1. INTRODUCTION
1.1 The Nitrogen
Nitrogen is commonly the most limiting element in agricultural production, and one of the most expensive to purchase as fertilizer (NifTAL, 1984). It is required by all living organisms for the synthesis of proteins, nucleic acids and other nitrogen-containing compounds (Anon, 2002). There is abundant supply of nitrogen in the air (the air is 80% nitrogen gas, amounting to about 8000 pounds of nitrogen in the air over every acre of land, or 6400 kilograms above every hectare). However, the nitrogen in the air is a stable gas, normally unavailable to plants until it has been fixed, that is reduced (combined with hydrogen), to ammonia. Green plants, the main producer of organic matter, use this supply of fixed nitrogen to make proteins that enter and pass through the food chain. Microorganisms (the decomposers) break down the proteins in excretions and dead organisms, releasing ammonium ions. These two processes form part of the nitrogen cycle. Many leguminous plants are able to utilize this atmospheric nitrogen through an association with rhizobia, bacteria that are hosted by the system of certain nitrogen fixing plants. Nitrogen can be fixed in three ways namely Atmospheric, Industrial and Biological fixation.
1.2 The Nitrogen Fixation Process
The element nitrogen, or “azote”, meaning “without life”, as Antonie Lavoisier called it about 200 years ago, has proved be anything but lifeless, since it is a component of food, poisons, fertilizers, and explosives (Schoot Uiterkamp, 1990). The atmosphere contains about 1015 tonnes of N2 gas, and the nitrogen cycle involves the transformation of some 3x 109 tonnes of N2 per year on a global basis (Postgate, 1982). However, transformations (e.g., N2 fixation) are not exclusively biological. Lightening probably accounts for about 10% of the world’s supply of fixed nitrogen (Sprent and Sprent, 1990). The fertilizer industry also provides very important quantities of chemically fixed nitrogen. World production of fixed nitrogen from dinitrogen for chemical fertilizer accounts for about 25% of the earth’s newly fixed N2, and biological processes account for about 60%. Globally the consumption of fertilizer-N increased from 8 to 17 kg/ha of agricultural land in the 15-year period from 1973 to 1988 (FAO, 1990). Significant growth in fertilizer-N usage has occurred in both developed and developing countries (Peoples et al., 1995). The requirements for fertilize-N are predicted to increase further in the future (Subba-Rao, 1980); however, with the current technology for fertilizer production and the inefficient methods employed for fertilizer application, both the economic and ecological costs of fertilizer usage will eventually become prohibitive.
For more than 100 years, biological nitrogen fixation (BNF) has commanded the attention of scientists concerned with plant mineral nutrition, and it has been exploited extensively in agricultural practice (Dixon and Wheeler, 1986; Burris, 1994). However, its importance as a primary source of N for agriculture has diminished in recent decades as increasing amounts of fertilizer-N have been used for the production of food and cash crops (Peoples et al., 1995). However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus attention on the potential role of BNF in supplying N for agriculture (Dixon and Wheeler, 1986; Peoples et al., 1995). The expanded interest in ecology has drawn attention to the fact that BNF is ecologically benign and that its greater exploitation can reduce the use of fossil fuels and can be helpful in reforestation and in restoration of misused lands to productivity (Burris, 1994; Sprent and Sprent, 1990).
Currently, the subject of BNF is of great practical importance because the use of nitrogenous fertilizers has resulted in unacceptable levels of water pollution (increasing concentrations of toxic nitrates in drinking water supplies) and the utrophication of lakes and rivers (Dixon and Wheeler, 1986; Sprent and Sprent, 1990 and Al-Sherif, 1998). Further, while BNF may be tailored to the needs of the organism, fertilizer is usually applied in a few large doses, up to 50% of which may be leached (Sprent and Sprent, 1990). This not only wastes energy and money but also leads to serious pollution problems, particularly in water supplies.
1.3 Biological Nitrogen Fixation
Biological nitrogen fixation (BNF) is an essential natural process that supports life on this planet. BNF is the process that changes inert N2 to biologically useful NH3. This process is mediated in nature mainly by bacteria. Other plants benefit from nitrogen fixing bacteria when bacteria die and release nitrogen to the environment, or when the bacteria live in close association with the plant. In legumes and a few other plants, the bacteria form small growth on the roots called nodules. Within these nodules, nitrogen fixation is done in association with bacteria and the NH3 is uptaken by the plant. Nitrogen fixation by legumes is a partnership between a bacteria and a plant.
1.4 Significance of Biological Nitrogen Fixation to Soil Fertility
BNF is an efficient source of nitrogen (Peoples et al., 1995)). The total annual terrestrial inputs of N from BNF as given by Burns and Hardy (1975) and Paul (1988) range from 139 million to 175 million tones of N, with symbiotic associations growing in arable land accounting for 25 to 30% (35 million to 44 million tons of N) and permanent pasture accounting for another 30% (45 million tons of N). While the accuracy of these figures may be open to question (Sprent and Sprent, 1990), they do help illustrate the relative importance of BNF in cropping and pasture systems and the magnitude of the task necessary if BNF is to be improved to replace a proportion of the 80 to 90 million tones of fertilizer-N expected to be applied annually to agricultural land by the end of the decade (Peoples et al., 1995; Peoples et al., 1995). Much land has been degraded worldwide, and it is time to stop the destructive uses of land and to institute a serious reversal of land degradation (Burris, 1994). BNF can play a key role in land remediation.
An examination of the history of BNF shows that interest generally has focused on the symbiotic system of leguminous plants and rhizobia, because these associations have the greatest quantitative impact on the nitrogen cycle. A tremendous potential for contribution of fixed nitrogen to soil ecosystems exists among the legumes (Brockwell et al., 1995; Peoples et al., 1995 and Tate, 1995). There are approximately 700 genera and about 13000 species of legumes, only a portion of which (about 20%[Sprent and Sprent, 1990]) have been examined for nodulation and shown to have the ability to fix N2. Estimates are that the rhizobial symbioses with the somewhat greater than 100 agriculturally important legumes contribute nearly half the annual quantity of BNF entering soil ecosystems (Tate, 1995). Legumes are very important both ecologically and agriculturally because they are responsible for a substantial part of the global flux of nitrogen from atmospheric N2 to fixed forms such as ammonia, nitrate, and organic nitrogen. Whatever the true figure, legumes symbioses contribute at least 70 million tons N per year, approximately half deriving from the cool and warm temperature zones and the remainder deriving from the tropics (Brockwell et al., 1995). Increased plant protein levels and reduced depletion of soil N reserves are obvious consequences of legume N2 fixation. Deficiency in mineral nitrogen often limits plant growth, and so symbiotic relationships have evolved between plants and a variety of nitrogen-fixing organisms (Freiberg et al., 1997).
Most of the attention in this thesis is directed toward N2 fixation inputs by legumes because of their proven ability to fix N2 and their contribution to integral agricultural production systems in both tropical and temperate climates (Peoples et al., 1995). Successful Rhizobium-legume symbioses will definitely increase the incorporation of BNF in to soil ecosystems. Rhizobium-legume symbioses are the primary source of fixed nitrogen in land-based systems (Tate, 1995) and can provide well over half of the biological source of fixed nitrogen (Tate, 1995).
Atmospheric N2 fixed symbiotically by the association between Rhizobium species and legumes represents a renewable source of N for agriculture (Peoples et al., 1995). Values estimated for various legume crops and pasture species are often impressive, commonly falling in the range of 200 to 300 Kg of N ha-1 year-1 (Peoples et al., 1995). Yield increases of crops planted after harvested of legumes are often equivalent to those expected from application of 30to 80 kg of fertilizer-N ha-1. Inputs of fixed N for alfalfa, red clover, pea, soybean, cowpea, and vetch were estimated to be about 65 to 335 kg of N ha-1 year-1 (Tate, 1995) or 23 to 300 kg of N ha-1 year-1 (Wani et al., 1995). However, the measured amount of N fixed by symbiotic systems may differ according to the method used to study N2 fixation (Sellstedt, 1993). Inputs into terrestrial ecosystems of BNF from the symbiotic relationship between legumes and their rhizobia amount to at least 70 million tones of N per year (Brockwell et al., 1995); this enormous quantity will have to be augmented as the world’s population increases and as the natural resources that supply fertilizer-N diminish. This objective will be achieved through the development of superior legume varieties, improvements in agronomic practice, and increased efficiency of the nitrogen-fixing process itself by better management of the symbiotic relationship between plants and bacteria.
The symbioses between Rhizobium or Bradyrhizobium and legumes are a cheaper and usually more effective agronomic practice for ensuring and adequate supply of N for legume-based crop and pasture production than the application of fertilizer-N. The introduction of legumes into these pastures is seen as the best strategy to improve nitrogen nutrition of the grasses. Large contribution (between 75 and 97 kg of N ha-1 in 97 days of growth) by Stylosanthes guianensis were found (Viera-Vargas et al., 1995). 15N data suggested that over 30% of the N accumulated by the grass in mixed swards could be derived from nitrogen fixed by the associated legume (Viera-Vargas et al., 1995). Other recent studies (Mandimba, 1995) revealed that the nitrogen contribution of Arachis hypogaea to the growth of Zea mays in intercropping system is equivalent to the application of 96 kg of fertilizer-N ha-1 at a ratio of plant population densities of one maize plant to four groundnut plants.
Actinorhizal interactions (Frankia-nonlegume symbioses) are major contributors to nitrogen inputs in forests, wetlands, fields, and disturbed sites of temperate and tropical regions (Tate, 1995). These associations involve more than 160 species of angiosperms classified among six or seven orders. The contributions of fixed nitrogen to native as well as managed ecosystems by the actinorhizal symbioses are comparable to those of the more extensively studied Rhizobium-legume interactions. Typical contributions by Alnus associations are 12 to 200 kg of N ha-1 year-1, and those by Hippophae associations are 27 to 179 kg of N ha-1 year-1 (Baker and Mullin, 1992).
The above overview clearly indicates the significance of Rhizobium-legume symbioses as major contributors to natural or biological N2 fixation.
1.5 Biological Nitrogen Fixation in Agroforestry
Biological nitrogen fixation is an important part of many agroforestry, sustainable agriculture, and land rehabilitation practices. Agroforestry is the cultivation of trees and crops together. Farmers have always dependent on trees for the regeneration of soil fertility. Many leguminous N2–fixing trees can grow rapidly on poor soils, due to at least partly to their ability to fix N2, and are thus chosen to form the tree component of agroforestry systems. As with any other tree species, they may be selected either because they produce a needed product-timber, feulwood or fodder- or simply because of their overall contribution to soil fertility. The use of N2 –fixing trees in agroforestry systems has shown considerable promise in terms of benefit to crop and livestock production (Kang et al., 1990; Danso et al., 1992).
The use of prunings from nodulated tree legumes has doubled cereal crop yields in Nigeria (Sanginga et al., 1986; Kang et al., 1990). In Malawi, Sesbania sesban is fast-growing multipurpose legume that nodules naturally with bradyrhizobia in soils, forms leaves that yield up to 80 kg N /ha and 6 kg P/ha after falling, and can produce 1.7t/ha of fuelwood for rural communities (Sanchez, 1995). The nutrient-supplying capacity of Sesbania sesban has therefore resulted in the development of a productive relay intercropping of maize with this species. Similarly, a doubling of maize yields was obtained in Zambia in Sesbania follows compared with continuous unfertilized maize (Kwesiga and Coe, 1994) Thus, interest has increased in exploiting both local and exotic tree legumes for timber, firewood and other agroforestry products in the world.
Tropical leguminous trees have been used as sources of fodder, fuel wood and timber for many years in Africa, Asia, Australia and the Americas (NAS, 1979). They also provide shade for plantation crops (NAS, 1977), function as support for climbing crops including vanilla (Alcenaro et al., 1973) and are important components in multiple cropping (Giller, 1992). Their ability to fix N2 in association with Rhizobium, Bradyrhizabium and Azorhizobium bacteria means they can meet their N requirements directly from symbiosis. Additionally, they can improve the N fertility of soils in which they grow through the release of symbiotic N from decomposing organic residues. Consequently, tree legumes play a vital role in rehabilitation of degraded and marginal soils and restoration of nutrient fertility in fields exhausted by intensive cultivation. With the ever-increasing population in Asia, the need to exploit these tree legumes for sustained productivity of soil and increased crop yields has also increased (Dakora and Keya, 1996).
BNF is an efficient source of fixed N2, which play an important role in land remediation. Interest in BNF has focused on the symbiotic systems of leguminous plants and rhizobia, because these associations have the greatest quantitative impact on the nitrogen cycle. Deficiency in mineral N often limits plant growth and so symbiotic relationships have evolved between plants and a variety of N2-fixing organisms (Freiberg et al., 1997). The symbiotically fixed N2 by the association between Rhizobium species and the legumes represents a renewable source of nitrogen for agroforestry. Values estimated for various legume crops and pasture species are often impressive, commonly falling in the range 200-300 kg N ha-1 per year (Peoples et al., 1995). This underlines the significance of Rhizobium-legume symbioses as a major contributor to BNF (Zahran, 1999). In addition to crops legumes; the nodulated wild (herb and tree) legumes have potential for nitrogen fixation, reforestation and to control soil erosion (Ahmed et al., 1984). It has been reported that novel, suitable wild legume-Rhizobium associations are useful in providing a vegetational cover in degraded lands (Jha et al., 1995).
1.6 Effect of Fertilizers on BNF and Seedling Growth
Legumes have complex association with the bacteria Rhizobium, which is affected by environmental factors including temperature, light, moisture, PH and fertilization (nutrient supply). The high global population and the corresponding need for agroforestry products have stimulated fertilizer production and application, especially nitrogen and phosphorus fertilizers.
Fertilizers have important effect on nodulation, N2 fixation and seedling growth. Nitrogen fixation should reduce or eliminate when the soils have high supplies of ammonium and nitrate (Hardarson, et al. 1991; MacDicken, 1994). Probably the most common limiting nutrient in many areas is phosphorus (P). Legumes require large amounts of P than any other plant and its effects are complex acting on nodulation, nitrogen fixation and plant growth. Nitrogen fixing plants have a high demand for phosphorus and this is also applicable for tropical nitrogen fixing trees. Biological nitrogen fixation studies, especially on fundamental process have been restricted to a limited number of agriculturally important annual crops like peas and beans (Akkermans and Houwers, 1983). However, recently there has been increasing interest in leguminous trees and shrubs because of their promising economic importance in the tropical countries (NAS, 1977; 1980). Many studies were done on this aspect in case of agroforestry crops in our country ( Prasad and Ram, 1986; Islam et al., 1987; Bhattacharyya and Mukherjee, 1988; Bhuiya et al., 1989; Solaiman and Habibullah, 1990; Gupta et al., 1993; Khanam et al., 1994; Rahman et al., 1994; Solaiman, 1999 etc.). No studies were made on this aspect in case of agroforestry trees in our country. Therefore, the present study will report the effect of fertilizers on nodulation, N2-fixation and seedling growth of several potentially useful agroforestry tree species in Bangladesh.
1.7 Objectives of the study:
1. To assess the competition and changes in the nodule forming abilities of potential legume agroforestry trees,
2. To examine the effect of fertilizers on nodule distribution on roots,
3. To explore and discuss the possibilities for enhancing N2 fixation by working on the agroforestry plant host.
4. To compare the nodulation and N2 fixing capacity of different agroforestry trees and to select the best one for getting maximum sustainable output,
5. To find out the type and optimum doze of fertilizer for maximizing biological nitrogen fixation,
6. To assess the quality index (root/shoot ratio, growth vigor, morphological superiority and biomass production) of seedlings raised in different fertilizer treatment,
7. To compare the biological nitrogen fixation ability/performance of seedlings raised in the nursery, and
8. To explore the nodulation and nitrogen fixing ability of some legumes in in-situ.
1.1 The Nitrogen
Nitrogen is commonly the most limiting element in agricultural production, and one of the most expensive to purchase as fertilizer (NifTAL, 1984). It is required by all living organisms for the synthesis of proteins, nucleic acids and other nitrogen-containing compounds (Anon, 2002). There is abundant supply of nitrogen in the air (the air is 80% nitrogen gas, amounting to about 8000 pounds of nitrogen in the air over every acre of land, or 6400 kilograms above every hectare). However, the nitrogen in the air is a stable gas, normally unavailable to plants until it has been fixed, that is reduced (combined with hydrogen), to ammonia. Green plants, the main producer of organic matter, use this supply of fixed nitrogen to make proteins that enter and pass through the food chain. Microorganisms (the decomposers) break down the proteins in excretions and dead organisms, releasing ammonium ions. These two processes form part of the nitrogen cycle. Many leguminous plants are able to utilize this atmospheric nitrogen through an association with rhizobia, bacteria that are hosted by the system of certain nitrogen fixing plants. Nitrogen can be fixed in three ways namely Atmospheric, Industrial and Biological fixation.
1.2 The Nitrogen Fixation Process
The element nitrogen, or “azote”, meaning “without life”, as Antonie Lavoisier called it about 200 years ago, has proved be anything but lifeless, since it is a component of food, poisons, fertilizers, and explosives (Schoot Uiterkamp, 1990). The atmosphere contains about 1015 tonnes of N2 gas, and the nitrogen cycle involves the transformation of some 3x 109 tonnes of N2 per year on a global basis (Postgate, 1982). However, transformations (e.g., N2 fixation) are not exclusively biological. Lightening probably accounts for about 10% of the world’s supply of fixed nitrogen (Sprent and Sprent, 1990). The fertilizer industry also provides very important quantities of chemically fixed nitrogen. World production of fixed nitrogen from dinitrogen for chemical fertilizer accounts for about 25% of the earth’s newly fixed N2, and biological processes account for about 60%. Globally the consumption of fertilizer-N increased from 8 to 17 kg/ha of agricultural land in the 15-year period from 1973 to 1988 (FAO, 1990). Significant growth in fertilizer-N usage has occurred in both developed and developing countries (Peoples et al., 1995). The requirements for fertilize-N are predicted to increase further in the future (Subba-Rao, 1980); however, with the current technology for fertilizer production and the inefficient methods employed for fertilizer application, both the economic and ecological costs of fertilizer usage will eventually become prohibitive.
For more than 100 years, biological nitrogen fixation (BNF) has commanded the attention of scientists concerned with plant mineral nutrition, and it has been exploited extensively in agricultural practice (Dixon and Wheeler, 1986; Burris, 1994). However, its importance as a primary source of N for agriculture has diminished in recent decades as increasing amounts of fertilizer-N have been used for the production of food and cash crops (Peoples et al., 1995). However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus attention on the potential role of BNF in supplying N for agriculture (Dixon and Wheeler, 1986; Peoples et al., 1995). The expanded interest in ecology has drawn attention to the fact that BNF is ecologically benign and that its greater exploitation can reduce the use of fossil fuels and can be helpful in reforestation and in restoration of misused lands to productivity (Burris, 1994; Sprent and Sprent, 1990).
Currently, the subject of BNF is of great practical importance because the use of nitrogenous fertilizers has resulted in unacceptable levels of water pollution (increasing concentrations of toxic nitrates in drinking water supplies) and the utrophication of lakes and rivers (Dixon and Wheeler, 1986; Sprent and Sprent, 1990 and Al-Sherif, 1998). Further, while BNF may be tailored to the needs of the organism, fertilizer is usually applied in a few large doses, up to 50% of which may be leached (Sprent and Sprent, 1990). This not only wastes energy and money but also leads to serious pollution problems, particularly in water supplies.
1.3 Biological Nitrogen Fixation
Biological nitrogen fixation (BNF) is an essential natural process that supports life on this planet. BNF is the process that changes inert N2 to biologically useful NH3. This process is mediated in nature mainly by bacteria. Other plants benefit from nitrogen fixing bacteria when bacteria die and release nitrogen to the environment, or when the bacteria live in close association with the plant. In legumes and a few other plants, the bacteria form small growth on the roots called nodules. Within these nodules, nitrogen fixation is done in association with bacteria and the NH3 is uptaken by the plant. Nitrogen fixation by legumes is a partnership between a bacteria and a plant.
1.4 Significance of Biological Nitrogen Fixation to Soil Fertility
BNF is an efficient source of nitrogen (Peoples et al., 1995)). The total annual terrestrial inputs of N from BNF as given by Burns and Hardy (1975) and Paul (1988) range from 139 million to 175 million tones of N, with symbiotic associations growing in arable land accounting for 25 to 30% (35 million to 44 million tons of N) and permanent pasture accounting for another 30% (45 million tons of N). While the accuracy of these figures may be open to question (Sprent and Sprent, 1990), they do help illustrate the relative importance of BNF in cropping and pasture systems and the magnitude of the task necessary if BNF is to be improved to replace a proportion of the 80 to 90 million tones of fertilizer-N expected to be applied annually to agricultural land by the end of the decade (Peoples et al., 1995; Peoples et al., 1995). Much land has been degraded worldwide, and it is time to stop the destructive uses of land and to institute a serious reversal of land degradation (Burris, 1994). BNF can play a key role in land remediation.
An examination of the history of BNF shows that interest generally has focused on the symbiotic system of leguminous plants and rhizobia, because these associations have the greatest quantitative impact on the nitrogen cycle. A tremendous potential for contribution of fixed nitrogen to soil ecosystems exists among the legumes (Brockwell et al., 1995; Peoples et al., 1995 and Tate, 1995). There are approximately 700 genera and about 13000 species of legumes, only a portion of which (about 20%[Sprent and Sprent, 1990]) have been examined for nodulation and shown to have the ability to fix N2. Estimates are that the rhizobial symbioses with the somewhat greater than 100 agriculturally important legumes contribute nearly half the annual quantity of BNF entering soil ecosystems (Tate, 1995). Legumes are very important both ecologically and agriculturally because they are responsible for a substantial part of the global flux of nitrogen from atmospheric N2 to fixed forms such as ammonia, nitrate, and organic nitrogen. Whatever the true figure, legumes symbioses contribute at least 70 million tons N per year, approximately half deriving from the cool and warm temperature zones and the remainder deriving from the tropics (Brockwell et al., 1995). Increased plant protein levels and reduced depletion of soil N reserves are obvious consequences of legume N2 fixation. Deficiency in mineral nitrogen often limits plant growth, and so symbiotic relationships have evolved between plants and a variety of nitrogen-fixing organisms (Freiberg et al., 1997).
Most of the attention in this thesis is directed toward N2 fixation inputs by legumes because of their proven ability to fix N2 and their contribution to integral agricultural production systems in both tropical and temperate climates (Peoples et al., 1995). Successful Rhizobium-legume symbioses will definitely increase the incorporation of BNF in to soil ecosystems. Rhizobium-legume symbioses are the primary source of fixed nitrogen in land-based systems (Tate, 1995) and can provide well over half of the biological source of fixed nitrogen (Tate, 1995).
Atmospheric N2 fixed symbiotically by the association between Rhizobium species and legumes represents a renewable source of N for agriculture (Peoples et al., 1995). Values estimated for various legume crops and pasture species are often impressive, commonly falling in the range of 200 to 300 Kg of N ha-1 year-1 (Peoples et al., 1995). Yield increases of crops planted after harvested of legumes are often equivalent to those expected from application of 30to 80 kg of fertilizer-N ha-1. Inputs of fixed N for alfalfa, red clover, pea, soybean, cowpea, and vetch were estimated to be about 65 to 335 kg of N ha-1 year-1 (Tate, 1995) or 23 to 300 kg of N ha-1 year-1 (Wani et al., 1995). However, the measured amount of N fixed by symbiotic systems may differ according to the method used to study N2 fixation (Sellstedt, 1993). Inputs into terrestrial ecosystems of BNF from the symbiotic relationship between legumes and their rhizobia amount to at least 70 million tones of N per year (Brockwell et al., 1995); this enormous quantity will have to be augmented as the world’s population increases and as the natural resources that supply fertilizer-N diminish. This objective will be achieved through the development of superior legume varieties, improvements in agronomic practice, and increased efficiency of the nitrogen-fixing process itself by better management of the symbiotic relationship between plants and bacteria.
The symbioses between Rhizobium or Bradyrhizobium and legumes are a cheaper and usually more effective agronomic practice for ensuring and adequate supply of N for legume-based crop and pasture production than the application of fertilizer-N. The introduction of legumes into these pastures is seen as the best strategy to improve nitrogen nutrition of the grasses. Large contribution (between 75 and 97 kg of N ha-1 in 97 days of growth) by Stylosanthes guianensis were found (Viera-Vargas et al., 1995). 15N data suggested that over 30% of the N accumulated by the grass in mixed swards could be derived from nitrogen fixed by the associated legume (Viera-Vargas et al., 1995). Other recent studies (Mandimba, 1995) revealed that the nitrogen contribution of Arachis hypogaea to the growth of Zea mays in intercropping system is equivalent to the application of 96 kg of fertilizer-N ha-1 at a ratio of plant population densities of one maize plant to four groundnut plants.
Actinorhizal interactions (Frankia-nonlegume symbioses) are major contributors to nitrogen inputs in forests, wetlands, fields, and disturbed sites of temperate and tropical regions (Tate, 1995). These associations involve more than 160 species of angiosperms classified among six or seven orders. The contributions of fixed nitrogen to native as well as managed ecosystems by the actinorhizal symbioses are comparable to those of the more extensively studied Rhizobium-legume interactions. Typical contributions by Alnus associations are 12 to 200 kg of N ha-1 year-1, and those by Hippophae associations are 27 to 179 kg of N ha-1 year-1 (Baker and Mullin, 1992).
The above overview clearly indicates the significance of Rhizobium-legume symbioses as major contributors to natural or biological N2 fixation.
1.5 Biological Nitrogen Fixation in Agroforestry
Biological nitrogen fixation is an important part of many agroforestry, sustainable agriculture, and land rehabilitation practices. Agroforestry is the cultivation of trees and crops together. Farmers have always dependent on trees for the regeneration of soil fertility. Many leguminous N2–fixing trees can grow rapidly on poor soils, due to at least partly to their ability to fix N2, and are thus chosen to form the tree component of agroforestry systems. As with any other tree species, they may be selected either because they produce a needed product-timber, feulwood or fodder- or simply because of their overall contribution to soil fertility. The use of N2 –fixing trees in agroforestry systems has shown considerable promise in terms of benefit to crop and livestock production (Kang et al., 1990; Danso et al., 1992).
The use of prunings from nodulated tree legumes has doubled cereal crop yields in Nigeria (Sanginga et al., 1986; Kang et al., 1990). In Malawi, Sesbania sesban is fast-growing multipurpose legume that nodules naturally with bradyrhizobia in soils, forms leaves that yield up to 80 kg N /ha and 6 kg P/ha after falling, and can produce 1.7t/ha of fuelwood for rural communities (Sanchez, 1995). The nutrient-supplying capacity of Sesbania sesban has therefore resulted in the development of a productive relay intercropping of maize with this species. Similarly, a doubling of maize yields was obtained in Zambia in Sesbania follows compared with continuous unfertilized maize (Kwesiga and Coe, 1994) Thus, interest has increased in exploiting both local and exotic tree legumes for timber, firewood and other agroforestry products in the world.
Tropical leguminous trees have been used as sources of fodder, fuel wood and timber for many years in Africa, Asia, Australia and the Americas (NAS, 1979). They also provide shade for plantation crops (NAS, 1977), function as support for climbing crops including vanilla (Alcenaro et al., 1973) and are important components in multiple cropping (Giller, 1992). Their ability to fix N2 in association with Rhizobium, Bradyrhizabium and Azorhizobium bacteria means they can meet their N requirements directly from symbiosis. Additionally, they can improve the N fertility of soils in which they grow through the release of symbiotic N from decomposing organic residues. Consequently, tree legumes play a vital role in rehabilitation of degraded and marginal soils and restoration of nutrient fertility in fields exhausted by intensive cultivation. With the ever-increasing population in Asia, the need to exploit these tree legumes for sustained productivity of soil and increased crop yields has also increased (Dakora and Keya, 1996).
BNF is an efficient source of fixed N2, which play an important role in land remediation. Interest in BNF has focused on the symbiotic systems of leguminous plants and rhizobia, because these associations have the greatest quantitative impact on the nitrogen cycle. Deficiency in mineral N often limits plant growth and so symbiotic relationships have evolved between plants and a variety of N2-fixing organisms (Freiberg et al., 1997). The symbiotically fixed N2 by the association between Rhizobium species and the legumes represents a renewable source of nitrogen for agroforestry. Values estimated for various legume crops and pasture species are often impressive, commonly falling in the range 200-300 kg N ha-1 per year (Peoples et al., 1995). This underlines the significance of Rhizobium-legume symbioses as a major contributor to BNF (Zahran, 1999). In addition to crops legumes; the nodulated wild (herb and tree) legumes have potential for nitrogen fixation, reforestation and to control soil erosion (Ahmed et al., 1984). It has been reported that novel, suitable wild legume-Rhizobium associations are useful in providing a vegetational cover in degraded lands (Jha et al., 1995).
1.6 Effect of Fertilizers on BNF and Seedling Growth
Legumes have complex association with the bacteria Rhizobium, which is affected by environmental factors including temperature, light, moisture, PH and fertilization (nutrient supply). The high global population and the corresponding need for agroforestry products have stimulated fertilizer production and application, especially nitrogen and phosphorus fertilizers.
Fertilizers have important effect on nodulation, N2 fixation and seedling growth. Nitrogen fixation should reduce or eliminate when the soils have high supplies of ammonium and nitrate (Hardarson, et al. 1991; MacDicken, 1994). Probably the most common limiting nutrient in many areas is phosphorus (P). Legumes require large amounts of P than any other plant and its effects are complex acting on nodulation, nitrogen fixation and plant growth. Nitrogen fixing plants have a high demand for phosphorus and this is also applicable for tropical nitrogen fixing trees. Biological nitrogen fixation studies, especially on fundamental process have been restricted to a limited number of agriculturally important annual crops like peas and beans (Akkermans and Houwers, 1983). However, recently there has been increasing interest in leguminous trees and shrubs because of their promising economic importance in the tropical countries (NAS, 1977; 1980). Many studies were done on this aspect in case of agroforestry crops in our country ( Prasad and Ram, 1986; Islam et al., 1987; Bhattacharyya and Mukherjee, 1988; Bhuiya et al., 1989; Solaiman and Habibullah, 1990; Gupta et al., 1993; Khanam et al., 1994; Rahman et al., 1994; Solaiman, 1999 etc.). No studies were made on this aspect in case of agroforestry trees in our country. Therefore, the present study will report the effect of fertilizers on nodulation, N2-fixation and seedling growth of several potentially useful agroforestry tree species in Bangladesh.
1.7 Objectives of the study:
1. To assess the competition and changes in the nodule forming abilities of potential legume agroforestry trees,
2. To examine the effect of fertilizers on nodule distribution on roots,
3. To explore and discuss the possibilities for enhancing N2 fixation by working on the agroforestry plant host.
4. To compare the nodulation and N2 fixing capacity of different agroforestry trees and to select the best one for getting maximum sustainable output,
5. To find out the type and optimum doze of fertilizer for maximizing biological nitrogen fixation,
6. To assess the quality index (root/shoot ratio, growth vigor, morphological superiority and biomass production) of seedlings raised in different fertilizer treatment,
7. To compare the biological nitrogen fixation ability/performance of seedlings raised in the nursery, and
8. To explore the nodulation and nitrogen fixing ability of some legumes in in-situ.
