Wetland crops

These are by-products of wetland management where areas are cleared of mainly excess reeds and mshes. This can be dried, compressed into briquettes and burnt as a raw material but other uses are being researched, including pyrolysation into a sustainable form of charcoal. 

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In random samples of soil taken from five Alabama counties, only 3 of 46 soil samples contained methyl parathion. The concentration in these samples was <0.1 ppm (Albright et al. 1974). Aspartofthe National Soils Monitoring Program, soil and crop samples from 37 states were analyzed for methyl parathion during 1972. Methyl parathion was detected in only 1 soil sample, at a concentration of <0.1 ppm and taken from South Dakota, out of 1,246 total samples taken from the 37 states (Carey et al. 1979). In soil and sediment samples collected from a watershed area in Mississippi, methyl parathion was not detected in the soil samples. In three wetland sediment cores, however, measurable concentrations of methyl parathion were detected during application season (Cooper 1991). 

The saturated soils that occur during wetland, or lowland, rice cultivation give rise to a set of physical, chemical, and biological properties that are quite different from upland soils. Rice is the only major row crop produced under flooded-soil conditions and the absence of air-filled pores along with reduced soil-atmosphere interactions result in an almost entirely different set of processes than those occurring in upland cropping systems. 

Because of rice s origins as a wetland plant, it is more sensitive to water deficiency than most other crops. But provided sufficient water is supplied to periodically inundate the land and the soil is able to retain the water, rice will thrive on almost any type of soil. The productivity of rice land therefore often depends more on position in the landscape and soil physical properties than on the finer attributes of the soil. Nonetheless, subtle differences in properties distinguish productive and problem soils and affect the behaviour of the soil in the environment. 

Figure 4.14 Changes in labile soil P (extractable with HCOs -form anion exchange resin) during 3 years of wetland rice cropping as affected by timing of tillage (early, late = start, end of fallow), incorporation of previous crop s straw, and application of P (20kgha in NPK plots). The overall P balances over 3 years were +37 and +7kgPha in the NPK plots with and without straw, and —90 and — llSkgPha in the PK plots. DS, WS, dry, wet season DAT, days after transplanting (Bucher, 2001). Reproduced by permission...

Greenland (1997) has compiled realistic average annual nutrient balances for wetland ricefields pre- and post-1960 from probable inputs and outputs. Inputs come from rainfall, R, irrigation and floodwater, F, sediments, S, nitrogen fixation, N, and manures and fertilizers, M. Outputs are due to crop removals in... 

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). 

Despite the burning of crop residues in the productive, irrigated rice areas of tropical and subtropical Asia, and their removal for other purposes in the low-producing rainfed rice areas, soil carbon levels are largely constant (Bronson et al., 1998). In any case, the amount of carbon in the shallow puddled layer of ricefields amounts to only a few per cent of the amount in natural wetlands. 

Quijano-Guerta C, Kirk GJD. 2002. Tolerauce of rice germplasm to saliuity aud other soil chemical stresses in tidal wetlands. Field Crops Research 76 111-121. 

Nava-Rodriguez, V. Hemandez-Bautista, B.E., Cruz-Ortega, R., Anaya, A.L. Allelopathic potential of beans (.Phaseolus spp.), other crops, and weeds from Mexico. Allelopathy J Neori, A., Reddy, K.R., Ciskova-Koncalova, H., Agami, M. Bioactive chemicals and biological-biochemical activities and their functions in rhizospheres of wetland plants. Bot Rev 2000 66 350-378. 

Bio-control using pathogens holds promise mostly in non-cropland situations because of the slow pace of control of weeds and the wider window for control as compared with the shorter window of the cropping season and associated disturbances under cropland situations. However, in aquatic systems they appear to be more realistic, due to the absence of such cropping barriers and enhanced mode of dispersal in water. Among the wetland weeds, Echinochloa crusgalli and Cyperus rotundas are being... 

The United Nation s report—prepared by over 1,000 scientists—predicts cultural and social disruptions, loss of wetlands, flooding of river deltas, bleaching of coral reefs, permafrost thawing, acidification of oceans, drop in crop output, widespread water shortage, and even starvation in parts of southern Europe, the Middle East, Africa, Mexico, Southern Asia, and the American Southwest. Deforestation, soil erosion, storms, droughts, and devastation of agriculture are likely to result as temperatures exceed the heat tolerance of crops. These trends can combine to cause migration, ethnic strife, social destabilization, and wars. 

As will be explained in more detail later, BNF can occur in both unmanaged and managed ecosystems. In the former, natural ecosystems produce Nr. In the latter, the cultivation of legumes enhances BNF and the cultivation of some crops (e.g., wetland rice) creates the necessary anaerobic environment to promote BNF. 

Weis, 2004). If the rhizosphere plays an important role in the sequestration of metals in wetland soils, fluxes of potentially toxic metals out of the wetland to adjacent aquatic systems will, in theory, be reduced. However, plants may also increase metal mobilization through rhizosphere acidification and the oxidation of metal-sulfide complexes (Jacob and Otte, 2003). They may also represent only a temporary sink for metals if rhizosphere Fe plaque is reduced following plant senescence. Furthermore, metals can be exported from the ecosystem if contaminated plant parts are consumed by people or wildlife. The pathways and possible health effects of metal consumption have been especially well studied in Southeast Asia, where metal contamination (notably As) of rice crops is a serious public health issue (e.g., Meharg and Rahman, 2003 Meharg, 2004). 

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