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?

Human production of food and energy is the dominant continental process that breaks the triple bond in molecular nitrogen (N2) and creates reactive nitrogen (Nr) species. Circulation of anthropogenic Nr in Earth’s atmosphere, hydrosphere, and biosphere has a wide variety of consequences, which are magnified with time as Nr moves along its biogeochemical pathway. The same atom of Nr can cause multiple effects in the atmosphere, in terrestrial ecosystems, in freshwater and marine systems, and on human health. We call this sequence of effects the nitrogen cascade. As the cascade progresses, the origin of Nr becomes unimportant. Reactive nitrogen does not cascade at the same rate through all environmental systems; some systems have the ability to accumulate Nr, which leads to lag times in the continuation of the cascade. These lags slow the cascade and result in Nr accumulation in certain reservoirs, which in turn can enhance the effects of Nr on that environment. The only way to eliminate Nr accumul...

http://bioeeos660-f12-bowen.wikispaces.umb.edu/file/view/Galloway+et+al.+2003.pdf

The Nitrogen Cascade

http://www.jlakes.org/news/hal-200812-sdarticle.pdf

Eutrophication and harmful algal blooms: A scientific consensus

Denitrification occurs in essentially all river, lake, and coastal marine ecosystems that have been studied. In general, the range of denitrification rates measured in coastal marine sediments is greater than that measured in lake or river sediments. In various estuarine and coastal marine sediments, rates commonly range between 50 and 250 µmol N m−2 h−1, with extremes from 0 to 1,067. Rates of denitrification in lake sediments measured at near-ambient conditions range from 2 to 171 µmol N m−2 h−1. Denitrification rates in river and stream sediments range from 0 to 345 µmol N m−2 h−1. The higher rates are from systems that receive substantial amounts of anthropogenic nutrient input. In lakes, denitrification also occurs in low oxygen hypolimnetic waters, where rates generally range from 0.2 to 1.9 µmol N liter−1 d−1. In lakes where denitrification rates in both the water and sediments have been measured, denitrification is greater in the sediments.

The major source of nitrate for denitrification in most river, lake, and coastal marine sediments underlying an aerobic water column is nitrate produced in the sediments, not nitrate diffusing into the sediments from the overlying water. During the mineralization of organic matter in sediments, a major portion of the mineralized nitrogen is lost from the ecosystem via denitrification. In freshwater sediments, denitrification appears to remove a larger percentage of the mineralized nitrogen. N2 fluxes accounted for 76–100% of the sediment-water nitrogen flux in rivers and lakes, but only 15–70% in estuarine and coastal marine sediments. Benthic N2O fluxes were always small compared to N, fluxes.

The loss of nitrogen via denitrification exceeds the input of nitrogen via N2 fixation in almost all river, lake, and coastal marine ecosystems in which both processes have been measured.

Denitrification is also important relative to other inputs of fixed N in both freshwater and coastal marine ecosystems. In the two rivers where both denitrification measurements and N input data were available, denitrification removed an amount of nitrogen equivalent to 7 and 35% of the external nitrogen loading. In six lakes and six estuaries where data are available, denitrification is estimated to remove an amount of nitrogen equivalent to between 1 and 36% of the input to the lakes and between 20 and 50% of the input to the estuaries.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1988.33.4part2.0702

Denitrification in freshwater and coastal marine ecosystems: Ecological and geochemical significance

Climate change: links to global expansion of harmful cyanobacteria.

Nitrogen fixation is mediated by a variety of autotrophic and heterotrophic bacteria. Cyanobacteria appear responsible for most planktonic fixation in aquatic ecosystems, and rates of fixation are high only when thcsc organisms make up a major percentage of the planktonic biomass, Planktonic nitrogen fixation tends to be low in oligotrophic and mesotrophic lakes (generally 20% of the nitrogen input to the Asko region of the Baltic Sea and 17% of the nitrogen input to the PeelHarvey estuary in Australia. Fixation in sediments of estuaries and eutrophic and mesotrophic lakes usually constitutes a small percentage of the nitrogen inputs to these systems. However, benthic fixation appears to be a major source of nitrogen for many oligotrophic tropical lagoons and for some oligotrophic lakes, even though fixation rates are moderate because other nitrogen inputs tend to be low. Nitrogen fixation probably is a fairly minor input of nitrogen to marine wetlands, which are generally open to other inputs, but contributes roughly half the total nitrogen input to some freshwater wetlands (bogs, cypress domes), where other inputs are more limited. Nitrogen fixation appears important in making up deficits in nitrogen availability relative to phosphorus availability in many lakes, contributing to the phosphorus-limited status of these systems. That many estuaries and coastal seas are nitrogen limited is due in part to the generally low rates of nitrogen fixation found in these systems.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1988.33.4part2.0669

Nitrogen fixation in freshwater, estuarine, and marine ecosystems. 1. Rates and importance1

Bacterial activities and abundance were measured seasonally in the water column of meromictic Big Soda Lake which is divided into three chemically distinct zones: aerobic mixolimnion, anaerobic mixolimnion, and anaerobic monimolimnion. Bacterial abundance ranged between 5 and 52 x 10h cells ml-‘, with highest biomass at the interfaces between these zones: 2-4 mg C liter’ in the photosynthetic bacterial layer (oxycline) and 0.8-2.0 mg C liter’ in the chemocline. Bacterial cell size and morphology also varied with depth: small coccoid cells were dominant in the aerobic mixolimnion, whereas the monimolimnion had a more diverse population that included cocci, rods, and large filaments. Heterotrophic activity was measured by [methyl-3H]thymidine incorporation and [14C]glutamate uptake. Highest uptake rates were at or just below the photosynthetic bacterial layer and were attributable to small (< 1 pm) heterotrophs rather than the larger photosynthetic bacteria. These high rates of heterotrophic uptake were apparently linked with fermentation; rates of other mineralization processes (e.g. sulfate reduction, methanogenesis, denitrification) in the anoxic mixolimnion were insignificant. Heterotrophic activity in the highly reduced monimolimnion was generally much lower than elsewhere in the water column. Therefore, although the monimolimnion contained most of the bacterial abundance and biomass (- 60%), most of the cells there were inactive.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1987.32.4.0781

Judith L. Lane

Papers

Nitrogen fixation in freshwater, estuarine, and marine ecosystems. 1. Rates and importance1

Big Soda Lake (Nevada). 1. Pelagic bacterial heterotrophy and biomass