Soil water flux

As already mentioned, molecular cross talk seems to be the prerequisite mechanism for most of root microbial infections. Indeed the initial step of any root colonization involves the movement of microbes to the plant root surface bacterial movement can be passive, via soil water flux, or active, via specific induction of flagellar activity by plant released compounds (chemotaxis) (Chaps. 4 and 7). Other important steps are adsorption and anchoring to the root surface. 

Figure 14 Comparison of actual volumetric water contents (measured by time domain reflectom-etry) and calculated soil-water flux values (Penman equation) at four soil depths...

Tyler S. W., Chapman J. B., Conrad S. H., Hammermeister D. P., Blout D. O., Miller J. J., Sully M. J., and Ginanni J. M. (1996) Soil-water flux in the southern Great Basin, United States temporal and spatial variations over the last 120,000 years. Water Resour. Res. 32, 1481-1499. 

A typical field site, varying in area from about 1 to 10 ha, may include several soil series. The model parameter values may be different not only for each of these soil series, but may also vary considerably within a single series. Such variability in a number of soil hydraulic properties (e.g., soil hydraulic conductivity, soil water flux, etc.) has been widely reported in the literature ( 5 - 1 ). The model parameter values for a given location in the field may also vary with profile depth depending upon soil horizonation as well as a function of the soil and environmental factors (e.g., soil aeration, temperature, etc.). Since soil and environmental factors undergo dynamic changes with time, model parameters are also expected to exhibit temporal variability. At present, only limited data are available to characterize such spatial and temporal variability in pesticide sorption and degradation parameters required in several simulation models. 

Because overland flow depth and infiltration rate are interdependent processes, the transient, spatially variable infiltration rate must also be accounted for. The two-dimensional Richard s equation can be used to calculate the soil water flux at any position x along the slope ... 

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

Fig. 1. Rates of CO2 assimilation, A (/miol s ) leaf conductance, g (mol m s ) intercellular partial pressure of CO2, Pi (Pa) soil water potential and leaf water potential, xp (MPa) during gas-exchange measurements of a 30-day-old cotton plant, plotted against day after watering was withheld. Measurements were made with 2 mmol m sec" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure difference between leaf and air (S.C. Wong, unpublished data).

For all the essential nutrient ions, the diffusion coefficient, Du is essentially the same with a value of around 10 cm s whereas the water flux at the root surface is typically of the order 10 cm s for soils at around field capacity. The tortuosity factor typically scales with the volumetric moisture content over quite a wide range of moisture content, i.e., / 0. As the soil becomes drier, the water flux will decline much faster than the tortuosity factor due to the typi-... 

Payer80 states that the UNSAT-H model was developed to assess the water dynamics of arid sites and, in particular, estimate recharge fluxes for scenarios pertinent to waste disposal facilities. It addresses soil-water infiltration, redistribution, evaporation, plant transpiration, deep drainage, and soil heat flow as one-dimensional processes. The UNSAT-H model simulates water flow using the Richards equation, water vapor diffusion using Fick s law, and sensible heat flow using the Fourier equation. 

Nouchi I, Hosono T, Aoki K, Minami K. Seasonal variation in methane flux from rice paddies associated with methane concentration in soil water, rice biomass and temperature, and its modeling. Plant Soil. 1994 161 195-208. 

PROFILE is a biogeochemical model developed specially to calculate the influence of acid depositions on soil as a part of an ecosystem. The sets of chemical and biogeochemical reactions implemented in this model are (1) soil solution equilibrium, (2) mineral weathering, (3) nitrification and (4) nutrient uptake. Other biogeochemical processes affect soil chemistry via boundary conditions. However, there are many important physical soil processes and site conditions such as convective transport of solutes through the soil profile, the almost total absence of radial water flux (down through the soil profile) in mountain soils, the absence of radial runoff from the profile in soils with permafrost, etc., which are not implemented in the model and have to be taken into account in other ways. 

Soil-related data (HM and BC content in soil parent materials) were included in calculations to account the values of HM weathering. Also we considered the influence of soil types on forest biomass productivity. Runoff data (at scale 0.5 x 0.50 were directly used to get input data on drainage water fluxes, Qie. Forest-type-related data (wood biomass growth and HM content in wood biomass) inserted into our database were subdivided depending on either coniferous, deciduous or mixed forests. 

BIOGEOCHEMICAL FLUXES AND EXPOSURE PATHWAYS IN SOIL-WATER SYSTEM OF BOREAL AND SUB-BOREAL ZONES... 

Figure 3. Water fluxes in soil with micropores. The following processes are relevant (1) infiltration into soil matrix, (2) lateral infiltration from macropores, and (3) exchange between aggregates (Richter 1999).

Transport of NH4+ to the roots in Kirk and Solivas experiment was mainly by diffusion. The additional transport resulting from mass flow of soil solution in the transpiration stream would have increased the influx across the roots by about QQaVa/0.5bD% where Va is the water flux (Tinker and Nye, 2000, pp. 146-148), or about 4% in Kirk and Solivas experiment. A sensitivity analysis showed that rates of diffusion will generally not limit uptake in well-puddled soils, but they may greatly limit uptake in puddled soils that have been drained and re-flooded and in unpuddled flooded soils. 

Summary of Landmine Flux Results Since no one has devised a method of directly measuring the flux of explosive molecules from a mine, whether in situ or in the laboratory, several laboratory measurements have been reported in which the mine was placed in a sealed container, surrounded by soil, water, or air. The concentrations of explosive molecules in the surrounding media were then measured at intervals of several days and the flux inferred from the total concentration divided by the elapsed time. This likely provides the best estimate that can be expected. The various measurements have substantial variation, depending on the techniques and media used. Phelan and Webb describe several experiments [1, pp. 23, 24], It appears that a reasonable expectation of flux of explosive compounds from a buried landmine that move into the surrounding soil will be in the range of 1 to 200 pg/day. There are some complications, of course, since the surrounding soil produces a level of resistance, or back pressure, to the flux of the molecules. While the mechanisms are complex, the net effect is that wet soil permits a lower diffusive flux than dry... 

Note that benzene sorption to the soil solids and dissolution in the soil water do not affect the steady-state flux, but they influence the time needed until-steady state is reached (see Box 18.5). 

Synthesis of concentrations and fluxes of DOC and DON in precipitation, throughfall, and soil organic and mineral horizons are often reported in the published literature (Willey et al., 2000 Michalzik et al., 2001 Neff and Asner, 2001 Neff et al., 2002). Dissolved organic matter from these sources contributes DOM to groundwater and surface water. Leaf litter leachate is documented as a major source of DOC in the soil water of forested watersheds (Hongve, 1999) and in surface waters (Wetzel and Manny, 1972 McDowell and Fisher, 1976). However, specific quantitative allocation of DOC and DON from roots, leaf litter, and the forest floor in soil solution remains uncertain. 

The relationship between surface water fluxes and ground water strongly depends on the flux of water infiltrating downward through the soil layer. This flux, called infiltration, accounting only for the vertical heterogeneity of the soil can be described in a general form by the equation ... 

Changes in the functional processes of the ecosystem (like mineralization and nutrient, soil, water, and energy fluxes). 

In Figure 9.21 all of the carbon eventually used in weathering of minerals by CC>2-charged soil water is shown as entering the atmosphere. The difference between the flux of CO2 owing to precipitation of carbonate minerals in the ocean and the total CO2 released from the ocean is that CO2 used to weather silicate minerals on land, and agrees with the calculations of riverine source materials made earlier in this chapter, in which it was shown that 30% of the HCC>3 in river water comes from weathering of silicate minerals. 

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