1.5MPa, and at stem water potentials of -3 MPa, the stems are over 80% embolized. This cultivar appears to be vulnerable to embolism relative to other studied woody species, and xylem dysfunction likely is a problem early in a drying period. However, a particular rootstocks ability to supply water during a dry period and a cultivar's ability to limit water loss by stomatal closure will dictate the exact water potentials in the stem and thus its level of embolism.

A Two-dimensional, Dynamic Model for Root Growth Distribution of Potted Plants

A two-dimensional mathematical model was developed to describe the time course of root growth and its spatial distribution for container-grown plants, using chrysanthemum [Dendranthema ×grandiflorum (Ramat.) Kitamura] as the model system. Potential root growth was considered as consisting of several concurrent processes, including branching, extension, and death. Branching rate was assumed to be related sigmoidally to existing root weight density. Root growth extension rate was assumed to be proportional to the existing root weight density above some threshold root weight density in adjacent cells. The senescence rate of root weight density was assumed to be proportional to existing root mass. The effects of soil matric potential and temperature on root growth were quantified with an exponential function and the modified Arrhenius equation, respectively. The actual root growth rate was limited by the amount of carbohydrate supplied by the canopy to roots. Parameters in the model were estimated by fitting the model to experimental data using nonlinear regression. Required inputs into the model included initial root dry weight density distribution, soil temperature, and soil water potential data. Being a submodel of the whole-plant growth model, the supply of carbohydrates from canopy to roots was required; the total root weight incremental rate was used to represent this factor. Rather than linking to a complex whole-plant C balance model, the total root weight growth over time was described by a logistic equation. The model was validated by comparing the predicted results with independently measured data. The model described root growth dynamics and its spatial distribution well. A sensitivity analysis of modeled root weight density to the estimated parameters indicated that the model was more sensitive to carbohydrate supply parameters than to root growth distribution parameters.

Simple Field Method for Determining Unsaturated Hydraulic Conductivity

A new method is proposed for determining in situ unsaturated hydraulic conductivities from unsaturated infiltration measurements made at several tensions on the same infiltration surface. Wooding's equation for steady-state unconfined infiltration rates is used in calculating hydraulic conductivities. Hydraulic conductivities calculated with the new method are consistent with unit gradient laboratory measurements of saturated and unsaturated hydraulic conductivity. This simple field method is potentially valuable because it is faster than unit gradient laboratory methods, and it is less disruptive of pore continuity than other field infiltration techniques. R OF SOIL WATER INFILTRATION and SUbSUrface water movement are important to researchers developing soil management practices to minimize potential groundwater contamination from land applied chemicals. A simple and rapid field technique of determining field unsaturated hydraulic conductivity would be useful in achieving this objective. Field and laboratory techniques for measurement of unsaturated hydraulic properties of soil were described by Green et al. (1986) and by Klute and Dirksen (1986), respectively. Solution of unsaturated flow problems generally requires experimental determination of the relationship between hydraulic conductivity and water potential or water content. Field methods used to obtain these relationships include the instantaneous profile method, steady-flux methods (with sprinkler irrigation or artificial crusts), sorptivity measurements, and use of tension infiltrometers (Clothier and White, 1981; Ankeny et al., 1988, 1989; Elrick et al., 1988a; White and Perroux, 1987, 1989; Smettem and Clothier, 1989). Because instantaneous profile and steady-flux techniques require laborious installation of tensiometers or neutron probe access tubes, sample numbers and the extensiveness of a site characterization can be limited. Sorptivity is an unsaturated soil parameter sometimes measured in the field (Green et al., 1986). Although sorptivity measurements are fast and simple, these measurements usually require that initial water content be known. White and Perroux (1989) have proposed a laboratory method for calculating unsaturated hydraulic conductivity from sorptivity measurements. Their method, however, requires air drying the sample between measurements at different tensions, which increases experimental time and may cause wetting/drying effects on soil structure. The Guelph infiltrometer (Soilmoisture Equipment Corp., Santa Barbara, CA) compares infiltration rates for difM.D. Ankeny, Daniel B. Stephens & Assoc., 4415 Hawkins NE, Albuquerque, NM 87109; M. Ahmed, Bangladesh Univ. of Engineering and Technology, Dhaka-1000, Bangladesh; T.C. Kaspar, National Soil Tilth Lab., Ames, IA 50011; and R. Horton, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. Joint contribution from USDA-ARS and Iowa State Univ. Journal Paper no. J-13716 of the Iowa Agric. and Home Economics Exp. Stn. Projects no. 2659 and 2715. Received 6 Nov. 1989. *Corresponding author. Published in Soil Sci. Soc. Am. J. 55:467-470 (1991). ferent radii surface disks and does not require driving a ring. Different soil surface areas, however, are being compared, which may introduce spatial variability associated with the different soil surfaces. A field method to measure in situ hydraulic conductivity at low water tensions is needed for studies of macroporosity and water flow in agricultural soils. The desired criteria for such a method are: 1. Only steady-state infiltration rate measurements are needed. Knowledge of the initial water potential or content should not be required. 2. Soil pore structure should not be disturbed by driving a ring into soil to obtain one-dimensional flow. This way, larger pores are not truncated or collapsed and infiltration through larger pores is less likely to be underestimated. 3. Measurements should be taken on the same soil surface. Measurements taken by using different radii (e.g., Elrick et al. 1988a) are more dependent on the assumption of soil homogeneity. 4. Calculation of hydraulic conductivities should be straightforward. We present a simple scheme for determination of in situ hydraulic conductivity that meets these criteria.