Chemistry of Lowland Rice Soils and Nutrient Availability

Transcription

1 Communications in Soil Science and Plant Analysis, 42: , 2011 Copyright Taylor & Francis Group, LLC ISSN: print / online DOI: / Chemistry of Lowland Rice Soils and Nutrient Availability N. K. FAGERIA, G. D. CARVALHO, A. B. SANTOS, E. P. B. FERREIRA, AND A. M. KNUPP National Rice and Bean Research Center of EMBRAPA (Empresa Brasileira de Pesquisa Agropecuaria), Santo Antônio de Goiás, Goiás, Brazil Rice is the staple food crop for about 50% of the world s population. It is grown mainly under two ecosystems, known as upland and lowland. Lowland rice contributes about 76% of the global rice production. The anaerobic soil environment created by flood irrigation of lowland rice brings several chemical changes in the rice rhizosphere that may influence growth and development and consequently yield. The main changes that occur in flooded or waterlogged rice soils are decreases in oxidation reduction or redox potential and increases in iron (Fe 2+ ) and manganese (Mn 2+ ) concentrations because of the reductions of Fe 3+ to Fe 2+ and Mn 4+ to Mn 2+. The ph of acidic soils increased and alkaline soils decreased because of flooding. Other results are the reduction of nitrate (NO 3 ) and nitrogen dioxide (NO 2 ) to dinitrogen (N 2 ) and nitrous oxide (N 2 O); reduction of sulfate (SO 4 2 ) to sulfide (S 2 ); reduction of carbon dioxide (CO 2 ) to methane (CH 4 ); improvement in the concentration and availability of phosphorus (P), calcium (Ca), magnesium (Mg), Fe, Mn, molybdenum (Mo), and silicon (Si); and decrease in concentration and availability of zinc (Zn), copper (Cu), and sulfur (S). Uptake of nitrogen (N) may increase if properly managed or applied in the reduced soil layer. The chemical changes occur because of physical reactions between the soil and water and also because of biological activities of anaerobic microorganisms. The magnitude of these chemical changes is determined by soil type, soil organic-matter content, soil fertility, cultivars, and microbial activities. The exclusion of oxygen (O 2 ) from the flooded soils is accompanied by an increase of other gases (CO 2,CH 4, and H 2 ), produced largely through processes of microbial respiration. The knowledge of the chemistry of lowland rice soils is important for fertility management and maximizing rice yield. This review discusses physical, biological, and chemical changes in flooded or lowland rice soils. Denitrification, Oryza sativa L., oxidation reduction potential, sub- Keywords merged soil Introduction Rice (Oryza sativa L.) is the staple food crop in the diet of about one-half of the world s population (Fageria, Slaton, and Baligar 2003). It is grown mainly under two ecosystems, known as upland and lowland. Upland rice, also known as aerobic rice, is generally grown on undulated and unbunded fields and totally depends on rainfall for water requirements. Received 20 January 2010; accepted 11 February Address correspondence to N. K. Fageria, National Rice and Bean Research Center of EMBRAPA (Empresa Brasileira de Pesquisa Agropecuaria), Caixa Postal 179, Santo Antônio de Goiás, Goiás, CEP , Brazil. fageria@cnpaf.embrapa.br 1913

2 1914 N. K. Fageria et al. Lowland rice, also known as irrigated rice or flooded rice, is grown on leveled lands with bunds and with irrigation facilities. Yields of lowland rice are much greater than those of upland rice because of the assured water supply and use of high inputs by farmers. For example, in Brazil, upland rice average yield is about 2.2 Mg ha 1, whereas lowland rice yield is more than 5 Mg ha 1. The lower yield of upland rice is associated with biotic and abiotic stresses (Fageria 2001). Upland rice has lower yields than lowland rice, but its cost of production also is lower. Because of the lower cost and lack of irrigation facilities, upland rice will continue to be an important component of cropping systems in South America, Africa, and Asia. Figures 1 and 2 show lowland and upland rice growth, respectively, in the central part of Brazil. Under normal conditions, lowland rice fields are flooded with water about 3 to 4 weeks after sowing. The water level of about 10 to 15 cm is maintained during the crop growth cycle and is drained before harvest. Because of flooding, lowland rice suffers less from disease, insects, and weeds compared to upland rice. These factors also contribute to the Figure 1. Lowland rice crop in the state of Tocantins, central Brazil. Figure 2. Upland rice crop grown on an Oxisol of central Brazil.

3 Chemistry of Lowland Rice 1915 greater yield of lowland rice compared to upland rice. Flooding or waterlogging eliminates oxygen from the rhizosphere and causes changes in the soil chemical properties. These chemical changes are associated with physical reactions between the soil and water and also because of biological processes set in motion as a result of excess water or oxygen deficiency (Patrick and Mahapatra 1968). The most important change in the soil as a result of flooding is the conversion of the root zone of the soil from an aerobic environment to an anaerobic or near-anaerobic environment where oxygen is absent or limiting (Patrick and Mahapatra 1968). Oxygen deficiency or exclusion in submerged soils can occur within a day after flooding. The oxygen movement through the flooding water is usually much slower than the rate at which oxygen can be reduced in the soil. This situation may result in the formation of two distinctly different layers being formed in a waterlogged soil. On the top is an oxidized or aerobic surface layer where oxygen is present, with a reduced or anaerobic layer underneath in which no free oxygen is present. Illustrated in Figure 3 is the thin oxidized layer (usually 1 to 20 mm in thickness) normally found at the interface between water and soil (Bouldin 1986). In addition, flooding also has major effects on the availability of macroand micronutrients. Some nutrients are increased in availability to the crop, whereas others are subject to greater fixation or loss from the soil as a result of flooding (Patrick and Mikkelsen 1971). The objective of this review is to discuss the chemistry of lowland or flooded rice soils, which may help in better nutrient management and consequently greater yields. Type of Soils Used for Lowland Rice Cultivation Lowland rice is produced on a variety of soils in different agroecological regions of the world. Because of the heterogeneity of agroecological regions, the pedogenetic and morphological characteristics of soils used to grow rice also vary considerably. The soils used for rice production worldwide are distributed over the 10 soil orders (Moormann Figure 3. Oxidized and reduced soil layer in the submerged rice soil.

4 1916 N. K. Fageria et al. 1978; Hudnall 1991). Moormann (1978) summarized that, worldwide, rice is grown on all soil orders identified in the soil classification system (USDA 1975). Worldwide, the wide array of soils used to produce rice results in an equally diverse assortment of management practices implemented for successful rice production on these soils. Murthy (1978) reported that the soils on which rice grows in India are so extraordinarily varied that there is hardly a type of soil, including salt-affected soils, on which it cannot be grown with some degree of success. In Brazil, flooded rice is mainly grown on Alfisols, Vertisols, Inceptisols, Histosols, and Entisols (Moraes 1999). In Sri Lanka, rice is grown on Alfisols, Ultisols, Entisols, Inceptisols, and Histosols (Panabokke 1978). In Indonesia, the main rice soils are Entisols, Inceptisols, Vertisols, Ultisols, and Alfisols (Soepraptohardjo and Suhardjo 1978). Raymundo (1978) reported that in the Philippines the soils used for wetland rice production are mainly Entisols, Inceptisols, Alfisols, and Vertisols. In Europe, rice is planted on limited areas in Albania, Bulgaria, France, Greece, Hungary, Italy, Portugal, Romania, Spain, and Yugoslavia, where the predominate soil orders are Inceptisols, Entisols, and Vertisols (Matsuo, Pecrot, and Riquier 1978). In the United States, rice is grown primarily on Alfisols, Inceptisols, Mollisols, and Vertisols (Flach and Slusher 1978). However, in Florida, a small hectarage of rice is produced on Histosols. Most of the soils used for rice production in the United States and some other geographic areas have properties that make them ideally suited for flood-irrigated rice. The soils are relatively young, contain significant amounts of weatherable minerals, and have relatively high base saturations despite the fact that some are in areas of high precipitation (Flach and Slusher 1978). Soil parameters for optimum rice yields are optimum soil depth, compact subsoil horizon, good soil moisture retention, good internal drainage, good fertility, and a favorable soil structure (Fageria, Slaton, and Baligar 2003a). Clayey to loamy clay texture soils are appropriate for lowland rice production. Permeable, coarse-textured soils are less suitable for flood-irrigated rice production because they have low water- or nutrient-holding capacities. In Brazil, there are about 35 million ha of poorly drained soils, known locally as Varzea, distributed throughout the country. Generally, Varzea soils have good initial soil fertility, but after 2 to 3 years of cultivation, the fertility level is known to decline (Fageria and Baligar 1996). Farming systems need to be developed with improved soil management technology to bring these areas under successful crop production. A sufficient supply of nutrients is one of the key factors required to improve crop yields and maintain sustainable agricultural production on these soils. Flood-irrigated rice is an important crop that needs to be included in the cropping system of these poorly drained areas during the rainy seasons. During dry periods, other crops can be planted in rotation, provided there is proper drainage. These soils generally have an adequate natural water supply throughout year, but are acidic and require routine applications of lime if legumes are grown in rotation with rice. Physical and chemical properties of varzeas soils of Brazil are presented in Tables 1 4. Data in these tables show that chemical and physical properties varied largely from state to state and from municipality to municipality within states. Physical, Biological, and Chemical Changes in the Flooded Soils Omission of oxygen from the large part of soil profile causes physical, biological, and chemical changes to occur in the submerged or flooded rice soils. These changes varied with the type of soil, presence of microbial biomass, quality and quantity of organic matter, cultivar planted, and level of soil fertility. In addition, these changes affect availability of essential plant nutrients and consequently plant growth and yield. Furthermore, the

5 Chemistry of Lowland Rice 1917 Table 1 Chemical properties of varzeas (lowland) soils of some states of Brazil Ca Mg Al MO ph P K (cmol c (cmol c (cmol c State (g kg 1 ) inh 2 O (mgkg 1 ) (mgkg 1 ) kg 1 ) kg 1 ) kg 1 ) Goiás Mato Grosso Mato Grosso do Sul Paraná Minas Gerais Rio Grande do Norte Piauí Maranhão Average Source: Fageria et al. (1991, 1994, 1997). Note. Values are from the 0- to 20-cm soil depth and lowland rice is generally grown on these soils during rainy season. Table 2 Micronutrient concentrations, cation exchange capacity (CTC), base saturation (V), and aluminum saturation (M) of várzeas (lowland) soils of some states of Brazil CTC V M Cu Zn Fe Mn (cmol c (cmol c (cmol c State (mg kg 1 ) (mgkg 1 ) (mgkg 1 ) (mgkg 1 ) kg 1 ) kg 1 ) kg 1 ) Goiás Mato Grosso Mato Grosso do Sul Paraná Minas Gerais Rio Grande do Norte Piauí Maranhão Média Source: Fageria et al. (1991, 1994, 1997). Note. Values are from the 0- to 20-cm soil depth and lowland rice is generally grown on these soils during rainy season. percolation rate decreases with flooding because of physical and chemical changes such as swelling, dispersion, disintegration of soil aggregates, reduction of soil pores by microbial activity, and organic-matter decomposition, which reduces the binding effect of aggregates and causes the soil to seal off (Wickham and Singh 1978).

6 1918 N. K. Fageria et al. Table 3 Textural analysis of várzeas (lowland) soils of some states of Brazil State Sand (g kg 1 ) Silt (g kg 1 ) Clay (gkg 1 ) Goiás Mato Grosso Mato Grosso do Sul Paraná Minas Gerais Rio Grande do Norte Piauí Maranhão Average Source: Fageria et al. (1991, 1994, 1997). Note. Values are from the 0- to 20-cm soil depth and lowland rice is generally grown on these soils during rainy season. Physical Changes As soon as soils of lowland rice are flooded, the oxygen level begins to decline. The rate of decline is very fast, and within 6 to 10 h after flooding, the O 2 level drops to near zero (Patrick and Mikkelsen 1971). The rapid declines of O 2 from the soil are accompanied by an increase of other gases produced through the microbial respiration. The major gases that accumulate in the flooded soils are carbon dioxide (CO 2 ), methane (CH 4 ), nitrogen (N 2 ), and hydrogen (H 2 ). Patrick and Mikkelsen (1971) reported that the composition of these gases may vary from 1 to 20% CO 2, 10 to 95% N 2, 15 to 75% CH 4, and 0 to 10% H 2.This variation may be associated with the presence of microbial biomass, organic matter, and inorganic substances and also the cultivar planted. Flooding may also alter the soil temperature and may disintegrate soil structure. At a given soil moisture content, and as bulk density increases, thermal conductivity increases (Ghildyal and Tripathi 1971). As the thermal conductivity of soil particles is greater than that of air, increased density decreases the volume of gases and increases thermal contact between the soil particles. As a result, thermal conductivity increases (Ghildyal 1978). Permeability to water may be reduced by clogging the soil pores, which results from physical, chemical, and biological changes. This may help to reduce percolation of water and leaching of nutrients. In the lowland rice production system, the subsoil layer is compacted with the help of a roller, a process known as puddling. According to the Soil Science Society of America (2008), puddling is defined as any process involving both shearing and compactive forces that destroys natural structure and results in a condition of greatly reduced pore space. Ghildyal (1978) defined puddling as mixing soil with water to render it impervious. Intensive tillage by repeated plowing of a wet soil breaks down coarse aggregates and mean particle size decreases. Soil compaction affects the waterretention characteristics, water-intake rates, and gas exchange. In compacted soil, bulk density, microvoids, thermal conductivity and diffusivity, and nutrient mobility increase, and macrovoids, hydraulic conductivity, and water-intake rates decrease. Medium-textured soils are most susceptible to compaction. Puddling is very common in Asian rice-producing countries. Puddling, intensive wetland cultivation, breaks the natural aggregates to finer fractions. It decreases the apparent

7 Table 4 Chemical and textural properties of várzeas (lowland) soils of state of Rio Grande do Sul of Brazil Location/ Ca+Mg Sand Silt Clay municipality MO (g kg 1 ) ph inh2o (cmolc kg 1 ) K(mgkg 1 ) Al (cmolc kg 1 ) V (%) M (%) (gkg 1 ) (gkg 1 ) (gkg 1 ) Palmares , Vacacaí , Pelotas , Meleiro , Colégio , Jundaí , Jacinto Machado , Curumim , Average , Source: Klamt et al. (1985). Note. Values are from the 0- to 20-cm soil depth and lowland rice is generally grown on these soils during rainy season. Rio Grande do Sul is the largest lowland-rice-producing state in Brazil. 1919

8 1920 N. K. Fageria et al. Figure 4. Puddling is done in the lowland rice plots in the state of Para in the Amazon basin (earlier project of Jari, funded by D. K. Ludwig), Brazil. specific volume and hydraulic conductivity, creates an anaerobic environment, and affects Eh and ph (Ghildyal 1978). Ghildyal (1978) reported that rice root growth, nutrient uptake, and water use are favorably affected by moderate compaction of a flooded soil where the soil strength is low. Figure 4 shows that puddling is done in the lowland rice plots before sowing the pregerminated seeds of rice. In many Asian countries, rice is transplanted in the puddle fields by small farmers. Biological Changes In waterlogged or flooded rice soils, aerobic microorganisms become quiescent or die, and facultative and obligate anaerobic bacteria proliferate. These new microorganisms bring many biological changes in the reduced soil environment. In the absence of oxygen, many facultative and obligate anaerobic bacteria oxidize organic compounds with the release of energy in a process called anaerobic fermentation (Patrick and Mikkelsen 1971). Anaerobic fermentation usually produces lactic acid as a first product. This is subsequently converted to acetic, formic, and butyric acids. Among aerobic organisms, oxygen serves as the electron acceptor, but in anaerobic forms, either an organic metabolic by-product or some inorganic substance must substitute for oxygen (Patrick and Mikkelsen 1971). In the flooded soils, organic-matter decomposition is retarded because of lower carbon assimilation rates of anaerobic bacteria. In a submerged soil, the facultative and obligate anaerobic organisms utilize nitrate (NO 3 ), manganese (Mn 4+ ), iron (Fe 3+ ), sulfate (SO 2 4 ), dissimilation products of organic matter, CO 2, and H + ions as electron acceptors in their respiration, reducing NO 3 to dinitrogen (N 2 ), Mn 4+ to Mn 2+,Fe 3+ to Fe 2+,SO 2 4 to sulfide (S 2 ), CO 2 to CH 4, and H + to H 2 gas (Patrick and Reddy 1978). Chemical Changes The most important chemical changes that occur in flooded or submerged rice soils are to ph, redox potential, and ionic strength or electrical conductivity. These changes occur as a result of oxygen depletion.

9 Chemistry of Lowland Rice 1921 ph. Soil ph is an important chemical property because of its influence on soil microorganisms and availability of nutrients to plants. It is determined by a ph meter using a glass electrode and in a specific soil solution ratio. Usually distilled water or 0.01 M calcium chloride (CaCl 2 ) or 1 M potassium chloride (KCl) solution is used for soil ph determination. Soil ph indicates acidity, alkalinity, or neutrality of a soil. Soil ph 7.0 is a neutral value. Above this ph, soils are designated as alkaline, and below this, soils are acidic in reaction. The ph of acidic soils increases and alkaline soils decreases as a result of flooding. Overall, ph of most soils tends to change toward neutral after flooding. An equilibrium ph in the range 6.5 to 7.5 is usually attained (Patrick and Reddy 1978). A majority of oxidation reduction reactions in flooded soils involve either consumption or production of H + /OH ions (Ponnamperuma 1972). The increase in ph of acidic soils is mainly determined by reduction of Fe and Mn oxides, which consume H + ions. These reduction processes are shown in the following equations: Fe 2 O 3 + 6H + + 2e 2Fe H 2 O MnO 2 + 4H + + 2e Mn H 2 O The decrease in the ph of alkaline soils is associated with the microbial decomposition of organic matter, which produces CO 2, and the produced CO 2 reacts with H 2 Oto form carbonic acid, which dissociates into H + and bicarbonate (HCO 3 ) ions. Patrick and Reddy (1978) reported that the decrease in ph of alkaline and calcareous submerged soils is associated with sodium carbonate (Na 2 CO 3 ) H 2 O CO 2 and calcium carbonate (CaCO 3 ) H 2 O CO 2 systems, respectively. Figure 5 shows the change in soil ph of lowland rice collected from four locations in the state of Rio Grande do Sul, Brazil. It can be seen from Figure 5 that soil ph increases with flooding and stabilized around 56 days after flooding in all the soils. However, the magnitude of ph change differs from soil to soil. The ph of Figure 5. Change in soil ph with the flooding of lowland rice. Adapted from Moraes and Freira (1974).

10 1922 N. K. Fageria et al. most agricultural soils is in the range of 4 to 9 (Fageria 2009). The most suitable ph for growth of annual crops such as soybean, corn, dry bean, and wheat in Brazilian lowland soils is around 6.5 (Fageria and Baligar 1999). Oxidation Reduction Potential Oxidation reduction or redox potential has significant influence on chemistry of iron and other nutrients in the submerged soils. It is the best single indicator of the degree of anaerobiosis in the flooded soil and allows reasonable predictions to be made concerning the behavior of several essential plant nutrients (Patrick and Mikkelsen 1971). Oxidation is the donation and reduction is the acceptance of electrons from other substances. Oxidizing agents accept electrons from other substances and thereby reduce themselves. Reducing agents donate electrons to other substances. For example, iron(ii) is an electron donor or a reducing agent when it oxidized to iron(iii). Hydrogen peroxide (H 2 O 2 ) is an oxidizing agent when it accepts electrons from organic matter and oxidizes it to CO 2 (Bohn, McNeal, and O Connor 1979). Oxidation reduction potential is measured in millivolts, and symbol used for this chemical change in flooded soil is Eh. Oxidized soils have redox potentials in the range of +400 to +700 millivolts, whereas waterlogged soils redox potential is generally in the range of 250 to 300 millivolts (Patrick and Mahapatra 1968). Important oxidation reduction processes that occur in the waterlogged soils are presented in Table 5. Some of the oxidized soil components that undergo reduction after oxygen is depleted are reduced sequentially; that is, all of the oxidized components of one system will be reduced before any of the oxidized components of another system begin to be reduced. Others overlap during reduction (Patrick and Reddy 1978). As the O 2 depletes from the waterlogged soils, reduction processes occur in sequence. Nitrate and manganese compounds are reduced first, then ferric compounds are reduced to the ferrous form, and at last sulfate is reduced to sulfide. Redox potential decreased with flooding of rice soils (Figure 6). Table 5 Thermodynamic sequence of reduction processes in the submerged soils Reaction Redox potential E 0 7 a (V) O 2 + 4H + + 4e 2H 2O NO 3 +12H + +10e N 2 + 6H 2 O 0.74 MnO 2 + 4H + + 2e Mn 2+ 2H 2 O 0.40 CH 3 COCOOH +2H + + 2e CH 3 CHOHCOOH 0.16 Fe(OH) 3 + 3H + + e Fe H 2 O 0.19 SO H + + 8e H 2 S + 4H 2 O 0.21 CO 2 + 8H + + 8e CH 4 + 2H 2 O 0.24 N 2 + 8H + + 6e + 2NH NADP + + 2H + + 2e NADPH 0.32 NAD + + 2H + + 2e NADH H + + 2e H Ferredoxin (ox) + e Ferrodoxin (red) 0.43 a E 0 corrected to ph 7.0. Sources: Ponnamperuma (1972); Ponnamperuma (1976), and Patrick and Reddy (1978).

11 Chemistry of Lowland Rice 1923 Figure 6. Influence of flooding on redox potential of some Mexican soils. Adapted from Moraes and Freira (1974). A rapid decline in redox potential is characteristic of soils with low contents of reducing Fe and Mn and high organic-matter content. Iron and Mn compounds serve as buffers against the development of reducing conditions in the soil (Patrick and Mahapatra 1968). The critical redox potentials for Fe reduction and consequent dissolution are between +300 mv and +100 mv at ph 6 and 7, and 100 mv at ph 8, while at ph 5 appreciable reductions occur at +300 mv (Gotoh and Patrick 1976). Oxidation reduction or potential reduction values for oxidized and submerged soils and reduction processes are given in Table 6. Ionic Strength Ionic strength is defined as the measure of the electrical environment of ions in a solution. Ionic strength can be calculated by using the following formula (Fageria et al. 2008): Ionic strength = 1 / 2 Mi Z 2 i where M is the molarity of the ion, Z i is the total charge of the ion (regardless of sign), and is a symbol meaning the sum of. The concentration of ions in the soil solution is measured by electrical conductivity. The ionic strength of the submerged soil increases with the release of macro- and micronutrients in the soil solution (Patrick and Mikkelsen 1971) (Figure 7). Nutrient Availability Reducing conditions in flooded rice soils change concentration and forms of applied as well as native soil nutrients. Hence, availability of essential macro- and micronutrients is significantly influenced in the flooded rice soils.

12 1924 N. K. Fageria et al. Table 6 Range of oxidation reduction potential values in oxidized and submerged soils and at which reduction processes occur Soil moisture/reduction processes Redox potential (mv) Well-oxidized soils +700 to +500 Moderately reduced soils +400 to +200 Reduced soils +100 to 100 Highly reduced soils 100 to 300 NO 3 to N to +220 Mn 4+ to Mn to +220 Fe 3+ to Fe to +150 SO 2 4 to S to 180 CO 2 to CH to 280 O 2 to H 2 O +380 to +320 Absence of free O Sources: Adapted from Patrick (1966), Patrick and Reddy (1978), Marschner (1995), and Fageria et al. (2008). Figure 7. Influence of flooding on electrical conductivity of some Mexican soils. Adapted from Moraes and Freira (1974). Nitrogen Nitrogen is a key nutrient in improving growth and yield of crop plants in all agroecosystems. Its main role is in increasing the photosynthesis process in the plants, which is associated with improving grain yield. Response of five lowland rice genotypes to N

13 Chemistry of Lowland Rice 1925 Figure 8. Responses of five lowland rice genotypes to nitrogen fertilization. Source: Fageria and Baligar (2006). fertilization is presented in Figure 8. Nitrogen is responsible for increasing yield components such as panicles or heads in cereals and pods in legumes (Fageria 2009). It also improves grain weight and reduces grain sterility. Figure 9 shows influence of N on yield components of lowland rice. Grain yield in rice is a function of panicles per unit area, number of spikelets per panicle, 1000-grain weight, and spikelet sterility or filled spikelets (Fageria 2007). Therefore, it is very important to understand the management practices that influence yield components and consequently grain yield. Nitrogen application up to 210 kg ha 1 influenced panicle length significantly (P < 0.01) and the relationship between N applied and panicle length was linear (Figure 9). The number of panicles m 2 and 1000-grain weight also increased significantly and quadratically with the application of N fertilizer. Spikelet sterility, however, decreased significantly and linearly with increasing N rates. Nitrogen treatment accounted for about 96% variation in panicle length, about 91% variation in panicles m 2, about 75% variation in spikelet sterility, and about 73% of variation in 1000-grain weight. Fageria (2007) also reported that panicles per unit area, filled spikelet percentage, and 1000-grain weight were major contributors to increased grain yield in modern high-yielding rice varieties. A major part of N in the flooded rice soils is lost through leaching and denitrification (Fageria and Baligar 2005). The major biological reaction involving nitrate in flooded soil is denitrification. Denitrification is the biological process in which nitrate reduces to N gas or nitrous oxide, or both. Patrick and Mikkelsen (1971) reported that denitrification losses of 50% or more of applied N are common in flooded rice soils. Frequent fluctuations in

14 1926 N. K. Fageria et al. Figure 9. Influence of nitrogen on number of panicles, panicle length, thousand-grain weight, and spikelet sterility of lowland rice. Source: Fageria and Baligar (2001). moisture content of a field as a result of flooding and drainage create ideal conditions for denitrification (Patrick and Wyatt 1964). Nitrogen converted to the nitrate form during the period when the soil is drained is lost through denitrification when soil is flooded. Deep placement of N in the flooded rice reduces N lost through denitrification. Nitrate produced in the surface oxidized layer of a waterlogged soil can easily move downward by diffusion and percolate into the underlying reduced layer, where it is rapidly denitrified (Patrick and Mahapatra 1968). Even with best management practices such as adequate rate, forms, methods, and timing of application, the utilization of added N is generally poorer in flooded rice soils. Fageria and Baligar (2001) and Fageria, Santos, and Cutrim (2007) studied N-recovery efficiency of lowland rice grown on Brazilian Inceptisols (Table 7). Average efficiency under different rates was 39%, whereas average N-recovery efficiency of five genotypes was 29%. Hence, a large part of applied N is lost in soil plant systems. Patrick and Mahapatra (1968) reported that in Japan 30 to 40% applied N is recovered by lowland rice as compared to an availability of 50 to 60% when applied to upland crops. In aerated soils, most of the N is in the form of NO 3 because of the nitrification process. In waterlogged soils, absence of O 2 inhibits the activity of the Nitrosomonas microorganisms that oxidize NH 4 +, and therefore N mineralization stops at the ammonium (NH 4 + ) form. Accumulation of NH 4 + in the waterlogged soils would mean that the N is not lost from the soil plant system, as is the case in denitrification. This may only happen if rice fields are constantly flooded during the crop growth cycle. If availability of water is not under farmers control because of lack of rainfall or storage facility, the situation may change in the transformation and availability of N to plants. Hence, if N is applied in the

15 Chemistry of Lowland Rice 1927 Table 7 Nitrogen recovery efficiency in lowland rice as influenced by N rate and genotypes N rate N recovery Lowland rice N recovery (kg ha 1 ) efficiency (%) genotype efficiency (%) CNAi CNAi BRSGO Guará BRS Jaburu BRS Giguá Average Average 39 R N uptake by plants in N fertilized plot N uptake by plants in control treatment Quantity of N applied Note. N recovery efficiency (%) = Significant at the 1% probability level. Sources: Adapted from Fageria and Baligar (2001) and Fageria et al. (2007). reduced soil layer and the water level is maintained in the rice field constantly, N uptake may improve in flooded rice. Phosphorus Phosphorus (P) plays an important role in the growth and development of crop plants. Its role is well documented in many physiological and biological processes in the plants (Fageria 2009). Phosphorus deficiency is one of the most important yield-limiting factors in annual crops grown on highly weathered acidic soils of the tropics (Sanchez and Salinas 1981; Dobermann, George, and Thevs 2002; Fageria and Barbosa Filho 2007). The P deficiency is associated with low natural P as well as with high P-fixation capacity of these soils. Added soluble P is usually rapidly adsorbed on the surfaces of Fe and aluminum (Al) oxides, which are followed by immobilization in other forms and within soil particles (Hedley, Kirk, and Santos 1994; Linquist et al. 1997). Data in Table 8 show that yield and yield components of lowland rice were significantly improved with the addition of P in a Brazilian Inceptisol. Phosphorus availability is increased in the flooded soils because of the reduction of ferric phosphate to the more soluble ferrous form and the hydrolysis of phosphate compounds. This may be more pronounced in acidic soils where P is immobilized by Fe and Al oxides. Similarly, P uptake in flooded alkaline soils also improves because of the liberation of P from Ca and calcium carbonate resulting from the decrease in ph. The formation of insoluble tricalcium phosphate is favored at a high ph. Potassium The influence of flooding is lesser on the chemistry of K than on the chemistry of N and P. The reducing conditions caused by flooding result in a larger fraction of the K ions being displaced from the exchange complex into the soil solution. The release of a relatively large amount of Fe and Mn ions and production of ammonium ions result in displacement

16 1928 N. K. Fageria et al. Table 8 Dry-matter yield of shoot, panicle number, panicle length, 1000-grain weight, spikelet sterility, and grain harvest index as influenced by phosphate treatments Shoot dry Panicle Panicle 1000-grain Spikelet Grain P rate weight number length weight sterility harvest (kg ha 1 ) (kg ha 1 ) (m 2 ) (cm) (g) (%) index F-test Year (Y) NS NS Prate(P) NS Y P NS NS NS NS NS NS CV (%) Regression analyses were as follows: P rate (X) vs shoot dry weight (Y) = X X 2,R 2 = P rate (X) vs panicle number (Y) = X X 2,R 2 = P rate (X) vs panicle length (Y) = X X 2,R 2 = P rate (X) vs 1000 grain weight (Y) = X X 2,R 2 = P rate (X) vs spikelet sterility (Y) = X X 2,R 2 = P rate (X) vs grain harvest index (Y) = X X 2, R 2 = Shoot dry weight (X) vs grain yield (Y) = X X 2, R 2 = Panicle number (X) vs grain yield (Y) = X X 2, R 2 = Panicle length (X) vs grain yield (Y) = X X 2, R 2 = grain weight (X) vs grain yield (Y) = X X 2, R 2 = Spikelet sterility (X) vs grain yield (Y) = X X 2, R 2 = NS Grain harvest index (X) vs grain yield (Y) = X X 2, R 2 = Notes. Values are averaged across 2 years.,, NS Significant at the 5% and 1% probability levels and nonsignificant, respectively. Source: Fageria and Santos (2008). of some of the K ions from the exchange complex to the soil solution. This may leads to greater availability of K to rice in flooded soils (Patrick and Mikkelsen 1971). Sulfur In flooded soils, SO 2 4 ion is reduced to hydrogen sulfide (H 2 S) by anaerobic microbial activities. Furthermore, in flooded soils, Fe 3+ reduction to Fe 2+ precedes SO 2 4 reduction;

17 Chemistry of Lowland Rice 1929 Fe 2+ will always be present in the soil solution by the time H 2 S is produced, so that H 2 S will be converted to insoluble iron sulfide (FeS). This reaction protects microorganisms and higher plants from the toxic effects of H 2 S (Patrick and Reddy 1978). Overall, availability of S is reduced in flooded soils due to formation of insoluble FeS. Calcium and Magnesium Calcium (Ca) and magnesium (Mg) deficiencies are rare in lowland rice. Rice is highly tolerant to soil acidity. Optimum soil ph for lowland rice grown on Brazilian Inceptisol was reported to be 4.9 (Fageria and Baligar 1999). In highly acidic soils, dolomitic lime can be added to supply Ca and Mg. Only a small amount of these elements are removed in the grain, and unless the straw is removed from the field, the total removal is small. Changes in Ca and Mg concentrations are minimum in flooded soils. Micronutrients The Fe 3+ reduces to Fe 2+ and Mn 4+ reduces to Mn 2+ ; hence uptake of these elements increased in the flooded rice soils. The reduction processes of Fe and Mn are shown under the section on ph changes. The greater concentration of Fe 2+ (>300 mg kg 1 ) may be toxic to rice plants under certain conditions (Fageria 1984; Fageria et al. 2008). Sims and Johnson (1991) reported that for most crops the critical deficiency soil Fe concentration range was mg kg 1 of diethylenetriaminepentaacetic acid (DTPA) extractable Fe but is also influenced by soil ph. In both field and pot experiments, the degree of bronzing in a given variety showed a highly significant correlation (r = 0.90 ) with yield (Breemen and Moormann 1978). Iron toxicity in rice plants, as indicated by bronzing of leaves, was reported when soluble Fe in the soil solution was more than mg kg 1 (Ponnamperuma, Bradfield, and Peech 1955; Tanaka, Loe, and Navasero 1966) by DTPA extracting solution. However, Breemen and Moormann (1978) reported that bronzing symptoms appear generally when Fe concentrations in the soil solution are in the range of mg kg 1 by DTPA extracting solution. Barbosa Filho, Fageria, and Stone (1983) reported that Fe toxicity in lowland rice occurred when Mehlich 1 extracting Fe in the soil was in the range of 420 to 730 mg kg 1. This means that Fe toxicity level in the soil is also dependent on the extracting solution used to extract the Fe from the soil. Values for the Mehlich 1 extracting solution are greater than for the DTPA solution. Iron toxicity in lowland rice has been reported in South America, Asia, and Africa (Sahu 1968; Barbosa Filho, Fageria, and Stone 1983; Fageria 1984; Fageria and Rabelo 1987; Fageria, Slaton, and Baligar 2003; Fageria, Stone, and Santos 2003; Sahrawat 2004). Metal toxicity in crop plants can be expressed in two ways. One is when metal is absorbed in greater amounts and becomes lethal to the plant cells. This is known as direct toxicity of metals. Another metal toxicity is associated with inhibition of uptake and utilization of essential nutrients by plants. This is known as indirect metal toxicity. Indirect toxicity creates nutrient imbalance in plants. This type of Fe toxicity is more common in lowland rice than direct toxicity (Fageria, Baligar, and Wright 1990; Fageria, Baligar, and Clark 2006). The most important nutrient deficiencies observed in irrigated or flooded rice in Brazil are P, K, and Zn (Barbosa Filho, Fageria, and Stone 1983). The yield reduction of rice cultivars due to Fe toxicity depends on tolerance or susceptibility of cultivars to toxicity. Ikehashi and Ponnamperuma (1978) reported that reduction of the yield on an Fe toxic soil ranged from a mean of 29% for five moderately tolerant lines to a mean of 74% for five susceptible lines.

18 1930 N. K. Fageria et al. Zinc and copper (Cu) concentrations generally decreased after flooding rice soils. The decrease in concentration with the flooding may be associated with increase in soil ph after flooding. Little is known about the behavior of B and Mo in the submerged soils. Boron concentration seems to remain more or less constant after submergence of rice soils (Ponnamperuma 1975). Molybdenum concentration in rice soils was found to increase after submergence (Ponnamperuma 1975), possibly because of the increased ph. In flooded soils, Si generally tends to increase after submergence. This increase is probably due to the release of adsorbed and occluded Si from oxyhydroxides of Fe and Al as well as to the effect of the increased ph resulting from submergence. Decompositing rice straw with its high silica content may also contribute to the increased Si content of the soil solution of flooded soils (Patrick and Reddy 1978). Conclusions Rice is mainly produced under upland and lowland ecosystems. Lowland ecosystem contributes the most rice production worldwide. Lowland rice is also known as flooded or submerged rice. Direct-seeded lowland rice fields are generally flooded about 3 to 4 weeks after sowing and remain flooded throughout the growing season; water is drained at harvest. Because of flooding, chemistry of lowland rice soils changes, which affect physical, chemical, and biological properties and consequently rice yields. The most significant chemical changes are increase in the ph of acidic soils and decrease in the ph of alkaline soils, reduction in the redox potential, and increase in the electrical conductivity or ionic strength. The magnitude of change of these chemical processes depend on soil type, microbial biomass, soil organic-matter content, and rice cultivar or genotype planted. All these changes influence availability of essential plant nutrients. Availability of essential nutrients is significantly influenced by flooding the rice soils. Availability of P, K, Si, Fe, Mn, and Mo increased in flooded soils, and availability of S, Zn, and Cu decreased. Availability of N depends on its proper management. If applied in the reduced soil zone, its uptake may improve as a result of fewer losses by denitrification. Both nitrate and ammonium ions can be assimilated by the rice plant, but better stability of the ammonium form in flooded soils makes it the superior form of N for lowland rice. In addition, the ammonium (NH 4 + )form of N requires less energy for absorption by plants compared to the nitrate (NO 3 )formof N. In addition, Al toxicity is decreased in flooded acidic soils because of the increase in soil ph. Flooding also favors microbial processes that release essential nutrients for plant growth. References Barbosa Filho, M. P., N. K. Fageria, and L. F. Stone Water management and liming in relation to grain yield and iron toxicity. Pesquisa Agropecuaria Brasileira 18: Bohn, H. L., B. L. McNeal, and G. A. O Connor Soil chemistry. New York: John Wiley & Sons. Bouldin, D. R The chemistry and biology of flooded soils in relations to the nitrogen economy in rice fields. Fertilizer Research 9:1 14. Breemen, N. V., and F. R. Moormann Iron-toxic soils. In Soils and rice, Los Bãnos, Philippines: IRRI. Dobermann, A., T. George, and N. Thevs Phosphorus fertilizer effects on soil phosphorus pools in acid upland soils. Soil Science Society of America Journal 66: Fageria, N. K Fertilization and mineral nutrition of rice. Goiânia, Brazil: EMBRAPA-CNPAF.

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