Anoxic soils

The identities of the solid phases that form remain a mystery. Direct identification is difficult because Fe(II) and Mn(II) solid phases are readily oxidized by O2 and it is therefore necessary to maintain scrupulously anoxic conditions to ensure that the material examined actually represents that in anoxic soil. An alternative is to make indirect assessments through measurements of pe, pH and [Fe +] in solution, but these too are difficult (see section on measurement of redox potential in soil). 

Further transformations of N take place at the oxic interfaces between the soil and floodwater and root and soil where NH4+ diffusing in from the neighbouring anoxic soil may be nitrified to NOs. Subsequently, NOs diffusing out into the anoxic soil may be denitrified to N2. This process results in significant losses of N from wet soils but its importance in submerged soils is unclear (Section 5.3). 

Kirk (2003) has developed a simple model to compare root requirements for aeration with those for efficient nutrient acquisition in rice. The main features of the rice root system are summarized in Figure 6.4. The model considers roots in the anoxic soil beneath the fioodwater—soil interface, receiving their oxygen solely from the aerial parts of the plant. 

Primary roots (with laterals) in anoxic soil... 

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). 

Of wetland plants, rice has been studied the most extensively, and nitrogen has been the most extensively studied element. In this section the rates at which rice roots can absorb nitrogen are discussed and whether this is affected by the morphological and physiological adaptations to anoxic soil conditions. 

Thieme, J., Prietzel, J., Tyufekchieva, N., Paterson, D., and McNulty, I. (2006). Speciation of sulfur in oxic and anoxic soils using X-ray spectromicroscopy. In Proceedings of the 8th International Conference on X-Ray Microscopy, IPAP Conference Series 7, pp. 318-320. 

The O2—H2O couple is the redox pair controlling reactions in aerated solutions, so that reaeration of anoxic soils drives reduced species (e.g., Fe " ) toward the oxidized state. The range of redox potentials over which Fe ", and NH4 have been found to oxidize and disappear on aeration of a reduced soil are denoted by the open boxes in Figure 7.5. Nitrate reappearance on aeration is also depicted by an open box. The measured redox potentials that follow re-aeration do not directly reflect the 02—H20 equilibrium state but rather the status of redox couples having faster electron exchange rates. Furthermore, while each redox couple would be expected (in theory) to undergo complete conversion to the reduced form (in flooded soils) or to the oxidized form (in re-aerated soils) before the adjacent redox couple on the Eh scale became active, actual behavior in soils is much less ideal. Several redox reactions are typically active simultaneously. This may reflect spatial variability in the aeration (and redox potential) of soil aggregates, caused by slow diffusion processes in micropores. 

In subsurface oxic soil near Los Alamos National Laboratory, USA, plutonium is relatively mobile and has been transported primarily by colloids in the 25-450 pm size range. Moreover, the association with these colloids is strong and removal of Pu from them is very slow. By contrast, near Sellafield in wet anoxic soil, most of the Pu is quickly immobilized in the sediments although a small fraction remain mobile. Differences in oxidation state (Pu(V) vs. Pu(lV)) as well as in humic content of the soils may explain these differences in mobility. 

Iodine Speciation in the Liquid Phase of Soil Yamaguchi et al. (2006) further investigated iodine sorption behavior in relation to by determining the speciation of the element in Japanese paddy field soils subject to oxic and anoxic conditions brought about by irrigation management. These workers observed the disappearance of added lOj from anoxic soils as 1 concentrations in soil solution increased, i.e., the transformation from oxic to anoxic... 

The vast majority of soil iodine is most likely adsorbed to the soil solid phase, although appreciable amounts may be released into the hquid and gaseous phases, particularly under anoxic soil conditions. 

Mobihty of iodine under anoxic soil conditions is markedly greater than under oxic conditions. Accumulations of iodine at the boundary between anoxic and oxic soil regions are likely to occur. 

Volatihzation of iodine, primarily as methyl iodide, occurs from both soil and plants. Again, the presence of anoxic soil conditions is likely to enhance this process. 

Masscheleyn, P. H., R. D. DeLaune, and W. H. Patrick, Jr. 1991. Biogeochemical behavior of selenium in anoxic soils and sediments an equilibrium thermodynamics approach. Environ. Sci. Health A26(4) 555-573. 

The main processes in soils demonstrate oxic influents (rainwater, surface water) and anoxic soil containing pyrite resulting in mean order of increasing duration displacement of native groundwate cation exchange, pyrite oxidation, acid buffering by... 

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