Wetland rice soils

Murthy A.S.P., Zinc fractions in wetland rice soils and their availability to rice. Soil Sci 1982 133 150-154. 

In data assembled by Greenland (1997), the mean level of organic carbon in the topsoils of wetland rice soils from across tropical Asia was 2%, and after excluding acid peaty soils the mean was 1 %. This compares with a range of 1.27-1.81 % for Oxisols and Ultisols of the Cerrado region of Brazil (Sanchez,... 

Deficiency of P is often the main nutrient limitation in natural wetlands, though it is rarely important in wetland rice soils that have at least some history of P... 

Ponnamperuma EN. 1985. Chemical kinetics of wetland rice soils relative to soil fertility. In Wetland Soils Characterization, Classification and Utilization. Manila International Rice Research Institute, 71-89. 

Gaunt J. L., Neue H. U., Bragais J., Grant I. F., and Giller K. E. (1997) Soil characteristics that regulate soil reduction and methane production in wetland rice soils. Soil Sci. Soc. Am. J. 61, 1526-1531. 

Savant, N. K. and S. K. DeDatta. 1982. Nitrogen transformations in wetland rice soils. Adv. Agron. 35 241-302. 

Denier van der Gon HAC, van Breemen N, Neue HU, Lantin RS, Aduna JB, Alberto MCR, Wassmann R. 1996. Release of entrapped methane from wetland rice fields upon soil drying. Global Biogeochem. Cycl. 1996 10 1-7. 

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...

As we have seen in Section 6.4, wetland rice is particularly efficient at absorbing N03. Kirk and Kronzucker (2000) developed a model to calculate the extent to which rice growing in submerged soil can capture NOs formed in the rhizosphere before it diffuses away and is denitrified in the soil bulk. The model allows for the following processes. 

Roger PA, Ladha JK. 1992. Biological N2 fixation in wetland rice fields-estimation and contribution to nitrogen balance. Plant and Soil 141 41-55. 

Banik S., Sen M., and Sen S. P. (1996) Effects of inorganic fertilizers and micronutrients on methane production from wetland rice Oryza sativa L.). Biol. Fertility Soils 21, 319-322. 

The sensitivity of plants to zinc deficiency is variable among species. For example, oats, wheat, barley, and rye are not very sensitive, whereas potatoes, tomatoes, lucerne, and red clover have a medium sensitivity. By contrast, maize, hops, flax, and soybeans require a high level of zinc supply (Viets 1966 see also Table 3.29-3, p. 612 in Henkin 1984). Soils associated with zinc deficiency in plant are usually low in organic matter content and neutral to alkaline in reaction (Pais and Jones 1997). In Asia, zinc deficiency has been identified as the most widespread micronutritional disorder of wetland rice which occurs on sodic, calcareous, and poorly drained soils. 

High moisture contents in alkaline soils have been shown to increase the availability of Mo (Nayyar, 1972). Poonamperuma (1972) observed an increase in soluble-Mo concentration under wetland rice conditions. Flooding of soils increases the availability of Mo. Soils in the tropics that remain flooded for considerable periods have relatively high contents of Mo (Lopes, 1980), because a high moisture content in the soil reduces ferric iron to the ferrous form, which favors fixation of Mo (Jones, 1956). 

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. 

Gamon M, Saez E, Gil J et al (2003) Direct and indirect exogenous contamination by pesticides of rice-farming soils in a Mediterranean wetland. Arch Environ Contam Toxicol 44 141-151... 

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. 

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. 

---