Showing papers on "Ecosystem published in 1984"

Marine ecological processes

Abstract: Contents Preface to the Third Edition Preface to the Second Edition Preface to the First Edition Part I Primary production in marine environments Chapter 1 Primary producers in the sea 1.1 Phytoplankton 1.2 Benthic producers Chapter 2 Production: the formation of organic matter 2.1 Photosynthesis 2.2 Chemosynthesis 2.3 Measurement of producer biomass and primary production 2.4 Contributions by different marine primary producers Chapter 3 Factors affecting primary production 2.1 Light 2.2 The Uptake and Availability of Nutrients 2.3 Temperature and Interactions with Other Factors 2.4 Distribution of Phytoplankton Production Over the World Ocean Part II Consumers in Marine Environments Chapter 3 Dynamics of Populations of Consumers 3.1 Elements of the Mathematical Description of Growth of Populations 3.2 Survival Life Tables 3.3 Fecundity Life Tables 3.4 Some Properties of Life Table Variables 3.5 Reproductive Tactics Chapter 4 Competition for Resources Among Consumers 4.1 Population Growth in Environments with Finite Resources 4.2 The Nature of Competition 4.3 Density-Dependent Control of Abundance 4.4 Density-Dependent Versus Density-Independent Effects on Abundance 4.5 Resource Partitioning 4.6 Niche Breadth and Species Packing Chapter 5 Feeding and Responses to Food Abundance 5.1 Introduction 5.2 Functional Response to Prey Density 8.3 Control Mechanisms in Benthic Communities Chapter 9 Trophic Structure 2: Components and Controls in Water Column Food Webs 9.1 Food Webs in Marine Water Columns 9.2 Microbial Food Webs 9.3 The Classic Microplankton Food Web 9.4 Speculations as to Control of Prey Populations by Larger Predators in the Marine Water Column Chapter 10 Taxonomic Structure: Species Diversity 10.1 Introduction 10.2 Measurement of Diversity 10.3 Factors Affecting Diversity 10.4 Integration of Factors Affecting Diversity and Some Consequences Chapter 11 Spatial Structure: Patchiness 11.1 Scales of Patchiness 11.2 Description of Spatial Distributions 11.3 Sources of Patchiness 11.4 Ecological Consequences of Patchiness 11.5 The Problem of Upscaling Chapter 12 Development of Structure in Marine Communities: Colonization and Succession 12.1 Introduction 12.2 Colonization Processes 12.3 Case Histories of Colonization and Succession 12.4 Interaction Among Communities at Different Stages of Succession 12.5 Generalized Properties of Succession in Marine Environments Part IV Functioning of Marine Ecosystems Chapter 13 The Carbon Cycle: Production and Transformations of Organic Matter 13.1 Inorganic Carbon 13.2 The Carbon Cycle in Aerobic Environments 13.3 The Carbon Cycle in Anoxic Environments Chapter 14 Nutrient Cycles and Ecosystem Stoichiometry 14.1 Phosphorus 14.2 Nitrogen 14.3 Sulfur 14.4 Ecosystem Energetics and Stoichiometry Chapter 15 Seasonal Changes in Marine Ecosystems 15.1 Introduction 15.2 Water Column Seasonal Cycles 15.3 Benthic Seasonal Cycles 15.4 Control of Seasonal Cycles Chapter 16 Long-Term and Large-Scale Change in Marine Ecosystems"/p> 16.1 Introduction 16.2 Large-Scale Effects of Long-Term Atmospheric Changes 16.3 Depletion of Fishery Stocks 16.4 Eutrophication 16.5 Toxic Contamination 16.6 Spread of Exotic Species 16.7 Harmful Algal Blooms 16.8 Interception of Freshwater Inputs and Sediment Loads 16.9 Multiple Factors in Concert: The Case of Black Sea 16.10 Implications of Long-Term, Large-Scale Changes References Index

Riparian Forests as Nutrient Filters in Agricultural Watersheds

Abstract: Riparian (streamside) vegetation may help control transport of sediments and chemicals to stream channels. Studies of a coastal plain agricultural watershed showed that riparian forest ecosystems are excellent nutrient sinks and buffer the nutrient discharge from surrounding agroecosystems. Nutrient uptake and removal by soil and vegetation in the riparian forest ecosystem prevented outputs from agricultural uplands from reaching the stream channel. The riparian ecosystem can apparently serve as both a shortand long-term nutrient filter and sink if trees are harvested periodically to ensure a net uptake of nutrients. (Accepted for publication 28 November 1983)

Phosphorus versus nitrogen limitation in the marine environment1

Abstract: Limnological and marine geochemical opinion favors phosphorus limitation of organic production in aquatic environments, while marine biological opinion favors nitrogen limitation. Clues in the literature and nutrient budgets for selected marine ecosystems suggest that phosphorus vs. nitrogen limitation is a function of the relative rates of water exchange and internal biochemical processes acting to adjust the ratio of ecosystem N:P availability.

Epiphyte biomass and nutrient capital of a neotropical Elfin forest

Abstract: The epiphyte communities of a Costa Rican cloud forest make up a conspicuous portion of the canopy, especially on large canopy dominants. Non-destructive sampling methods were used to assess the composition, biomass, and nutrient concentration of live and dead epiphytes on representative host trees to determine the mineral capital contained in the epiphyte components of the standing vegetation. Epiphyte standing crop on a single large Clusia alata (Guttiferae) tree is 141.9 kg. The nutrient capital (in g) is: N = 3062; P = 97; K = 678; Ca = 460; Mg = 126; Na = 207. Using information on forest structure and epiphyte distribution, stand-level estimates of epiphyte mat nutrient capital were made. Although epiphyte biomass constitutes less than 2 percent of total elfin forest ecosystem dry weight, the nutrients they contain are equivalent to up to 45 percent of nutrients contained in ecosystem foliage of similar ecosystems. Assessment of epiphyte nutrient capital gives a more complete and accurate idea of the aboveground vegetation pools, and supports the idea that epiphytes may play a greater role in ecosystem nutrient dynamics than has been previously considered. ALTHOUGH THE IMPORTANCE OF MINERAL NUTRITION for plants and animals has been recognized for centuries, only recently has there been a systematic approach to mineral element cycling in entire ecosystems (Pomeroy 1970, Jordan et al. 1972, Golley et al. 1975). Estimates of the total amount of mineral elements and the rate of elemental cyding within a complete landscape unit during a period of time have resulted in the formulation of several basic mineral cycling concepts (Rodin and Bazilevich 1967, Jordan and Kline 1972, Bormann and Likens 1979). The general approach has been to divide the ecosystem into a series of compartments, or pools, and to measure the quantity and chemical composition of each pool and the pathways and rates of flux between each component in as great detail as possible. A small number of ecosystemlevel studies (e.g., Ovington and Madgwick 1959, Cole et al. 1968, Woodwell and Whittaker 1968, Duvigneaud and Denaeyer-DeSmet (1975), an even smaller number of which were tropical (e.g., Nye 1961, Odum and Pigeon 1970, Golley et al. 1975, Grubb 1977, Cole and Johnson 1978, Jordan 1982), have been important in the development of theories of community stability, nutrient use efficiency, and ecosystem resilience. Because of the greater diversity and complexity of tropical forests, and the greater logistical problems encountered there, the sizes of many of the compartments that make up tropical ecosystems remain poorly known. In most whole-ecosystem studies, epiphytes-plants deriving support but not nutrients directly from their host trees-have been discounted or ignored, as their biomass was considered insignificant in proportion to other forest components. However, vascular and non-vascular epiphytes make up a conspicuous portion of many rain forest canopies, reaching their greatest diversity and abundance in neotropical cloud and elfin forests, which are regularly enshrouded in mist and lack a prolonged dry season (Richards 1964, Sanford 1968, Madison 1977, Sugden and Robins 1979). Although the epiphytic flora of these forests has attracted a good deal of botanical attention, most of it has been focused on aspects of taxonomy (Dressler 1979, Benzing 1981a), phytosociology (Eggeling 1947, Sanford 1968, Johnson and Awan 1972, Russell and Miller 1977, Madison 1979, Sugden 1981, Yeaton and Gladstone 1982), and physiology (Hosokawa and Odani 1957, Medina 1972, Benzing and Ott 1981, Huber 1978). These canopy-dwelling plants must overcome greater extremes of insolation, temperature, humidity, and wind than their terrestrial counterparts. They lack organic connection to the bank of nutrients and water stored in forest soils. Resources are pulse-supplied from atmospheric sources, and canopy surfaces may be characterized by frequent and/or prolonged deprivations of moisture and nutrients (Benzing 198 1b). Many aspects of epiphyte morphology, physiology, and life history contribute to their efficiency at garnering and retaining airborne nutrients (Benzing 198 1b, 1982). By virtue of their powers of mineral accretion and their location along primary nutrient flux routes, epiphytes can be major participants in the impoundment and movements of mineral elements in a forest ecosystem (Nadkarni 1983). The effects of epiphytes on ecosystem-level interactions have been investigated in only a few studies, all in temperate forests (Denison et al. 1972, Pike 1972, 1978, Lang et al. 1976, 1980, Schlesinger and Marks 1977, Benzing and Seeman 1978). Assessing the mineral IReceived 3 November 1983, revised 3 February 1984, accepted 7 February 1984. BIOTROPICA 16(4): 249-256 1984 249 This content downloaded from 157.55.39.104 on Sun, 19 Jun 2016 06:38:59 UTC All use subject to http://about.jstor.org/terms capital contained in epiphytes is an important first step in determining their role in rain forest nutrient dynamics and in obtaining a more complete and accurate picture of all aboveground components. Minerals contained in their living and dead tissues constitute a nutrient pool, distinct from host tree minerals, which are immobilized for some span of time within the canopy. These can be transferred to other ecosystem pools via litterfall, crown wash, and in some cases, by host tree canopy root systems (Nadkarni 1981). In this paper, as part of a comparative study of within-canopy nutrient dynamics in temperate and tropical rain forests, I assess the composition, biomass, and nutrient content of epiphyte communities on large canopy trees in a neotropical elfin forest, compare the nutrient capital contained in epiphytes with other ecosystem components of similar forests, and discuss some of the processes by which epiphytes accrue and retain their nutrient capital. Some of the terms used in this paper have multiple or ambiguous meanings and are defined here: biomassthe dry weight of living matter present in a given plant community; standing crop-the dry weight of living and dead components of a given plant community; epiphyte mat-the composite unit of living arboreal plants and their associated detrital matter found within host tree

The ecology of tidal freshwater marshes of the United States East Coast: a community profile

Abstract: Tidal freshwater marshes are a distinctive type of estuarine ecosystem located upstream from tidal saline marshes and downstream from non-tidal freshwater marshes. They are characterized by freshwater or nearly freshwater conditions most of the time, flora and fauna dominated by freshwater species, and daily lunar tidal flushing. This report examines the ecology of this community as it occurs along the Atlantic seaboard from southern New England to northern Florida. This marsh community is heavily influenced by river flow, which maintains freshwater conditions and deposits sediments high in silt and clay. The plant assemblage in tidal freshwater marshes is diverse both in numbers of species and structural plant types. Plant community structure is markedly diverse spatially and seasonally, and reflects the dynamic processing of energy and biomass in the marsh through high productivity, rapid decomposition and seasonal nutrient cycling. The diverse niches in this heterogeneous environment are exploited by a very diverse animal community of as many as 125 species of fish, 102 species of amphibians and reptiles, 280 species of birds, and 46 species of mammals over the community's broad range. Although fewer species are permanent residents or marsh breeders, use of his community for food and cover is high.more » This use, coupled with the marshes' capacity to be natural buffers and water filters, argue for their high value as natural resources. 349 references, 31 figures, 24 tables.« less

The Influence of Arthropods on Ecosystems

Abstract: Arthropod interactions with plants and microbes influence the amounts of living and dead organic matter and transfers of nutrients in terrestrial ecosystems. Arthropods in the canopy have their greatest effect on mobile elements such as potassium, whereas soil detritivores influence mineralization rates of less mobile elements such as nitrogen, phosphorus, and calcium. Nominal (baseline) herbivory and detritivory combine to speed nutrient cycling and reduce standing crops of decaying plant materials. 49 references, 1 figure, 3 tables.

Ecological effects of acid deposition upon peatlands: a neglected field in «acid-rain» research

Abstract: Certain types of peatlands are probably highly susceptible to anthropogenic acidification, yet very little research is being done on the vulnerability of bogs and fens to acid deposition. We have documented the need for such research and for studies of the role of acidification — natural and anthropogenic — in determining nutrient availability, metal mobilization, and biogeochemical cycling by fauna and microflora. Possible effects of hydrological changes, and of drainage from acid peatlands to lakes and streams, are noted. We provide an outline of possible responses of plants and animals to acidification; these should be investigated at species, community, and ecosystem levels. Studies of peatlands as possible sources of the gaseous precursors of acid deposition are needed. Different approaches to examining the responses of peatlands to acid deposition include geographical surveys, experimental studies, short-term, long-term, and paleoecological investigations, and analysis of biogeochemical mass-balance...

Acidic deposition: Effects on aquatic ecosystems

Abstract: The literature pertaining to the influence of the atmospheric deposition of acidifying substances on aquatic ecosystems is examined in this article. Important chemical species in atmospheric deposition are first identified, and their influence on stream and ultimately on lake water chemistry discussed. Changes that have accompanied acidification among biological communities of decomposers, primary and higher order producers are discussed. Where they exist, hypotheses relating to causes of community changes are examined.

Effects of pollutants at the ecosystem level.

Abstract: Surveys current knowledge of the effects of man-made chemicals and other disturbances on the ecosystem, focusing on the effects on the structure and function of the system rather than on individual organisms or populations. Covers community structure, diversity, nutrient cycling, litter decomposition, and more. Examines the question of detectability of change and includes a number of case histories of aquatic, marine, and terrestrial systems.

Flows of Energy and Materials in Marine Ecosystems: Theory and Practice

Abstract: One - Invited Papers.- The Importance of Measuring Fluxes in Marine Ecosystems.- Community Measures of Marine Food Networks and their Possible Applications.- Thermodynamics of the Pelagic Ecosystem: Elementary Closure Conditions for Biological Production in the Open Ocean.- The Structure of Aquatic Ecosystems and its Dependency on Environmental Conditions.- Eutrophication of a Coastal Marine Ecosystem - An Experimental Study Using the MERL Microcosms.- Oceanic Nutrient Cycles.- State-of-the-Art in the Measurement of Primary Production.- Functional Types of Marine Planktonic Primary Producers and their Relative Significance in the Food Web.- Measuring the Metabolism of the Benthic Ecosystem.- The Measurement of the Enthalpy of Metabolism of Marine Organisms.- Bacterial Production in the Marine Food Chain: The Emperor's New Suit of Clothes?.- Suspended Marine Bacteria as a Food Source.- The Biological Role of Detritus in the Marine Environment.- The Cycling of Organic Matter by Bacterioplankton in Pelagic Marine Ecosystems: Microenvironmental Considerations.- An Overview of Secondary Production in Pelagic Ecosystems.- The Quantitative Significance of Gelatinous Zooplankton as Pelagic Consumers.- Fish Production in Open Ocean Ecosystems.- Ecological Efficiency and Activity Metabolism.- Detrital Organic Fluxes Through Pelagic Ecosystems.- The Supply of Food to the Benthos.- Coupling the Sub-Systems - the Baltic Sea as a Case Study.- On the Relation of Primary Production to Grazing during the Fladen Ground Experiment 1976.- Two - Working Group Reports.- Studies on Marine Autotrophs: Recommendations for the 1980s.- Respiration.- Nutrient Cycling in Estuarine and Coastal Marine Ecosystems.- Excretion and Mineralisation Processes in the Open Sea.- Herbivory.- Detritivory.- Carnivory.- The Role of Free Bacteria and Bactivory.- Participants.

Interaction of increasing atmospheric carbon dioxide and soil nitrogen on the carbon balance of tundra microcosms

Abstract: Natural cores of vegetation and soils of arctic tundra were collected in frozen condition in winter near Barrow, Alaska (71°20′N). These cores were used as microcosms in a phytotron experiment to measure the interactions, if any, between increasing atmospheric CO2 concentration and fertilization by ammonium nitrate on net ecosystem CO2 exchange and net yield of tundra vegetation. Increased soil N significantly enhanced net ecosystem CO2 uptake. The effect of increased CO2 concentration had little or no effect on mean net ecosystem carbon balance of the tundra microcosms. Added N significantly increased leaf area and phytomass of vascular plants in the microcosms while increased atmospheric CO2 had no effect on these parameters. We conclude that atmospheric CO2 is not now limiting net ecosystem production in the tundra and that its direct effects will be slight even at double the present concentration. the most probable effects of carbon dioxide in the coastal tundra will be through its indirect effects on temperature, water table, peat decomposition, and the availability of soil nutrients.

Natural Disturbance and Nitrogen Mineralization: Wave-Form Dieback of Mountain Hemlock in the Oregon Cascades

Abstract: Wave—form dieback of relatively pure stands of mountain hemlock provided an opportunity to study changes in nutrient availability following natural disturbance. Nitrogen mineralization rates of forest floor and mineral soil were estimated using laboratory and in situ incubations. Nitrogen mineralization rates in both the mineral soil and O2 horizon were at least doubled following the pathogen—induced disturbance. As the regenerating stands developed, rates declined again to the very low predisturbance levels. These changes in nitrogen availability may in turn influence tree resistance to the pathogen, suggesting that in this system the pattern of nitrogen availability is both a consequence and a case of natural disturbance. See full-text article at JSTOR

Phosphorus limitation of net production in a confined aquatic ecosystem

Abstract: The sediment composition of confined aquatic ecosystems with little terrigenous input reflects net ecosystem production. Here we present nutrient budgets for one such system, Shark Bay in Western Australia, which suggest that net ecosystem production is limited by the oceanographic delivery of P, while the system meets its N requirements by nitrogen fixation. We suggest that N versus P limitation of the net production of aquatic ecosystems reflects the degree of confinement of those systems.

Integrated Acidification Study (ILWAS): A Mechanistic Ecosystem Analysis

Abstract: An integrated, interdisciplinary, intensive study of three forested watersheds was started in 1977 to quantify the relationship between the deposition of atmospheric acids and surface water acidity. Results indicate the importance of using an integrated ecosystem perspective to assess the vulnerability of surface waters to acidification and the value of analysing relative flowpath contributions to understanding surface water alkalinity levels and dynamics. Important ecosystem properties affecting surface water acidity are soil depth, soil mineralogy and stage of forest development.

The Biological Role of Detritus in the Marine Environment

Abstract: The heterotrophic fate of detrital material in the marine environment has attracted widespread interest since the pioneer work of Teal (1962) and others established that primary production by wetland vegetation in coastal saltmarshes of Georgia, U.S.A., greatly exceeded direct consumption by grazing herbivores. Such material was thus available for decomposition or export as a potential trophic resource for consumer organisms in the shallow waters bordering such wetland ecosystems (see Darnell, 1967a,b; Odum, 1971; Keefe, 1972; Day et al., 1973; Gosselink and Kirby, 1974; Woodwell et al., 1977; for reviews, Newell, 1979; Nixon, 1980). Much the same situation exists on many rocky shores which are dominated by kelp beds. In contrast to the situation in the open sea, where much of the primary production is thought to be consumed directly by herbivores (Steele, 1974), the discrepancy between the primary production and the material which is directly removed by grazers near to kelp beds suggests that as in wetland ecosystems, large quantities of photo-assimilated material may become available to consumer organisms after fragmentation and partial decomposition (see Mann, 1972, 1973; Field et al., 1977; Newell et al., 1982).

Dynamics of Pools of Dissolved Organic Carbon

Abstract: Bacteria living and growing in the plankton of marine waters are clearly doing so at the expense of organic matter in the dissolved state. In recent years our understanding of this process has improved greatly. This paper will explore the interaction of bacteria and dissolved organic matter from an ecological viewpoint. In energy flow terms, the DOM (dissolved organic matter) of natural waters can be construed as a trophic level, the base of a food chain involving only heterotrophs. The next (higher) trophic level is occupied by the heterotrophic microorganisms dependent on DOM, and there is good evidence now that in planktonic ecosystems the bacteria uniquely occupy this trophic level. They in turn contribute energy to higher trophic levels when they are fed on by planktonic and benthic animals. From this point the energy flow joins that of the food chain based on particulate matter such as algae and detritus.

Animals as indicators of ecosystem responses to air emissions

Abstract: With existing and proposed air-quality regulations, ecological disasters resulting from air emissions such as those observed at Copperhill, Tennessee, and Sudbury, Ontario, are unlikely. Current air-quality standards, however, may not protect ecosystems from subacute and chronic exposure to air emissions. The encouragement of the use of coal for energy production and the development of the fossil-fuel industries, including oil shales, tar sands, and coal liquification, point to an increase and spread of fossil-fuel emissions and the potential to influence a number of natural ecosystems. This paper reviews the reported responses of ecosystems to air-borne pollutants and discusses the use of animals as indicators of ecosystem responses to these pollutants. Animal species and populations can act as important indicators of biotic and abiotic responses of aquatic and terrestrial ecosystems. These responses can indicate long-term trends in ecosystem health and productivity, chemical cycling, genetics, and regulation. For short-term trends, fish and wildlife also serve as monitors of changes in community structure, signaling food-web contamination, as well as providing a measure of ecosystem vitality. Information is presented to show not only the importance of animals as indicators of ecosystem responses to air-quality degradation, but also their value as air-pollution indices, that is, as air-quality-related values (AQRV), required in current air-pollution regulation.

The Role of Ocean Biota in Accelerated Ecological Cycles: A Temporal View

Abstract: Over the last century humans have accelerated both the return of carbon to the atmosphere via burning of wood, coal, and oil and the removal of nitrogen via use of fertilizers. Previous unbalanced global carbon and nitrogen budgets have not properly considered the role of marine biota in mediating such element fluxes among ocean, land, and atmosphere. Although anthropogenic carbon dioxide emissions is tenfold more than carbon dioxide emission in glacial periods, methane emission and nitrogen fixation may have declined. 119 references, 3 tables.

Classification and comparison of upland hardwood and conifer ecosystems of the Cyrus H. McCormick Experimental Forest, upper Michigan

Abstract: An ecological method of forest site classification was used to classify and map local ecosystems of the Cyrus H. McCormick Experimental Forest near Marquette in the upper peninsula of Michigan, U.S.A. The field method of site classification was effective in distinguishing 11 ecosystem units that differed markedly in their vegetation as well as in their topographic and soil properties. Each ecosystem repeatedly occurred in a characteristic topographic position within the landscape. Ecosystems were identified in the field using combinations of biophysical properties such as slope, aspect, soil texture, soil drainage, and forest composition. Ecological species groups, groups of indicator plants, were especially helpful in distinguishing ecosystems in the field. Soils of ecosystems identified using the ecological method of classification differed significantly in their physical and chemical properties, and principal component analysis and numerical clustering confirmed that soils of the 11 upland ecosystems w...

Nitrogen and phosphorus content in plant species of Mediterranean ecosystems in Greece

Abstract: The values of nitrogen phosphorus ratios measured in green parts of evergreen sclerophylls, seasonal dimorphic, deciduous and herbaceous plants found in Mediterranean-type ecosystems of Greece, form a homogeneous group, which is comparable only with those of analogous ecosystems of California but not with Mediterranean systems of the southern hemisphere.

Development and Use of an Aquatic Micro‐Ecosystem as a Test System for Toxic Substances. Properties of an Aquatic Micro‐Ecosystem IV

Abstract: Experiments were performed using several modifications of a micro-ecosystem made up of three subsystems connected by a unidirectional flow of recirculating water. The three subsystems represent roughly the trophic levels of autotrophs (algae), herbivores (Daphnia magna), and decomposers (sandfilter with bacteria). The particle concentration was determined weekly, using a Coulter Counter, and the Daphnia population was counted. At regular intervals, the species composition of the algae was determined. The changes in the algal concentration and Daphnia population are described, and the different modifications are discussed. In several micro-ecosystems, the herbicide dichlobenil was introduced once or several times, and in one micro-ecosystem, the herbicide diuron was added. The response of the micro-ecosystems is discussed. For the periods without a toxic substance present, a normal operating range is suggested in relation to which the response of the systems to the introduced herbicide is discussed. Besides the separate analysis of the data, the algal concentrations and the Daphnia numbers were correlated to each other. This was done by regression analysis and according to a dynamic system approach. In both cases, the effect of herbicides was better discerned than in the separate analysis of the data. In the dynamic systems approach, a normalized ecosystem strain was defined as a parameter to measure effects of stress on the ecosystem level.

Terrestrial nitrogen cycles: Some unanswered questions

Abstract: Nitrogen is generally considered to be the element which most often limits the growth of plants in both natural and agricultural ecosystems. It regulates plant growth because photosynthetic rates are strongly dependent on the concentration of nitrogen in leaves, and because relatively large mounts of protein are required for cell division and growth. Yet nitrogen is abundant in the biosphere - the well-mixed pool in the atmosphere is considered inexhaustible compared to biotic demand, and the amount of already fixed organic nitrogen in soils far exceeds annual plant uptake in terrestrial ecosystems. In regions where natural vegetation is not nitrogen limited, continuous cultivation induces nitrogen deficiency. Nitrogen loss from cultivated lands is more rapid than that of other elements, and nitrogen fertilization is generally required to maintain crop yield under any continuous system. The pervasiveness of nitrogen deficiency in many natural and most managed sites is discussed.