Soils floodwater

There may be a cycling of S compounds of different oxidation state between anaerobic and aerobic zones in the soil, such as at the soil—floodwater interface. In reduced lake and marine sediments this leads to accumulation of insoluble sulfides as S04 carried into the sediment from the water above is immobilized. Such deposits function as sinks for heavy metals. Plants absorb S through their roots as S04 H2S is toxic to them. Therefore HS must be oxidized to S04 in the rhizosphere before it is absorbed. 

These general features of NOs reduction in submerged rice soils are bom out by field observations. Buresh et al. (1993b) found that from 60 to 75 % of N-labelled NOs" applied on the surface of flooded ricefields was lost by denitrification over 2-3 weeks, as measured by the not recovered in the soil, floodwater and plants. The recovery of (N2 + N20)- N in chambers placed over the floodwater was less than the estimated denitrification loss because gas bubbles became entrapped in the soil. More N2 + N2O was recovered when the chambers were placed over the rice plants showing that some of the gas escaped through the plants. The not lost by denitrification was presumed to have... 

Note that the above conclusions refer to uptake of soil N by the main body of the rice root system in the anoxic soil beneath the soil-floodwater interface. Uptake of fertilizer N broadcast into ricefield floodwater and absorbed by roots in the floodwater or soil near the floodwater is not likely to be limited by root uptake properties or transport (Kirk and Solivas, 1997). 

The model shows that the spread of urea and NH4+ into the soil is typically only a centimetre or two in a week (Fignre 8.9). The recovery of broadcast fertilizer N in the crop mnst therefore depend entirely on the superficial root system in the soil-floodwater interface. The good recovery of broadcast fertilizer N obtained if the fertilizer is added when the crop demand is maximal (Peng and Cassman, 1998) therefore indicate rapid uptake by roots in the soil-floodwater interface. 

The soil-floodwater interface in wetland systems is extremely important in cycling of... 

In soils, the pH of soil pore water tends to decrease with depth. The pH values are usually above 7 at the soil-floodwater interface and decrease to native soil pH at a lower depth. Some examples of soil pore water pH as a function of depth in selected Florida s wetland soils are shown in Figure 4.15. High pH values at the soil-floodwater interface are due to the photosynthesis activity of algae. 

Distinct Eh gradients are present (i) at the soil-floodwater interface, (ii) at the root zone of wetland plants, and (iii) aronnd the soil aggregates in drained portions of wetlands dnring low water table depths. 

The thickness of the aerobic layer varies from <1 mm to 3 cm. In relation to anaerobic soil volume, the aerobic soil volume at the soil-floodwater interface is small. However, this thin aerobic interface in the proximity of anaerobic soil is key to many unique biogeochemical processes functioning in wetlands. The differentiation of a wetland soil or sediment into two distinct zones as a result of limited oxygen penetration into the soil was first described by Pearsall and Mortimer (1939) and Mortimer (1941). 

The thickness of the aerobic layer can be determined by measuring Eh as a function of depth redox potentials show sharp gradients at the soil-floodwater interface. Laboratory studies have indicated complete disappearance of Oj at Eh <300 mV (pH = 7.0). This Eh value was used as a boundary between aerobic and anaerobic layers. A simple technique to determine redox profiles as a function of depth is described by Patrick and DeLaune (1972). This method involves a special motor-driven assembly that advances a platinum electrode at a rate of 2 mm h" through a soil profile. Redox potential is recorded continuously on a recorder or a data logger. Examples of Eh profiles are shown in Figure 6.22. 

The dominant and most reported component of root plaques is various oxidized compounds of iron. Microscopic observations of root plaques show a highly heterogenous morphology composed mostly of an amorphous material dispersed throughout nodules (50-300 nm in diameter), needles (50-100 nm in length), and filaments with variable lengths. This iron plaque formation on roots results from diffusion of Fe + toward the root zone in response to concentration gradients at the interface (similar to those observed at the soil-floodwater interface). The oxidized rhizosphere functions as a sink for Fe + and other reduced substances. 

Pathway 6 shows dinitrogen fixation, which is the reduction of atmospheric, inert nitrogen to ammonia that plants and microbes can then use. It occurs in the water column, soil-floodwater interface, root zone, and anaerobic soil in wetlands. It is carried out by free-living bacteria (Clostridium), bacteria living in symbiotic association with plants (Rhizobium), cyanobacteria (Anabaena), and periphyton. 

Because of limited oxygen availability in wetland environments, nitrification is restricted to the (1) aerobic water column, (2) aerobic soil-floodwater interface, and (3) aerobic root zone (Figure 8.34). In all these zones, nitrification is supported predominantly by chemoautotrophic bacteria, which use oxygen as their electron acceptor and ammonium as their energy source. Nitrification in these zones is often limited by the availability of ammonium. 

Ammonium flux from anaerobic soil layer is governed by the (1) concentration gradient established as a result of ammonium consumption in the aerobic zone due to nitrification and ammonia volatilization, (2) ammonium regeneration rate in the anaerobic soil layer, (3) adsorption coefficient for ammonium, (4) soil CEC, (5) intensity of soil reduction and accumulation of reduced cations, (6) bioturbation at the soil-floodwater interface, and (7) soil porosity. 

Wetland soils and aquatic sediments are uniquely characterized by aerobic and anaerobic interfaces at the soil-floodwater interface or in the root zone of wetland plants (see Chapter 4 for details). Aerobic oxidation of Fe(II) and Mn(ll) is restricted to the thin aerobic layer at the soil-floodwater interface or in the root zone. Thus, the extent of aerobic oxidation of Fe(ll) and Mn(ll) is dependent on the flux of dissolved species from anaerobic soil layers to aerobic zones. At circumneutral pH, concentrations of dissolved Fe(ll) and Mn(II) are very low, thus restricting flux into aerobic portions of the soil. At this pH level, the majority of Fe(II) and Mn(ll) compounds are present as immobile solid phases such as FeCOj, MnCOj, FeS2, Fe(OH)2, and Mn(OH)2. These compounds can be oxidized only when the water table is lowered, thus exposing top portion of the soil profile to aerobic conditions. 

Wetlands exhibit distinct redox gradients between the soil and overlying water column and in the root zone (Chapter 4), resulting in aerobic interfaces. For example, the aerobic layer at the soil-floodwater interface is created by a slow diffusion of oxygen and the rapid consumption at the interface. The thin aerobic layer at the soil-floodwater interface and around roots functions as an effective zone for aerobic oxidation of Fe(ll) and Mn(II). Below this aerobic layer there exists the zone of anaerobic oxidation of Fe(ll) and Mn(ll) and reduction of Fe(III) and Mn(IV). The juxtaposition of aerobic and anaerobic zones creates conditions of intense cycling of iron and manganese mediated by both biotic and abiotic reactions. 

The intensity of bioturbation is expressed as biodiffusivity (Dg) and the effect of bioturbation versus the molecular diffusion is described as the ratio between Dg and Dj. A vast amount of literature is available on the role of macrobenthos on the exchange of solutes across sediment-water interface in freshwater and marine sediments. It should be noted that in wetlands, rooting of vegetation in soils creates additional complexity on exchange of solutes across soil-floodwater interface. The macrobenthos can influence the vertical distribution of sediments and POM, and the... 

The exchange of dissolved solutes across the soil-floodwater interface is calculated using Pick s first law (see Section 14.3.1) (Berner, 1980) and Equation [14.12]... 

The exchange of solutes such as nutrients, dissolved organic substances, and metals and their associated processes at soil-floodwater interfaces are measured using laboratory-incubated cores or in situ pore water equilibrators as has been described. But these methods often underestimate fluxes because they do not account for processes such as bioturbation and bioirrigation at the soil-floodwater interface. To overcome these limitations, autonomous benthic chambers installed on top... 

The second reason is that as soil gases are transported to the surface they must pass through the thin aerobic soil-floodwater interface where methane oxidation occurs. 

Schipper, L. A. and K. R. Reddy. 1994. In situ determination of plant detritus breakdown in a wetland soil-floodwater profile. Soil Sci. Soc. Am. J. 59 565-568. 

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