Nitrogen is a key element controlling the species composition, diversity, dynamics, and functioning of many terrestrial, freshwater, and marine ecosystems. Many of the original plant species living in these ecosystems are adapted to, and function optimally in, soils and solutions with low levels of available nitrogen. The growth and dynamics of herbivore populations, and ultimately those of their predators, also are affected by N. Agriculture, combustion of fossil fuels, and other human activities have altered the global cycle of N substantially, generally increasing both the availability and the mobility of N over large regions of Earth. The mobility of N means that while most deliberate applications of N occur locally, their influence spreads regionally and even globally. Moreover, many of the mobile forms of N themselves have environmental consequences. Although most nitrogen inputs serve human needs such as agricultural production, their environmental conse- quences are serious and long term. Based on our review of available scientific evidence, we are certain that human alterations of the nitrogen cycle have: 1) approximately doubled the rate of nitrogen input into the terrestrial nitrogen cycle, with these rates still increasing; 2) increased concentrations of the potent greenhouse gas N 2O globally, and increased concentrations of other oxides of nitrogen that drive the formation of photochemical smog over large regions of Earth; 3) caused losses of soil nutrients, such as calcium and potassium, that are essential for the long-term maintenance of soil fertility; 4) contributed substantially to the acidification of soils, streams, and lakes in several regions; and 5) greatly increased the transfer of nitrogen through rivers to estuaries and coastal oceans. In addition, based on our review of available scientific evidence we are confident that human alterations of the nitrogen cycle have: 6) increased the quantity of organic carbon stored within terrestrial ecosystems; 7) accelerated losses of biological diversity, especially losses of plants adapted to efficient use of nitrogen, and losses of the animals and microorganisms that depend on them; and 8) caused changes in the composition and functioning of estuarine and nearshore ecosystems, and contributed to long-term declines in coastal marine fisheries.

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Human alteration of the global nitrogen cycle: sources and consequences

This paper contrasts the natural and anthropogenic controls on the conversion of unreactive N2 to more reactive forms of nitrogen (Nr). A variety of data sets are used to construct global N budgets for 1860 and the early 1990s and to make projections for the global N budget in 2050. Regional N budgets for Asia, North America, and other major regions for the early 1990s, as well as the marine N budget, are presented to highlight the dominant fluxes of nitrogen in each region. Important findings are that human activities increasingly dominate the N budget at the global and at most regional scales, the terrestrial and open ocean N budgets are essentially dis- connected, and the fixed forms of N are accumulating in most environmental reservoirs. The largest uncertainties in our understanding of the N budget at most scales are the rates of natural biological nitrogen fixation, the amount of Nr storage in most environmental reservoirs, and the production rates of N2 by denitrification.

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Nitrogen cycles: past, present, and future

The number of prokaryotes and the total amount of their cellular carbon on earth are estimated to be 4-6 3 10 30 cells and 350-550 Pg of C (1 Pg 5 10 15 g), respectively. Thus, the total amount of prokaryotic carbon is 60-100% of the estimated total carbon in plants, and inclusion of prokaryotic carbon in global models will almost double estimates of the amount of carbon stored in living organisms. In addition, the earth's prokaryotes contain 85-130 Pg of N and 9-14 Pg of P, or about 10-fold more of these nutrients than do plants, and represent the largest pool of these nutrients in living organisms. Most of the earth's prokaryotes occur in the open ocean, in soil, and in oceanic and terrestrial subsurfaces, where the numbers of cells are 1.2 3 10 29 , 2.6 3 10 29 , 3.5 3 10 30 , and 0.25-2.5 3 10 30 , respectively. The numbers of het- erotrophic prokaryotes in the upper 200 m of the open ocean, the ocean below 200 m, and soil are consistent with average turnover times of 6-25 days, 0.8 yr, and 2.5 yr, respectively. Although subject to a great deal of uncertainty, the estimate for the average turnover time of prokaryotes in the subsurface is on the order of 1-2 3 10 3 yr. The cellular production rate for all prokaryotes on earth is estimated at 1.7 3 10 30 cellsyyr and is highest in the open ocean. The large population size and rapid growth of prokaryotes provides an enormous capacity for genetic diversity. Although invisible to the naked eye, prokaryotes are an essential component of the earth's biota. They catalyze unique and indispensable transformations in the biogeochemical cy- cles of the biosphere, produce important components of the earth's atmosphere, and represent a large portion of life's genetic diversity. Although the abundance of prokaryotes has been estimated indirectly (1, 2), the actual number of pro- karyotes and the total amount of their cellular carbon on earth have never been directly assessed. Presumably, prokaryotes' very ubiquity has discouraged investigators, because an esti- mation of the number of prokaryotes would seem to require endless cataloging of numerous habitats. To estimate the number and total carbon of prokaryotes on earth, several representative habitats were first examined. This analysis indicated that most of the prokaryotes reside in three large habitats: seawater, soil, and the sedimentysoil subsur- face. Although many other habitats contain dense populations, their numerical contribution to the total number of pro- karyotes is small. Thus, evaluating the total number and total carbon of prokaryotes on earth becomes a solvable problem. Aquatic Environments. Numerous estimates of cell density, volume, and carbon indicate that prokaryotes are ubiquitous in marine and fresh water (e.g., 3-5). Although a large range of cellular densities has been reported (10 4 -10 7 cellsyml), the

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Prokaryotes: The unseen majority

Biofilms are complex communities of microorganisms attached to surfaces or associated with interfaces. Despite the focus of modern microbiology research on pure culture, planktonic (free-swimming) bacteria, it is now widely recognized that most bacteria found in natural, clinical, and industrial settings persist in association with surfaces. Furthermore, these microbial communities are often composed of multiple species that interact with each other and their environment. The determination of biofilm architecture, particularly the spatial arrangement of microcolonies (clusters of cells) relative to one another, has profound implications for the function of these complex communities. Numerous new experimental approaches and methodologies have been developed in order to explore metabolic interactions, phylogenetic groupings, and competition among members of the biofilm. To complement this broad view of biofilm ecology, individual organisms have been studied using molecular genetics in order to identify the genes required for biofilm development and to dissect the regulatory pathways that control the plankton-to-biofilm transition. These molecular genetic studies have led to the emergence of the concept of biofilm formation as a novel system for the study of bacterial development. The recent explosion in the field of biofilm research has led to exciting progress in the development of new technologies for studying these communities, advanced our understanding of the ecological significance of surface-attached bacteria, and provided new insights into the molecular genetic basis of biofilm development.

Microbial Biofilms: from Ecology to Molecular Genetics

Phosphate solubilizing bacteria and their role in plant growth promotion

Soil Microbiology and Biochemistry in Perspective. Soil as a Habitat for Organisms and Their Reactions. Methods for Studying Soil Organisms. Components of the Soil Biota. Occurrence and Distribution of Soil Organisms. Carbon Cycling and Soil Organic Matter. Dynamics of Residue Decomposition and Soil Organic Matter Turnover. Ammonification and Nitrification. The Fate of Nitrates. Closing the Nitrogen Cycle: Return of Nitrogen to the Soil. Mycorrhizal Relationships. The Commercialization of Organisms. Phosphorus Transformations. Sulfur Transformations in Soil. Microbial Transformations of Metal. Chapter References and Suggested Reading. Subject Index.

Soil microbiology and biochemistry.

Aerobic sporeforming bacteria

Publisher Summary The role of microscopically small organisms in numerous soil processes, particularly those affecting plant productivity, is well recognized. Less well known is the quantity of recently gathered information concerning microorganisms as the components of major plant communities and the extent to which they participate in the total energy flow therein. Although the soil microflora is the single most important group in the annual turnover of energy trapped by photosynthesis, no broad general review of the microflora of grassland has heretofore been compiled. There is a rich microflora associated with the surfaces of living plants that is either nonpathogenic or at most functioning at an extremely marginal level of pathogenicity. This microflora feeds primarily on exudates and sloughed cellular material. It commonly is divided into two categories: (a) the aboveground, shoot-associated microflora, or broadly referred as the “phyllosphere” and (b) the belowground, root-associated microflora, broadly referred as the “rhizophere.” This chapter examines the microflora of grassland litter, the microflora of grassland soils, biomass and bioactivity measurements, the humic component of grassland soil, and nitrogen transformations in grassland soils.

The microflora of grassland.

Publisher Summary The diversity of interests in rhizosphere microbiology has led to a profusion of experimental techniques and to differences in the expression and interpretation of results The relationships between microorganisms and plant roots can be classified into three categories: symbiotism, parasitism, and commensalism These relationships are variable, depending upon environmental and physiological conditions Soil microorganisms have been considered capable of affecting the growth of higher plank in numerous ways They may act as general agents of decomposition of the organic constituents of the soil leading to the formation of water, carbon dioxide, ammonia, sulfates, and phosphates; act as transformers of such mineral constituents of soil as ammonia and sulfur, thereby oxidizing them to nitrate and sulfate, respectively; act as agents of the assimilation of nutrients that are thus removed at least temporarily from the zone of absorption of plants; act as agents lowering the oxygen concentration in the soil system, thereby creating conditions unfavorable to root growth; produce toxic substances or reduce such substances as nitrates and sulfates to gaseous nitrogen and sulfides; act as solvent agents through the organic and inorganic acids that are produced in various transformations; and act as nitrogen-fixing organisms, the nitrogen sooner or later becoming available to higher plants The increased microbial flora within the rhizosphere is predominantly bacterial Some fungi are also found in the rhizosphere Modification of the root surface microflora can be achieved by seed or soil inoculations, soil treatments, plant treatments, and the application of herbicides and insecticides

Francis E. Clark

Papers

Soil microbiology and biochemistry.

Aerobic sporeforming bacteria

The microflora of grassland.

Soil Microorganisms and Plant Roots

Dissimilar nitrifying capacities of soils in relation to losses of applied nitrogen.