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Soil Microbiology
In this revised edition, the chapters on pesticides and biotechnology in agriculture have been redone and brought up to date. Recent developments in other areas have also been incorporated.
424 pages, Paperback
First published January 1, 1999
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A Darwinian perspective on improving nitrogen-fixation efficiency of legume crops and forages
R. Ford Denison, in Crop Physiology (Second Edition), 2015
Abstract
Symbiotic nitrogen fixation by rhizobia in root nodules of crop and forage legumes provides substantial economic and environmental benefits. Nitrogen fixation could be increased in various ways, but most of these would cause a proportional increase in photosynthate costs. This might decrease rather than increase yields, as indicated by the poor performance of crops that make extra nodules. A hypothesis explaining such failures is that past natural selection is unlikely to have missed improvements to nitrogen fixation that are both simple (i.e. arising frequently through mutation) and free of fitness-reducing trade-offs. Various plant and rhizobial mutants that indiscriminately increase resource allocation to nitrogen fixation have presumably arisen frequently, but died out because fitness costs exceeded fitness benefits. Increasing nitrogen-fixation efficiency (gN/gC) may be possible, however, via more complex genetic changes or by accepting trade-offs rejected by natural selection. Two rhizobia strains had greater efficiency in legumes that caused swelling of rhizobial bacteroids in their nodules relative to the same strains in hosts that did not cause swelling. Increasing nodule occupancy by more efficient rhizobial strains could provide major benefits, once we recognize that less efficient strains may evolve or acquire competitive traits of more efficient strains without their greater efficiency. Some legume crops and forages reduce the relative reproduction of less beneficial rhizobia in their nodules. These ‘host sanctions’ are based on actual nitrogen fixation, not easily mimicked recognition signals. Further enhancing host sanctions could lead to legumes that selectively enrich soils with only the most beneficial local rhizobia.
Read full chapter
Symbiotic Systems
T. Fenchel, ... T.H. Blackburn, in Bacterial Biogeochemistry (Third Edition), 2012
Symbiotic N2 Fixation in Legumes
This is the best known and most important type of symbiotic nitrogen fixation. Legumes (about 1700 species) belong to the Fabaceae (Leguminosae). They form root, or in a few cases stem, nodules containing N2-fixing rhizobia (Rhizobium and Braydrhizobium), most of which belong to the α-Proteobacteria. These symbionts are related to Agrobacterium, an organism that can invade plant tissue and form tumerous growth. This might explain how the rhizobium-legume relationship originally evolved. However, a small number of β-Proteobacteria has also been documented as tropical legume symbionts, so the relationship with Agrobacterium is uncertain. Among non-legumes, only a tree belonging to the elm family (genus Parasaponia) is known to form rhizobia nodules that contain Bradyrhizobium or Rhizobium symbionts.
Rhizobia occur as free-living bacteria in soils. They are relatively rare in soils in which legumes have not been grown over a period of many years, but are especially numerous in the rhizosphere (the soil surrounding roots) of legumes; presumably they are stimulated by root exudates. Under microaerobic conditions they can be induced to fix N2 to a variable degree. The extent to which they fix N2 in soils is not known, but it would seem likely that this property is adaptive under some circumstances.
Rhizobia multiply around germinating legumes. Infection and subsequent nodule formation requires adhesion to root hairs, and whether this takes place depends on the species of legume and the symbiont strain. Some strains (cross-inoculation groups) can infect several species of legumes, and some legumes can form nodules with different rhizobia strains. Adhesion depends on specific lectins produced by the host plant, and on specific polysaccharide cell coatings produced by the bacteria (Young, Johnston, 1989). Following adhesion, the root hair forms an infection thread through which the bacteria enter the roots. The bacteria then invade root cells and transform into bacteroids; they swell and become deformed in various ways, and they lose the ability to divide. These events also induce the root to form nodules that host the infected cells. Nodulation is inhibited by high ambient concentrations of combined nitrogen, acidic conditions, and low phosphate availability.
Leghemoglobin represents one of the more notable features of the rhizobia-legume symbiosis. It is a true hemoglobin, the synthesis of which depends on symbiont genes for the heme moiety and plant host genes for the protein. Leghemoglobin is responsible for the pink colour seen when mature nodules are sectioned. Leghemoglobin, which has a high affinity for O2, maintains low oxygen tensions within nodules, thus protecting the highly oxygen-sensitive nitrogenase, while at the same time supplying the symbionts with enough oxygen to maintain a high rate of aerobic metabolism. This is essential for producing the large supply of ATP needed for nitrogen fixation.
A substantial part (13–28%) of the photosynthate of legumes is supplied to the nodules (Minchin et al., 1981). This is, in part, in the form of carbohydrates serving as an energy source for N2 fixation in the rhizobia, and also as carbon skeleton for ammonia assimilation in the surrounding root cells. The plant assimilates ammonia as glutamine, other amino acids, or urea derivatives.
The impacts of the rhizobia-legume symbiosis extend far beyond the plant and its symbionts. Legumes affect the composition and population structure of bulk soil bacterial communities, in addition to promoting symbiont growth in the rhizosphere. The latter occurs even though bacteroids are incapable of growth, because some untransformed rhizobia are always present in nodules and in infection threads. It has been suggested that these cells are liberated when nodules senesce and decay, thus contributing to the maintenance of a high local population density in the rhizosphere. Other mechanisms affecting rhizobia and additional populations might also be involved. For example, legume nodules often emit significant amounts of molecular hydrogen and carbon monoxide into the rhizosphere. These gases originate from the activity of nitrogenase, and from leghemoglobin and peribacteroid membrane turnover, respectively. Some rhizobia and many other taxa can use one or both of these gases to support maintenance metabolism or even growth. Accordingly, several studies have shown an increase in hydrogen oxidizers in the legume rhizosphere. In addition, the turnover of legume nodules and biomass can increase N availability, thereby affecting patterns of organic matter decomposition and the dynamics of microbial communities.
Read full chapter
Ecologically Based Nutrient Management
Laurie E. Drinkwater, ... Louise E. Jackson, in Agricultural Systems (Second Edition), 2017
Biological N-Fixation: A Key Source of Nitrogen
Effective management of biological N fixation is central to ecologically based nutrient management. The most familiar example of symbiotic nitrogen fixation is the close association between legumes and rhizobial bacteria (Rhizobium, Mesorhizobium, Sinorhizobium, and Bradyrhizobium) although associative and free-living diazotrophs are potentially important in several monocot crops.
Legumes can be incorporated into crop rotations either intercropped with nonlegumes or in sequential (relay) rotations. A disadvantage of relay cropping is that mineralization of N may not coincide with the subsequent crop N demand. Beneficial effects of relay cropping systems include the addition of OM and mineralization of N from residual legume biomass that can support the growth of subsequent, nonlegume crops. Grain legumes, such as soybeans, are typically grown as monocultures in rotation with nonlegume grain crops, such as maize. Grain legumes are the most common type of legume in cropping systems, because they provide essential human and livestock protein sources in a form that is easily stored and transported. Grain legumes, such as soybeans, can fix up to 200 kg N/ha per year (Table 7.4). However, most of this N is exported off the farm in the protein-rich seeds, resulting in low or negative net soil N balance. Most estimates of N fixation, however, do not include root biomass, which can be 16–77% of total plant N (Table 7.5). Root biomass is difficult to measure; however, from the limited data available, it is clear that legume species can vary greatly in root-to-shoot ratios. Perennial species tend to have a higher root:shoot ratios than annual species (Antos and Halpern, 1997). This is generally supported by recent below-ground N results (Table 7.5), where perennial legumes tend to have a higher below-ground N as a percentage of total plant N (average of 43%) than the annual grain legumes (average of 32%). Environmental conditions can also influence root biomass and root architecture. Generally, plant allocation to roots increases under drought conditions. If estimates of root biomass are included, grain legumes can provide modest positive N balances, even with high grain N exports.
Table 7.4. Average and Upper Range of Biological N-Fixation Contributions to Tropical Cropping Systems
Associated Crop Average N Fixed (kg N/ha per year) Upper Range of N Fixed (kg N/ha per year)
Rice: Cyanobacteriab 30 Up to 80
Azolla: Anabaena in ricea 32 −
Sugarcane: Acetobacter c − Up to 150
Grain legumesa 77 Up to 200
Green manure legumesa 85 Up to 300
Pasture legumesa 78 Up to 250
Leguminous trees and shrubsa 150 Up to 275
a
From Giller (2001). Legume nitrogen fixation values do not include below-ground biomass and are, therefore, underestimates.
b
From Roger and Ladha (1992) As cited in Reis (2000).
c
From Boddey et al. (1995).
Table 7.5. Measured Legume Below-Ground N Biomass as a Percentage of Total Plant N
Legume Primary Use BGN as % of Total Plant N Sources
Chick pea (Cicer arietinum) Grain 29 Turpin et al. (2002)
Fava bean (Vicia faba) Grain 25 Khan et al. (2003)
Fava bean (Vicia faba) Grain 17 Mayer et al. (2003)
Field pea (Pisum sativum) Grain 17 Mayer et al. (2003)
Jack bean (Canavalia ensiformis) Green manure/forage 39 Ramos et al. (2001)
Mucuna (Mucuna aterrima) Green manure 49 Ramos et al. (2001)
Intercropping systems incorporate legumes into agroecosystems by planting legumes and nonlegumes together in close proximity in the same field. Examples of an annual intercropping system include maize−pigeon pea mixtures (Snapp et al., 2003). Legume intercrops can supply a slow, but steady supply of N for the nonlegume crop. Furthermore, intercropping can also reduce soil erosion and nutrient leaching, contribute to suppression of weeds and pathogens, and provide food and shelter for beneficial insects. To provide these benefits while increasing yields, intercrops must combine crop species that maximize complementarity and minimize competition for light, nutrients, and water. One of the major constraints to the adoption of legumes in cropping systems is the opportunity cost of taking land out of production in either space, as part of an intercrop, or in time as part of a legume relay cropping rotation. For this reason, successful adoptions are more likely when legumes serve multiple purposes of producing a net positive N balance, while still producing consumable products or livestock forage. Pigeon pea is one such example of a green manure crop that produces a high-protein vegetable product while maintaining a positive N balance (Ghosh et al., 2007).
In contrast with grain legumes, green manures are grown for the primary purpose of improving soil N fertility, and are typically incorporated into the soil at a maximal stage of biomass production. Tropical green manures, such as Canavalia, Crotalaria, and Mucuna, commonly fix over 100 kg N/ha per year, all of which is retained in the system, resulting in more positive N balances than grain legumes. Green manures as relay crops are more commonly used in temperate systems, because of lower land pressures and because they can be grown during the colder winter months when crop production is not possible. In tropical systems, relay green manures are less common due to high land pressures, limited labor supply, the ability to produce crops year-round in some regions, or the lack of water to support green manure growth during the dry seasons between cropping seasons. Intercropping of green manure crops to supply N to a simultaneously growing cash crop have been adopted in some systems. The aquatic fern, Azolla, and its symbiotic association with the cyanobacteria Anabaena provides an example of a green manure that is used exclusively as an N source when intercropped in lowland rice systems. With 80–95% of Azolla N derived from fixation, rice−Azolla intercrops can fix approximately 30 kg N/ha (Yoneyama et al., 1987; Choudhury and Kennedy, 2004). Some constraints to more widespread adoption of Azolla are pest pressures, P limitation, and limited irrigation availability in some regions (Giller, 2001).
Farmers that have limited land, labor, and other resources are interested in “dual purpose” legumes, which have an intermediate phenology. That is, they provide a product, such as leaf, vegetable, or grain, while at the same time providing long-term benefits through residues that suppress weeds and build soil fertility (Fig. 7.14). There is a trade-off, as carbohydrate and nutrient invested in residues provides less resources for yield potential, thus residue biomass is inversely related to harvest index across legume species (see Fig. 3.12). Examples of dual purpose, low harvest index legumes include long-duration pigeon pea, forage soybean, and mucuna. Such species provide returns to farmers in the short-term—and thus the economic feasibility of adoption—while simultaneously contributing to ecosystem services.
Sign in to download full-size image
Figure 7.14. A Bolivian farmer shows off his fava bean crop. The previous potato crop failed due to unfavorable climatic conditions, leaving behind P from the manure application that is normally applied to potatoes but not to bean crops. As a result, the fava beans produced a very large biomass.
Over the long-term, dual purpose plant types contribute to resilient cropping systems. This is both through the soil building properties of high quality residues, and the inherent ability of indeterminant growth types to recover from pest epidemics. Plant breeding efforts have historically focused on producing high yield potential phenotypes. Examples include the development of new varieties of pigeon pea and cowpea that are extra-early, and extra short duration. These crops often incorporate high harvest index traits, which has had the unintended consequence of reducing biomass available for fodder, weed suppression, and soil fertility enrichment. Producing a wider range of dual purpose genotypes with intermediate phenology, and experimenting with intercrops of short and long-duration crops are approaches that require careful consideration in the future.
Alley cropping involves the use of woody or shrub perennial legumes between “alleys” of nonlegume crops. Prunings from the legumes are used as livestock forage, and/or added to the soil as a N source for the nonlegume. Inclusion of perennials in cropping systems provides important ecological benefits due to their extensive rooting systems that persist across multiple cropping seasons. Perennials can reduce soil erosion, access deeper soil pools of nutrients and water, provide critical microbial habitat between annual cropping seasons, and increase SOM. Leucaena and Gliricidia are two common leguminous alley crop species. Leucaena intercropped with sorghum increased sorghum yields by 73%, as compared to sorghum grown without N fertilizer, and yields were 43% greater than with a low rate N fertilizer application (Ghosh et al., 2007). Alley cropped legumes can fix between 200 and 300 kg N/ha per year (Giller, 2001). Some of the challenges in the adoption of alley cropping systems include the competition of the legume with the cash crop for moisture in dry years, the pruning labor required, and the use of land for a noncash crop. Selection of species that have complementary rooting systems with cash crops (i.e., a deep-rooted perennial legume cropped with a shallow-rooted annual), and species that grow at a manageable pace to supply N while not requiring excessive pruning inputs, are important considerations in the selection of legume species for alley cropping.
Lastly, while reliable data on the contributions of nonsymbiotic diazotrophs (free-living and those found in the rhizosphere) is limited, there are circumstances where it may be possible to increase N fixed by these microbes. Management practices that affect the availability of soil carbon should significantly impact the potential for BNF. For example, the retention of the carbon in straw from a wheat crop with a yield of 2 t/ha could theoretically fuel the production of 50–150 kg N/ha if utilized by diazotrophs to drive N fixation (Kennedy and Islam, 2001). In addition, crop selection and breeding can affect BNF potential, because plant species differ greatly in the quantity and quality of root exudates produced.
Read full chapter
Advances in Host Plant and Rhizobium Genomics to Enhance Symbiotic Nitrogen Fixation in Grain Legumes
Sangam L. Dwivedi, ... Rodomiro Ortiz, in Advances in Agronomy, 2015
Abstract
Legumes form symbiotic relationship with root-nodule, rhizobia. The nitrogen (N2) fixed by legumes is a renewable source and of great importance to agriculture. Symbiotic nitrogen fixation (SNF) is constrained by multiple stresses and alleviating them would improve SNF contribution to agroecosystems. Genetic differences in adaptation tolerance to various stresses are known in both host plant and rhizobium. The discovery and use of promiscuous germplasm in soybean led to the release of high-yielding cultivars in Africa. High N2-fixing soybean cultivars are commercially grown in Australia and some countries in Africa and South America and those of pea in Russia. SNF is a complex trait, governed by multigenes with varying effects. Few major quantitative trait loci (QTL) and candidate genes underlying QTL are reported in grain and model legumes. Nodulating genes in model legumes are cloned and orthologs determined in grain legumes. Single nucleotide polymorphism (SNP) markers from nodulation genes are available in common bean and soybean. Genomes of chickpea, pigeonpea, and soybean; and genomes of several rhizobium species are decoded. Expression studies revealed few genes associated with SNF in model and grain legumes. Advances in host plant and rhizobium genomics are helping identify DNA markers to aid breeding of legume cultivars with high symbiotic efficiency. A paradigm shift is needed by breeding programs to simultaneously improve host plant and rhizobium to harness the strength of positive symbiotic interactions in cultivar development. Computation models based on metabolic reconstruction pathways are providing greater insights to explore genotype–phenotype relationships in SNF. Models to simulate the response of N2 fixation to a range of environmental variables and crop growth are assisting researchers to quantify SNF for efficient and sustainable agricultural production systems. Such knowledge helps identifying bottlenecks in specific legume–rhizobia systems that could be overcome by legume breeding to enhance SNF. This review discusses the recent developments to improve SNF and productivity of grain legumes.
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Gene Editing in Plants
Longlong Wang, ... Deqiang Duanmu, in Progress in Molecular Biology and Translational Science, 2017
Abstract
Nitrogen-fixing rhizobia have established a symbiotic relationship with the legume family through more than 60 million years of evolution. Hundreds of legume host genes are invo
A Darwinian perspective on improving nitrogen-fixation efficiency of legume crops and forages
R. Ford Denison, in Crop Physiology (Second Edition), 2015
Abstract
Symbiotic nitrogen fixation by rhizobia in root nodules of crop and forage legumes provides substantial economic and environmental benefits. Nitrogen fixation could be increased in various ways, but most of these would cause a proportional increase in photosynthate costs. This might decrease rather than increase yields, as indicated by the poor performance of crops that make extra nodules. A hypothesis explaining such failures is that past natural selection is unlikely to have missed improvements to nitrogen fixation that are both simple (i.e. arising frequently through mutation) and free of fitness-reducing trade-offs. Various plant and rhizobial mutants that indiscriminately increase resource allocation to nitrogen fixation have presumably arisen frequently, but died out because fitness costs exceeded fitness benefits. Increasing nitrogen-fixation efficiency (gN/gC) may be possible, however, via more complex genetic changes or by accepting trade-offs rejected by natural selection. Two rhizobia strains had greater efficiency in legumes that caused swelling of rhizobial bacteroids in their nodules relative to the same strains in hosts that did not cause swelling. Increasing nodule occupancy by more efficient rhizobial strains could provide major benefits, once we recognize that less efficient strains may evolve or acquire competitive traits of more efficient strains without their greater efficiency. Some legume crops and forages reduce the relative reproduction of less beneficial rhizobia in their nodules. These ‘host sanctions’ are based on actual nitrogen fixation, not easily mimicked recognition signals. Further enhancing host sanctions could lead to legumes that selectively enrich soils with only the most beneficial local rhizobia.
Read full chapter
Symbiotic Systems
T. Fenchel, ... T.H. Blackburn, in Bacterial Biogeochemistry (Third Edition), 2012
Symbiotic N2 Fixation in Legumes
This is the best known and most important type of symbiotic nitrogen fixation. Legumes (about 1700 species) belong to the Fabaceae (Leguminosae). They form root, or in a few cases stem, nodules containing N2-fixing rhizobia (Rhizobium and Braydrhizobium), most of which belong to the α-Proteobacteria. These symbionts are related to Agrobacterium, an organism that can invade plant tissue and form tumerous growth. This might explain how the rhizobium-legume relationship originally evolved. However, a small number of β-Proteobacteria has also been documented as tropical legume symbionts, so the relationship with Agrobacterium is uncertain. Among non-legumes, only a tree belonging to the elm family (genus Parasaponia) is known to form rhizobia nodules that contain Bradyrhizobium or Rhizobium symbionts.
Rhizobia occur as free-living bacteria in soils. They are relatively rare in soils in which legumes have not been grown over a period of many years, but are especially numerous in the rhizosphere (the soil surrounding roots) of legumes; presumably they are stimulated by root exudates. Under microaerobic conditions they can be induced to fix N2 to a variable degree. The extent to which they fix N2 in soils is not known, but it would seem likely that this property is adaptive under some circumstances.
Rhizobia multiply around germinating legumes. Infection and subsequent nodule formation requires adhesion to root hairs, and whether this takes place depends on the species of legume and the symbiont strain. Some strains (cross-inoculation groups) can infect several species of legumes, and some legumes can form nodules with different rhizobia strains. Adhesion depends on specific lectins produced by the host plant, and on specific polysaccharide cell coatings produced by the bacteria (Young, Johnston, 1989). Following adhesion, the root hair forms an infection thread through which the bacteria enter the roots. The bacteria then invade root cells and transform into bacteroids; they swell and become deformed in various ways, and they lose the ability to divide. These events also induce the root to form nodules that host the infected cells. Nodulation is inhibited by high ambient concentrations of combined nitrogen, acidic conditions, and low phosphate availability.
Leghemoglobin represents one of the more notable features of the rhizobia-legume symbiosis. It is a true hemoglobin, the synthesis of which depends on symbiont genes for the heme moiety and plant host genes for the protein. Leghemoglobin is responsible for the pink colour seen when mature nodules are sectioned. Leghemoglobin, which has a high affinity for O2, maintains low oxygen tensions within nodules, thus protecting the highly oxygen-sensitive nitrogenase, while at the same time supplying the symbionts with enough oxygen to maintain a high rate of aerobic metabolism. This is essential for producing the large supply of ATP needed for nitrogen fixation.
A substantial part (13–28%) of the photosynthate of legumes is supplied to the nodules (Minchin et al., 1981). This is, in part, in the form of carbohydrates serving as an energy source for N2 fixation in the rhizobia, and also as carbon skeleton for ammonia assimilation in the surrounding root cells. The plant assimilates ammonia as glutamine, other amino acids, or urea derivatives.
The impacts of the rhizobia-legume symbiosis extend far beyond the plant and its symbionts. Legumes affect the composition and population structure of bulk soil bacterial communities, in addition to promoting symbiont growth in the rhizosphere. The latter occurs even though bacteroids are incapable of growth, because some untransformed rhizobia are always present in nodules and in infection threads. It has been suggested that these cells are liberated when nodules senesce and decay, thus contributing to the maintenance of a high local population density in the rhizosphere. Other mechanisms affecting rhizobia and additional populations might also be involved. For example, legume nodules often emit significant amounts of molecular hydrogen and carbon monoxide into the rhizosphere. These gases originate from the activity of nitrogenase, and from leghemoglobin and peribacteroid membrane turnover, respectively. Some rhizobia and many other taxa can use one or both of these gases to support maintenance metabolism or even growth. Accordingly, several studies have shown an increase in hydrogen oxidizers in the legume rhizosphere. In addition, the turnover of legume nodules and biomass can increase N availability, thereby affecting patterns of organic matter decomposition and the dynamics of microbial communities.
Read full chapter
Ecologically Based Nutrient Management
Laurie E. Drinkwater, ... Louise E. Jackson, in Agricultural Systems (Second Edition), 2017
Biological N-Fixation: A Key Source of Nitrogen
Effective management of biological N fixation is central to ecologically based nutrient management. The most familiar example of symbiotic nitrogen fixation is the close association between legumes and rhizobial bacteria (Rhizobium, Mesorhizobium, Sinorhizobium, and Bradyrhizobium) although associative and free-living diazotrophs are potentially important in several monocot crops.
Legumes can be incorporated into crop rotations either intercropped with nonlegumes or in sequential (relay) rotations. A disadvantage of relay cropping is that mineralization of N may not coincide with the subsequent crop N demand. Beneficial effects of relay cropping systems include the addition of OM and mineralization of N from residual legume biomass that can support the growth of subsequent, nonlegume crops. Grain legumes, such as soybeans, are typically grown as monocultures in rotation with nonlegume grain crops, such as maize. Grain legumes are the most common type of legume in cropping systems, because they provide essential human and livestock protein sources in a form that is easily stored and transported. Grain legumes, such as soybeans, can fix up to 200 kg N/ha per year (Table 7.4). However, most of this N is exported off the farm in the protein-rich seeds, resulting in low or negative net soil N balance. Most estimates of N fixation, however, do not include root biomass, which can be 16–77% of total plant N (Table 7.5). Root biomass is difficult to measure; however, from the limited data available, it is clear that legume species can vary greatly in root-to-shoot ratios. Perennial species tend to have a higher root:shoot ratios than annual species (Antos and Halpern, 1997). This is generally supported by recent below-ground N results (Table 7.5), where perennial legumes tend to have a higher below-ground N as a percentage of total plant N (average of 43%) than the annual grain legumes (average of 32%). Environmental conditions can also influence root biomass and root architecture. Generally, plant allocation to roots increases under drought conditions. If estimates of root biomass are included, grain legumes can provide modest positive N balances, even with high grain N exports.
Table 7.4. Average and Upper Range of Biological N-Fixation Contributions to Tropical Cropping Systems
Associated Crop Average N Fixed (kg N/ha per year) Upper Range of N Fixed (kg N/ha per year)
Rice: Cyanobacteriab 30 Up to 80
Azolla: Anabaena in ricea 32 −
Sugarcane: Acetobacter c − Up to 150
Grain legumesa 77 Up to 200
Green manure legumesa 85 Up to 300
Pasture legumesa 78 Up to 250
Leguminous trees and shrubsa 150 Up to 275
a
From Giller (2001). Legume nitrogen fixation values do not include below-ground biomass and are, therefore, underestimates.
b
From Roger and Ladha (1992) As cited in Reis (2000).
c
From Boddey et al. (1995).
Table 7.5. Measured Legume Below-Ground N Biomass as a Percentage of Total Plant N
Legume Primary Use BGN as % of Total Plant N Sources
Chick pea (Cicer arietinum) Grain 29 Turpin et al. (2002)
Fava bean (Vicia faba) Grain 25 Khan et al. (2003)
Fava bean (Vicia faba) Grain 17 Mayer et al. (2003)
Field pea (Pisum sativum) Grain 17 Mayer et al. (2003)
Jack bean (Canavalia ensiformis) Green manure/forage 39 Ramos et al. (2001)
Mucuna (Mucuna aterrima) Green manure 49 Ramos et al. (2001)
Intercropping systems incorporate legumes into agroecosystems by planting legumes and nonlegumes together in close proximity in the same field. Examples of an annual intercropping system include maize−pigeon pea mixtures (Snapp et al., 2003). Legume intercrops can supply a slow, but steady supply of N for the nonlegume crop. Furthermore, intercropping can also reduce soil erosion and nutrient leaching, contribute to suppression of weeds and pathogens, and provide food and shelter for beneficial insects. To provide these benefits while increasing yields, intercrops must combine crop species that maximize complementarity and minimize competition for light, nutrients, and water. One of the major constraints to the adoption of legumes in cropping systems is the opportunity cost of taking land out of production in either space, as part of an intercrop, or in time as part of a legume relay cropping rotation. For this reason, successful adoptions are more likely when legumes serve multiple purposes of producing a net positive N balance, while still producing consumable products or livestock forage. Pigeon pea is one such example of a green manure crop that produces a high-protein vegetable product while maintaining a positive N balance (Ghosh et al., 2007).
In contrast with grain legumes, green manures are grown for the primary purpose of improving soil N fertility, and are typically incorporated into the soil at a maximal stage of biomass production. Tropical green manures, such as Canavalia, Crotalaria, and Mucuna, commonly fix over 100 kg N/ha per year, all of which is retained in the system, resulting in more positive N balances than grain legumes. Green manures as relay crops are more commonly used in temperate systems, because of lower land pressures and because they can be grown during the colder winter months when crop production is not possible. In tropical systems, relay green manures are less common due to high land pressures, limited labor supply, the ability to produce crops year-round in some regions, or the lack of water to support green manure growth during the dry seasons between cropping seasons. Intercropping of green manure crops to supply N to a simultaneously growing cash crop have been adopted in some systems. The aquatic fern, Azolla, and its symbiotic association with the cyanobacteria Anabaena provides an example of a green manure that is used exclusively as an N source when intercropped in lowland rice systems. With 80–95% of Azolla N derived from fixation, rice−Azolla intercrops can fix approximately 30 kg N/ha (Yoneyama et al., 1987; Choudhury and Kennedy, 2004). Some constraints to more widespread adoption of Azolla are pest pressures, P limitation, and limited irrigation availability in some regions (Giller, 2001).
Farmers that have limited land, labor, and other resources are interested in “dual purpose” legumes, which have an intermediate phenology. That is, they provide a product, such as leaf, vegetable, or grain, while at the same time providing long-term benefits through residues that suppress weeds and build soil fertility (Fig. 7.14). There is a trade-off, as carbohydrate and nutrient invested in residues provides less resources for yield potential, thus residue biomass is inversely related to harvest index across legume species (see Fig. 3.12). Examples of dual purpose, low harvest index legumes include long-duration pigeon pea, forage soybean, and mucuna. Such species provide returns to farmers in the short-term—and thus the economic feasibility of adoption—while simultaneously contributing to ecosystem services.
Sign in to download full-size image
Figure 7.14. A Bolivian farmer shows off his fava bean crop. The previous potato crop failed due to unfavorable climatic conditions, leaving behind P from the manure application that is normally applied to potatoes but not to bean crops. As a result, the fava beans produced a very large biomass.
Over the long-term, dual purpose plant types contribute to resilient cropping systems. This is both through the soil building properties of high quality residues, and the inherent ability of indeterminant growth types to recover from pest epidemics. Plant breeding efforts have historically focused on producing high yield potential phenotypes. Examples include the development of new varieties of pigeon pea and cowpea that are extra-early, and extra short duration. These crops often incorporate high harvest index traits, which has had the unintended consequence of reducing biomass available for fodder, weed suppression, and soil fertility enrichment. Producing a wider range of dual purpose genotypes with intermediate phenology, and experimenting with intercrops of short and long-duration crops are approaches that require careful consideration in the future.
Alley cropping involves the use of woody or shrub perennial legumes between “alleys” of nonlegume crops. Prunings from the legumes are used as livestock forage, and/or added to the soil as a N source for the nonlegume. Inclusion of perennials in cropping systems provides important ecological benefits due to their extensive rooting systems that persist across multiple cropping seasons. Perennials can reduce soil erosion, access deeper soil pools of nutrients and water, provide critical microbial habitat between annual cropping seasons, and increase SOM. Leucaena and Gliricidia are two common leguminous alley crop species. Leucaena intercropped with sorghum increased sorghum yields by 73%, as compared to sorghum grown without N fertilizer, and yields were 43% greater than with a low rate N fertilizer application (Ghosh et al., 2007). Alley cropped legumes can fix between 200 and 300 kg N/ha per year (Giller, 2001). Some of the challenges in the adoption of alley cropping systems include the competition of the legume with the cash crop for moisture in dry years, the pruning labor required, and the use of land for a noncash crop. Selection of species that have complementary rooting systems with cash crops (i.e., a deep-rooted perennial legume cropped with a shallow-rooted annual), and species that grow at a manageable pace to supply N while not requiring excessive pruning inputs, are important considerations in the selection of legume species for alley cropping.
Lastly, while reliable data on the contributions of nonsymbiotic diazotrophs (free-living and those found in the rhizosphere) is limited, there are circumstances where it may be possible to increase N fixed by these microbes. Management practices that affect the availability of soil carbon should significantly impact the potential for BNF. For example, the retention of the carbon in straw from a wheat crop with a yield of 2 t/ha could theoretically fuel the production of 50–150 kg N/ha if utilized by diazotrophs to drive N fixation (Kennedy and Islam, 2001). In addition, crop selection and breeding can affect BNF potential, because plant species differ greatly in the quantity and quality of root exudates produced.
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Advances in Host Plant and Rhizobium Genomics to Enhance Symbiotic Nitrogen Fixation in Grain Legumes
Sangam L. Dwivedi, ... Rodomiro Ortiz, in Advances in Agronomy, 2015
Abstract
Legumes form symbiotic relationship with root-nodule, rhizobia. The nitrogen (N2) fixed by legumes is a renewable source and of great importance to agriculture. Symbiotic nitrogen fixation (SNF) is constrained by multiple stresses and alleviating them would improve SNF contribution to agroecosystems. Genetic differences in adaptation tolerance to various stresses are known in both host plant and rhizobium. The discovery and use of promiscuous germplasm in soybean led to the release of high-yielding cultivars in Africa. High N2-fixing soybean cultivars are commercially grown in Australia and some countries in Africa and South America and those of pea in Russia. SNF is a complex trait, governed by multigenes with varying effects. Few major quantitative trait loci (QTL) and candidate genes underlying QTL are reported in grain and model legumes. Nodulating genes in model legumes are cloned and orthologs determined in grain legumes. Single nucleotide polymorphism (SNP) markers from nodulation genes are available in common bean and soybean. Genomes of chickpea, pigeonpea, and soybean; and genomes of several rhizobium species are decoded. Expression studies revealed few genes associated with SNF in model and grain legumes. Advances in host plant and rhizobium genomics are helping identify DNA markers to aid breeding of legume cultivars with high symbiotic efficiency. A paradigm shift is needed by breeding programs to simultaneously improve host plant and rhizobium to harness the strength of positive symbiotic interactions in cultivar development. Computation models based on metabolic reconstruction pathways are providing greater insights to explore genotype–phenotype relationships in SNF. Models to simulate the response of N2 fixation to a range of environmental variables and crop growth are assisting researchers to quantify SNF for efficient and sustainable agricultural production systems. Such knowledge helps identifying bottlenecks in specific legume–rhizobia systems that could be overcome by legume breeding to enhance SNF. This review discusses the recent developments to improve SNF and productivity of grain legumes.
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Gene Editing in Plants
Longlong Wang, ... Deqiang Duanmu, in Progress in Molecular Biology and Translational Science, 2017
Abstract
Nitrogen-fixing rhizobia have established a symbiotic relationship with the legume family through more than 60 million years of evolution. Hundreds of legume host genes are invo
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