Matric potential

The importance of including soil-based parameters in rhizosphere simulations has been emphasized (56). Scott et al. u.sed a time-dependent exudation boundary condition and a layer model to predict how introduced bacteria would colonize the root environment from a seed-based inoculum. They explicitly included pore size distribution and matric potential as determinants of microbial growth rate and diffusion potential. Their simulations showed that the total number of bacteria in the rhizosphere and their vertical colonization were sensitive to the matric potential of the soil. Soil structure and pore size distribution was also predicted to be a key determinant of the competitive success of a genetically modified microorganism introduced into soil (57). The Scott (56) model also demonstrated that the diffusive movement of root exudates was an important factor in determining microbial abundance. Results from models that ignore the spatial nature of the rhizosphere and treat exudate concentration as a spatially averaged parameter (14) should therefore be treated with some caution. 

Numerical models are used to predict the performance and assist in the design of final cover systems. The availability of models used to conduct water balance analyses of ET cover systems is currently limited, and the results can be inconsistent. For example, models such as Hydrologic Evaluation of Landfill Performance (HELP) and Unsaturated Soil Water and Heat Flow (UNSAT-H) do not address all of the factors related to ET cover system performance. These models, for instance, do not consider percolation through preferential pathways may underestimate or overestimate percolation and have different levels of detail regarding weather, soil, and vegetation. In addition, HELP does not account for physical processes, such as matric potential, that generally govern unsaturated flow in ET covers.39 42 47... 

Tensiometer Measures the matric potential of a given soil, which is converted to soil moisture content Commonly consists of a porous ceramic cup connected to a pressure-measuring device through a rigid plastic tube... 

Performance data Percolation is being measured with a lysimeter connected to flow monitoring systems, soil moisture is being measured with water content reflectometers, and soil matric potential and soil temperature are being monitored with heat dissipation units. From November 1999 to July 2002, the capillary barrier cover system had a cumulative percolation of 0.5 mm. Total precipitation was 837 mm over the 32-month period. Additional field data were collected through 2005. 

Meickle, A et al. (1995) Matric potential and the survival and activity of a Pseudomonas fluorescens inoculum in soil. Soil Biology and Biochemistry, 27, 881-892. 

Soil moisture can have a significant effect on decomposition (Swift et al. 1979). This is due, in part, to the fact that soil moisture can affect the metabolism of decomposer microorganisms. This effect can be modified by soil texture because bioavailable moisture is determined, in part, by the suction with which water is held between soil particles (matric potential). Thus, the calibration of soils to a known matric potential can lead to the assessment of the effect of bioavailability of moisture in soil (Hillel 1982) and allow for the comparison of process rates between soils at the same matric potential (Orchard and Cook 1983). 

Figure 8 Hydrologic parameters defining pore-water flow in the Luquillo regolith (a) moisture saturation, (b) matric potential describing capillary tension, and (c) experimental hydrologic conductivities as functions of moisture saturation at several regolith depths (after White et aL, 1998).

The moisture characteristics for three granular materials is illustrated in Figure 3.(2) Note the steep slopes at high moisture contents for the sand and solid Los Alamos tuff. This results from a non-uniform pore size distribution the sand contains a preponderance of large pores which drain at relatively high matric potentials the solid tuff contains mostly smaller pores which drain at lower potential values. The relatively constant slope for the crushed tuff results from a fairly uniform pore size distribution. The moisture characteristics demonstrate the fact that materials at the same water content can display significantly different moisture potentials. 

The modified unitary approach of Philip (1977a), supplemented by adsorption terms presented by Iwamatsu and Horii (1996), provides a means of calculating equilibrium liquid-vapor interfaces for various chemical potentials during drainage and imbibition. Four major steps are discerned during the transition from adsorption to capillary-dominated imbibition (Fig. 1-6). At low matric potentials,... 

Fig. 1-6. A sketch illustrating liquid-vapor interfacial configurations during transition Imm adsorption to capillary-dominated imbibition in the proposed unit cell () spontaneous slit fill up (capillary condensation), (c) pore snap-off, and < /) full unit cell.

A physically based unitary approach for adsorptive and capillary contributions to the matric potential—including the incorporation of disjoining pressure representation of interfacial forces was introduced. 

Free energy change, which consists of matric potential, osmotic potential and gravity potential, is equal to potential swelling pressure (Nakano et al.(1984)). In the case of buffer material, it is approximately equal to the suction, because the... 

Microparticles of hydroxyapatite and inulin polysaccharide were incorporated in precipitation cast polycaprolactone matrices. Potential applications in hard tissue repair and macromolecular drug release are illustrated. In vitro degradation characteristics were monitored over 45 months. 31 refs. 

Soil Matric Potential and Seed-Soil Contact... 

One suggestion is that the low germination at low matric potential is due to a decrease in hydraulic conductivity of the soil, severely limiting the amount of water arriving at the surface of the germinating seed. More important however appears to be the area of contact between the seed and the soil. 

Many seeds placed in distilled water in Petri dishes under optimum conditions for germination show a triphasic pattern of water uptake as shown in Figure 4.6. Initial uptake of water in Phase I (i.e. imbibition) is a consequence of the matric forces (i/cell walls and cell contents of the seed, and this uptake occurs irrespective of whether a seed is dormant or non-dormant, viable or non-viable. Phase II is the lag period of water uptake, when the matric potential is high (less negative), as is the solute or osmotic potential Dead and dormant seeds maintain this level of hydration typical of Phase II, but unlike germinating seeds they do not enter Phase III, which is associated with visible germination. 

After the initial imbibition phase and hydration of the cell walls and cell contents the matric potential (i/ J plays a minor role in attracting water into the cell. Indeed, some of the matrices, e.g. protein and carbohydrate storage products in the storage organs, are later hydrolysed to low molecular weight, osmotically-active substrates which decrease (make more negative) the of both the seed as a whole, and, when transported there, of the growing embryonic axis (Chap. 6). The increase in water uptake in Phase III is thus initially asso-... 

---