+ PA15






Measurements were made 60 days post-inoculation of plants. Reproduced from Kennedy et al. (2000) by permission of the publisher.

Values followed by different superscript letters are statistically significantly different at p < 0.05.

phototrophic bacteria

Because BNF is an energy-expensive process, it is not surprising that photosyn-thetic microorganisms are major suppliers of newly fixed N in certain soil ecosystems. In rice paddies, for example, cyanobacteria and other photosynthetic bacteria provide substantial inputs of N because of the flooded soil conditions early in the growing season, the presence of abundant light until canopy closure, adequate phosphate fertilization, and the low O2 conditions found at the sediment-water interface. There is a long history of BNF in rice production in Asia through use of the water fern Azolla and its cyanobacterial microsymbiont, Anabaena azollae, either as a green manure or in coculture with the rice crop. N inputs of >100 kg N ha' year''1

have been recorded (Wagner, 1997). In sharp contrast to the tropical rice paddy ecosystem, biological soil crusts are highly specialized photosynthetic communities of cyanobacteria, algae, lichens, and mosses that are commonly found in arid and semiarid environments throughout the world (Belnap and Lange, 2003). Because they are concentrated in the top few millimeters of soil, biological crusts contribute to soil stability and water infiltration as well as soil N status. Estimates of annual BNF rates in biological soil crusts have ranged widely from 1 to 350 kg N ha-1 year-1, but generally fall in the range of 10 kg N ha-1 year-1. Because of rapid wetting and drying, the N2 fixed tends to be released quickly and provides immediate benefit to the vascular plants in the surrounding ecosystem.

symbiotic n2-fixing associations between legumes and rhizobia

Most terrestrial N2-fixing symbioses involve a N2-fixing prokaryote and a pho-tosynthetic host. Because the prokaryote gains its energy from the photosynthetic host, the energy cost of BNF is compensated adequately, and considerable amounts of N can be fixed if other factors are not limiting (Table 14.5). In agroe-cosystems, an intensively managed perennial legume, such as alfalfa (Medicago sativa), can fix several hundred kilograms of N per hectare per year. Legumes are the most widely recognized N2-fixing symbioses because of their importance as a food source. In 1886, Hellreigel and Willfarth demonstrated the ability of legumes to convert N2 into organic N. Quickly thereafter, Beijerinck in 1888 isolated bacteria (rhizobia) from legume root nodules and showed they were able to reinfect the legume, form nodules, and fix N2 in the symbiosis. Studies carried out in the early part of the 20th century illustrated that rhizobia recovered from root nodules of different legume species expressed different phenotypic characteristics and expressed plant host specificity. Today we recognize that rhizobia fall into several genera and species within the Alphaproteobacteria and are currently subdivided into six genera and >20 species (Table 14.7).

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