L. M. Dudley

Adsorption and capillary condensation in porous media: Liquid retention and interfacial configurations in angular pores

Conventional models of liquid distribution, flow, and solute transport in partially saturated porous media are limited by the representation of media pore space as a bundle of cylindrical capillaries (BCC). Moreover, the capillary model ignores the dominant contribution of adsorptive surface forces and liquid films at low potentials. We propose two new complementary elements for improving our understanding of liquid configuration in porous media: (1) an approach for considering the individual contributions of adsorptive and capillary forces to the matric potential and (2) a more realistic model for pore space geometry. Modern interface science formalism is applied to determine the thickness of adsorbed liquid films as a function of thermodynamic conditions and specific surface area of the medium. The augmented Young-Laplace (AYL) equation provided the necessary framework for combining adsorptive and capillary processes. A new pore space geometry composed of an angular pore cross section (for capillary processes) connected to slit-shaped spaces with internal surface area (for adsorption processes) offers a more realistic representation of natural porous media with explicit consideration of surface area (absent in the standard BCC model). Liquid-vapor configuration, saturation, and liquid-vapor interfacial area were calculated for different potentials and pore (unit cell) dimensions. Pore dimensions may be easily related to measurable soil properties such as specific surface area and porosity. Rigorous calculations based on the AYL equation were simplified and led to the development of algebraic expressions relating saturation and interfacial area of liquid in the proposed pore space geometry to chemical potential. These simple expressions are amenable to upscaling procedures similar to those presently used with the BCC model.

The role of calcium oxalate in the availability of phosphorus in soils of semiarid regions: a thermodynamic study

Evidence is presented for the presence of Ca-oxalate crystals at the soil-hyphae interface of mycorrhizal Pascopyrum smithii. This prompted the development of a thermodynamic model that uses the ability of oxalate to scavenge Ca2+ ion from the soil solution. The model predicts the increased solubility of Ca-apatite when oxalate is present in both the calcite-apatite (calcareous) system and in the exchangeable Ca-apatite (noncalcareous) system. The result is a marked increase of soluble (available) P in solution relative to the situation where oxalate is absent. The model is formulated on conditions prevalent in semiarid soils and presents a possible mechanism by which phosphorus uptake of plants is enhanced by mycorrhizal infection.

Soil solution phosphate, root uptake kinetics and nutrient acquisition: implications for a patchy soil environment

SummaryThe importance of increased root phosphate (P) uptake kinetics, root proliferation and local increases of soil solution P (P1) for P acquisition from fertile soil microsites was explored with a simulation model and calculated uptake was compared with experimental data. Based on the partitioning of added P in microsites to P1 and P adsorbed on soil particles and the results of a dual-isotope-labeling experiment (Caldwell et al. 1991a), acquisition of P from the fertile microsites was some 20 X that of uptake from an equal volume of soil which received only water. Simulations were in general agreement and also showed that elevation of root P uptake kinetics could contribute more to the increased acquisition than did root proliferation under these circumstances. Although increased physiological uptake capacity for P has generally been considered to be of little benefit because of diffusion limitation, in patchy soil environments selective elevation of P uptake kinetics in fertile microsites may be of considerable benefit. These tests were conducted in calcareous soil which releases much less P into the soil solution than do many other soils. In many noncalcareous soils the benefits of selective elevation of root uptake kinetics would likely be greater.

Enhanced root system C-sink activity, water relations and aspects of nutrient acquisition in mycotrophic Bouteloua gracilis subjected to CO2 enrichment

In order to better elucidate fixed-C partitioning, nutrient acquisition and water relations of prairie grasses under elevated [CO2], we grew the C4 grass Bouteloua gracilis (H.B.K.) lag ex Steud. from seed in soil-packed, column-lysimeters in two growth chambers maintained at current ambient [CO2] (350 μL L−1) and twice enriched [CO2] (700 μL L−1). Once established, plants were deficit irrigated; growth chamber conditions were maintained at day/night temperatures of 25/16°C, relative humidities of 35%/90% and a 14-hour photoperiod to simulate summer conditions on the shortgrass steppe in eastern Colorado. After 11 weeks of growth, plants grown under CO2 enrichment had produced 35% and 65% greater total and root biomass, respectively, and had twice the level of vesicular-arbuscular mycorrhizal (VAM) infection (19.8% versus 10.8%) as plants grown under current ambient [CO2]. The CO2-enriched plants also exhibited greater leaf water potentials and higher plant water use efficiencies. Plant N uptake was reduced by CO2 enrichment, while P uptake appeared little influenced by CO2 regime. Under the conditions of the experiment, CO2 enrichment increased root biomass and VAM infection via stimulated growth and adjustments in C partitioning below-ground.

Effects of Soil Osmotic Potential on Nitrification, Ammonification, N-Assimilation, and Nitrous Oxide Production

Previous studies have examined the effects of soil osmotic potential (Ψ s ) on net rates of mineralization and nitrification. Because net rates represent the difference between gross production and consumption processes, it is unclear which process is being affected. We used an 15 N isotopic dilution method to evaluate the effects of Ψ s , on gross rates of nitrification, ammonification, NH + 4 assimilation, and NO - 3 assimilation, and net rates of nitrous oxide production in a Penoyer sandy loam at field capacity. To avoid creating specific ion toxicities that normally do not occur in this soil, we used a chemical equilibrium model to predict how solute concentrations in the soil solution change during evapo-concentration; then we used solutions containing these mixtures of solutes to create individual Ψ s treatments. A nitrification potential assay was also performed to determine the effect of Ψ s on nitrification rates at high substrate concentrations. In soil slurries with elevated NH + 4 concentration (1110 μ M) s , nitrification rates declined exponentially with reduced Ψ s (increased salt concentration); however, in soil samples incubated at field capacity without added NH - 4 (9.7 μM, or 2 mg N kg -1 ), the gross nitrification rate was independent of Ψ s . The differential response between slurries and soil at field capacity was attributed to differences in NH + 4 concentrations, and indicated that the effects of Ψ were secondary to NH + 4 concentrations in controlling nitrification rates. Nitrification rates in slurries declined more when a single salt (K 2 SO 4 ) was used than when the mixture of salts that more closely approximated the solute composition predicted to occur in the field was used to lower Ψ s . This suggests that nitrifying bacteria are capable of adapting to specific ion toxicities. Gross rates of ammonification declined exponentially with decreased Ψ s between 0 and -500 kPa but were independent of Ψ s at potentials of -500 to -1750 kPa. Rates of microbial assimilation of NO - 3 exceeded NH + 4 assimilation by a factor of 4, indicating that under NH + 4 limited conditions substantial NO - 3 assimilation can occur. Microbial assimilation of both NH + 4 and NO - 3 declined exponentially with decreased Ψ s , and were insignificant at <-1500 kPa Ψ s . Because NO - 3 assimilation declined more rapidly than gross nitrification, net nitrification rates actually increased with declining Ψ s . Rates of nitrous oxide (N 2 O) production were also inversely correlated with Ψ s . Our results indicate that in previous studies, measurement of net rates, use of inappropriate salts, and addition of substrate may have resulted in over-estimation of the adverse effects of low Ψ s on rates of N-transformations.

Interactive effects of sodium chloride, sodium sulfate, calcium sulfate, and calcium chloride on snapbean growth, photosynthesis, and ion uptake

Abstract Excessive sodium (Na) accumulation in soil, which can be a problem for production agriculture in arid and semiarid regions, may be ameliorated by calcium (Ca). The mechanisms of Ca amelioration of Na stress in plants have received much more attention than has the effect of the anion of the Ca salt. Our objective was to determine the relative effects of the chloride (Cl‐) and sulfate (SO4 2‐) anions on Ca amelioration of Na stress. We exposed Phaseolus vulgaris L., cv. Contender seedlings growing in 1‐L styrofoam pots under greenhouse conditions to sodum chloride (NaCl) or sodium sulfate (Na2SO4) at concentrations of 0, 15, 30, 45, and 60 mmol/L combined with either 15 and 30 mmol/L of calcium sulfate (CaSO4) or calcium chloride (CaCl2). Plants in each styrofoam pot were irrigated with 300 mL of salt solution (leaching fraction = 0.25) every fourth day for four weeks. Increasing Na concentration decreased shoot dry weight, number and weight of pods, and number of nodules. The photo‐ synthesis rate...

The effects of oxalates produced by Salsola tragus on the phosphorus nutrition of Stipa pulchra

Oxalic acid is produced by some species of plants and mycorrhizal fungi and it may solubilize unavailable soil phosphorus (P) bound by cations (Ca++, Al++, Fe+++). Field and greenhouse experiments were conducted to show whether oxalate produced by the annual Salsola tragus or added oxalic acid would solubilize P from the inorganic-bound soil P pool, making it available for uptake by Stipa pulchra. Oxalate could be leached in the laboratory from the senescent canopy of Salsola, and leaching by rainfall was hypothesized to be a source of potential increased soil P under the Salsola canopy. Both oxalate leached from the canopy of Salsola and added oxalic acid increased the availability of soil P in greenhouse experiments. The source of the increase in available soil P in the greenhouse experiment was shown to be the inorganic-bound P pool, as the total P concentration of the soil decreased with increasing oxalate. There were significant increases in Stipa shoot P in response to Salsola leachates and in response to added oxalate in the greenhouse studies. These results suggest an important role for oxalate in P cycling. On disturbed sites where Salsola invades it may act to facilitate the establishment of later seral species like Stipa by creating a nutrient island of available P.

Drainage water reuse: biological, physical, and technological considerations for system management.

Previous reviews of drainage water reuse have discussed principles of water reuse and disposal; provided examples of reuse practices; offered reuse criteria for salinity, for trace elements, and for bacteria; discussed mitigation of dissolved trace elements in reuse strategies; and summarized the California experience with a focus on discussion of salinity, sodicity, B, Mo, and Se issues. This review emphasizes recent literature contributing to understanding physical and biological constraints to drainage water reuse. The potential for drip irrigation and, particularly, low-flow/high-frequency systems to enhance the use of drainage water while minimizing the deleterious effects on yield and on water and soil resources is examined using the numeric HYRDUS-2d model. Additionally, an analytical model is used to illustrate physical and biological limitations to drainage water management that result from the self-regulating nature of the soil-plant-water system. The models suggest that crop, soil, irrigation frequency, and delivery systems might be manipulated to reduce the quantity of drainage water, but they also suggest that the nature of the system may seriously constrain the amount of reduction that might be achieved.

Modeling Plant Response to Drought and Salt Stress: Reformulation of the Root-Sink Term

Because transpiration is often the largest component of the water budget in arid systems, the efficacy of computer simulation models as predictors of water and salt movement is predicated on their ability to predict transpiration. The objective of this study was to improve the root-sink term for water extraction and thus improve predictions of transpiration. The root-sink terms often use either a transpiration-apportioning (TA) empirical function for the plant response to the soil matric and osmotic potential or a potential-flow (PF) function. Three root-sink terms, a TA formulation, a PF formulation, and a combination of the two (PFTA, a TA function for computing water uptake in response to salinity and a PF function for computing water uptake in response to water availability) were coded into a simulation model, and model predictions were compared with field-collected data. When predicted and measured relative yields (yield/yield max ) were compared, the PF produced the poorest agreement with data ( y = 0.8741 x + 0.0251), the TA gave better agreement ( y = 0.9283 x + 0.0476), and PFTA provided the best agreement ( y = 0.9909 x + 0.0430). The plant and plant–soil based formulation predicted the salinity profile at the end of the field experiment under conditions of high salinity (EC irrigation water = 6.0 dS m −1 ) and irrigation equal to potential evaporation. The plant–soil formulation was the better predictor under the same salinity condition with irrigation at 40% of potential evaporation. The simulated evolution of the water content and salinity profile across time was also examined.

Calcium amelioration of NaCl effects on plant rowth, chlorophyll, and ion concentration n Phaseolus vulgaris

Abstract This study was conducted to determine whether different forms of Ca amendments ameliorated the effects of NaCl stress on growth, chlorophyll content, and ion concentration of snapbeans, Phaseolus vulgaris L. cv Contender. The plants were grown in a greenhouse and irrigated with solutions containing 0, 44, 88, or 132 mM NaCl combined with 0, 5, and 8 mM of either CaSO4 or CaCl2. Without Ca amendments, shoot and root dry weights, chlorophyll concentration, and leaf K+/ Na+ ratio decreased more as NaCl increased from 0 and 44 mM than when it increased from 44 to 88 mM. Leaf K+, Na+, Ca2+, and Mg2+ increased and the K+/Na+ ratio decreased with each increase in NaCl concentration. The addition of Ca as either CaCl2 or CaSO4 to all levels of NaCl increased shoot and root dry weights and leaf chlorophyll concentration. The addition of CaSO4 or CaCl2 to different levels of NaCl did not affect leaf K+, Na+, or Mg2+ levels of the leaf. Among the ions, leaf Ca2+ increased significantly as concentration of e...