The widespread occurrence of nitrogen limitation to net primary production in terrestrial and marine ecosystems is something of a puzzle; it would seem that nitrogen fixers should have a substantial competitive advantage wherever nitrogen is limiting, and that their activity in turn should reverse limitation. Nevertheless, there is substantial evidence that nitrogen limits net primary production much of the time in most terrestrial biomes and many marine ecosystems. We examine both how the biogeochemistry of the nitrogen cycle could cause limitation to develop, and how nitrogen limitation could persist as a consequence of processes that prevent or reduce nitrogen fixation. Biogeochemical mechansism that favor nitrogen limitation include: A number of mechanisms could keep nitrogen fixation from reversing nitrogen limitation. These include: The possible importance of these and other processes is discussed for a wide range of terrestrial, freshwater, and marine ecosystems.

Nitrogen limitation on land and in the sea: How can it occur?

Riparian zones possess an unusually diverse array of species and environmental processes. The ecological diversity is related to variable flood regimes, geographically unique channel processes, altitudinal climate shifts, and upland influences on the fluvial corridor. The resulting dynamic environment supports a variety of life-history strategies, biogeochemical cycles and rates, and organisms adapted to disturbance regimes over broad spatial and temporal scales. Innovations in riparian zone management have been effective in ameliorating many ecological issues related to land use and environmental quality. Riparian zones play essential roles in water and landscape planning, in restoration of aquatic systems, and in catalyzing institutional and societal cooperation for these efforts.

The Ecology of Interfaces: Riparian Zones

We discuss the dynamics of plant litter, the effects of litter on the chemical and physical environment, the direct and indirect effects of plant litter on plant populations and communities, and different adaptative traits that may be related to litter accumulation. The production of litter depends primarily on the site productivity, but other properties of the environment, as well as chance, may introduce important variation. The existence of time lags between the production of plant organs and their transformation into litter appears as a relevant character of litter dynamics seldom included in models. Herbivory, and other processes that destroy biomass or reduce productivity, may reduce the amount of litter produced. The destruction of litter encompasses a complex of interactions. The main processes, including physical and chemical degradation, consumption by invertebrates and decomposition, are differentially affected by the environment and by the physical and chemical characteristics of the litter itself. The relative importance of those processes varies among systems.

Plant litter : its dynamics and effects on plant community structure

Much of the recent progress in ecosystem ecology can be traced to studies which have examined the responses of ecosystems to disturbance (Odum 1969). For example, early studies of forest clear-felling (cf. Hesselman 1917, in Stalfelt (1960) demonstrated that soil nutrient availability is usually enhanced in harvested sites. More recently, studies at the Hubbard Brook Experimental Forest (Likens et al. 1970, Bormann and Likens 1979), in Sweden (Tamm et al. 1974, Wiklander 1981), and elsewhere documented that forest cutting alters watershed-level hydrology and nutrient losses; longer-term measurements have documented the reestablishment of biotic regulation of water and nutrient cycling during secondary succession (Bormann and Likens 1979).

Biological invasions and ecosystem processes : towards an integration of population biology and ecosystem studies

Herbivores are taxonomically and ecologically diverse, ranging in size from microscopic zooplankton to the largest of land vertebrates. Aquatic grazers include zooplankton (28 , 182) , larger invertebrates such as snails, insects, and crayfish, and vertebrates such as waterfowl, tadpoles, fish, muskrats, and moose ( 1 1 , 27, 73, 1 15, 162, 1 63). Insects and mammals are the most conspicuous terrestrial herbivores (2, 46-49, 125) , but nematodes (20), crustaceans ( 152), molluscs (78) , birds, and reptiles (69) can also be signifi­ cant. Marine ecosystems are grazed primarily by crustaceans (57, 79, 1 12 , 143), molluscs (15 , 126), fish (83 , 84, 92), echinoderms (24, 25 , 65), and a few insects ( 170) , reptiles and mammals ( 123, 185). These herbivores affect plant communities in many ways. Feeding selectiv­ ity and feeding modes are highly varied; the terrestrial insect herbivores alone include phloem and xylem feeders, root grazers, gall formers, and folivores that mine, chew, roll, rasp, or pit leaves ( 1 94). Herbivores may select among plant parts, individuals, species, patches, and portions of landscapes (7 , 55 , 83 , 95 , 120, 126 , 137, 138 , 140). Many herbivores also clip or tear loose much plant biomass that is not consumed (5 , 70, 104, 105 , 122, 169). Herbivores change the environment by their trails, burrows, wallows, den building, foraging, social behavior, and other activities ( 1 , 4, 15, 40 , 50, 53 , 94, 96, 98 , 138, 148 , 165 , 167, 177). They convert plants into dung, frass,

https://www.jstor.org/stable/pdfplus/2097271.pdf

Herbivores and the dynamics of communities and ecosystems

Chronic defoliation by the fall cankerworm, Alsophila pometaria (Harris), accompanied substantial increases in the stream export of nitrate nitrogen (NO3-N) from three mixed hardwood forests in the southern Appalachians. These integrated results clearly demonstrate a measurable effect of insect consumers on ecosystem processes, and provide support for the regulatory importance of insects on a landscape scale.

Insect defoliation enhances nitrate export from forest ecosystems

Exceptions to the AET Model: Deserts and Clear-Cut Forest

A series of plots in an Amazonian Rain Forest were trenched and treated with calcium to determine the effects of perturbation on numbers and activity of nitrifying bacteria. Although treatment resulted in between 2 and 26% of the nitrogen being lost from the humus layer, virtually all of it was in the NH4+form. Numbers of nitrifying bacteria in the plots were relatively low. The low numbers and low activity of nitrifying bacteria is attributed to the low pH and high concentration of tannins in the root mat. It is hypothesized that the suppression of nitrifying bacteria results in nitrogen conservation in the rain forest.

R. L. Todd

Papers

Insect defoliation enhances nitrate export from forest ecosystems

Exceptions to the AET Model: Deserts and Clear-Cut Forest

Nitrogen conservation in a tropical rain forest.