Wetland roots

Metal cations in the soil solution may be immobilized by sorption onto iron plaque on root surfaces in submerged soils, in the same way that solubilized Zn + was re-adsorbed on ferric oxide in the experiments in Figure 6.22. Sequestering of metals on the external surfaces of wetland roots in this way limits uptake... 

This chapter has shown the complexity of the chemical and biological processes around wetland plant roots and the effects of the extreme electrochemical gradient between the root surface and surrounding soil. Models of nutrient uptake by plants in aerobic soil, which treat the root as a simple sink to which nutrients are delivered by mass flow and diffusion but the root not otherwise influencing the surrounding soil, work reasonably well for the more soluble nutrient ions. However, the complexity of the wetland root environment is such that such models are inadequate and more elaborate treatments are necessary. Many of the mechanisms involved are still poorly defined and speculative, but their potential significance is clear. 

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... 

Phytofiltration, a specific strategy of phytoremediation, is the use of plants to remove contaminants from water and aqueous waste streams. Three different systems (Figure 10.1) can be considered within this strategy (a) rhizofiltration (the use of hydroponically cultivated plant roots),31112 (b) constructed wetlands (CWs) and lagoons, and (c) bioadsorbents-based systems.1... 

In the wetlands of Idaho, the formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots (Typha latifolia, cat tail and Phalaris arundinacea, reed canary grass) may provide a means of attenuation and external exclusion of metals and trace elements (Hansel et al, 2002). Iron oxides were predominantly ferrihydrite with lesser amounts of goethite and minor levels of siderite and lepidocrocite. Both spatial and temporal correlations between As and Fe on the root surfaces were observed and arsenic existed as arsenate-iron hydroxide complexes (82%). 

Radionuelides can be also used to study the accumulation and degradation of organic pollutants. In our experiments we have followed the uptake and degradation of labelled TNT by wetland plants (Nepovim et al., 2005), and showed that about 63% of the localized in the roots of Ph. australis was bound (Fig. 6) and the remainder was acetone-extractable. An HPLC analysis of the acetone extract failed to detect any TNT, showing that all TNT had been chemically transformed. Thus TNT was not adsorbed on the root surface. In similar experiments performed in wheat (Triticum aestivum). Sens et al. (1999) found that 57% of the taken up was bound... 

In wetlands N2 fixation can occur in the water colnmn, in the aerobic water-soil interface, in the anaerobic soil bulk, in the rhizosphere, and on the leaves and stems of plants. Phototrophic bacteria in the water and at the water-soil interface are generally more important than non-photosynthetic, heterotrophic bacteria in the soil and on plant roots (Buresh et al, 1980 Roger 1996). The phototrophs comprise bacteria that are epiphytic on plants and cyanobacteria that are both free-living and epiphytic. A particularly favourable site for cyanobacteria is below the leaf surface of the water fern Azolla, which forms a very efficient symbiosis with the cyanobacterinm Anabaena azollae. This symbiosis and those in various leguminous plants have been exploited in traditional rice prodnction systems to sustain yields of 2 to 4 t ha of grain withont fertilizer for hnndreds of years. 

Transport of gases through the aerenchyma may occur by diffusion and, where pressure gradients develop, by convection. Pressurized flow is important in wetland plants with root systems permitting a throughflow of gases, but is insignificant in other plants (Beckett et al., 1988 Skelton and Alloway,... 

NUTRIENT ABSORPTION PROPERTIES OF WETLAND PLANT ROOTS... 

Nutrient Absorption Properties of Wetland Plant Roots... 

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. 

The removal of fertilizer N in the crop as NH4+ does not lead to acidification. Hydrolysis of urea fertilizer—by far the main form of N fertilizer used in wetland rice, together with ammonium bicarbonate in some countries—consumes 1 mol of H+ per mol of NH4+ formed (Table 7.1, Process 1). So although absorption of N as NH4+ leads to a net export of H+ from the roots to balance the resulting excess intake of cations over anions (Table 7.1, Process 5), this acidity is matched by the H+ consumed in urea hydrolysis. Likewise there is no net generation of acidity as a result of NH3 volatilization, although 1 mol of H+ is left behind per mol of NH4+ converted to NH3 (Table 7.1, Process 3). 

Armstrong W, Armstrong J, Beckett PM. 1990. Measurement and modelling of oxygen release from roots of Phragmites australis. In Cooper SC, Findlater BC, eds. The Use of Constructed Wetlands in Water Pollution Control. Oxford Pergamon, 41-52. 

Armstrong W, Cousins D, Armstrong J, Turner DW, Beckett PM. 2000. Oxygen distribution in wetland plant roots and permeability barriers to gas-exchange with the rhizosphere a microelectrode and modelling study with Phragmites australis. Annals of Botany 86 687-703. 

Bedford BL, Bouldin DR, Beliveau BD. 1991. Net oxygen and carbon dioxide balances in solutions bathing roots of wetland plants. Journal of Ecology 79 943-959. 

Reddy KR, Patrick WH, Jr., Lindau CW. 1989. Nitrification-denitrification at the plant root-sediment interface in wetlands. Limnology and Oceanography 34 1004-1013. 

Sorrell BK, Armstrong W. 1991. On the difficulties of measuring oxygen release by root systems of wetland plants. Journal of Ecology 82 177-183. 

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