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.

/pdf/nitrogen-cycles-past-present-and-future-1zdbw0976r.pdf

Nitrogen cycles: past, present, and future

Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM decomposes readily—and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming.

/pdf/persistence-of-soil-organic-matter-as-an-ecosystem-property-ago0khy192.pdf

Persistence of soil organic matter as an ecosystem property

http://www.tinread.usarb.md:8888/tinread/fulltext/lal/soil_structure.pdf

Soil structure and management: a review

I. CONTEXT * The Ecosystem Concept * Earth's Climate System * Geology and Soils * II. MECHANISMS * Terrestrial Water and Energy Balance * Carbon Input to Terrestrial Ecosystems * Terrestrial Production Processes * Terrestrial Decomposition * Terrestrial Plant Nutrient Use * Terrestrial Nutrient Cycling * Aquatic Carbon and Nutrient Cycling * Trophic Dynamics * Community Effects on Ecosystem Processes * III. PATTERNS * Temporal Dynamics * Landscape Heterogeneity and Ecosystem Dynamics * IV. INTEGRATION * Global Biogeochemical Cycles * Managing and Sustaining Ecosystem * Abbreviations * Glossary * References

Principles of Terrestrial Ecosystem Ecology

N itrogen emissions to the atmosphere due to human activity remain elevated in industrialized regions of the world and are accelerating in many developing regions (Galloway 1995). Although the deposition of sulfur has been reduced over much of the United States and Europe by aggressive environmental protection policies, current nitrogen deposition reduction targets in the US are modest. Nitrogen deposition remains relatively constant in the northeastern United States and is increasing in the Southeast and the West (Fenn et al. in press). The US acid deposition effects

/pdf/nitrogen-saturation-in-temperate-forest-ecosystems-29ixssxci4.pdf

Nitrogen Saturation in Temperate Forest Ecosystems

Concern is resurfacing in the United States over the long-term effects of excess nitrogen (N) deposition and mobility in the environment We present here a new synthesis of existing data sets for the northeastern United States, intended to answer a single question: Is N deposition altering the N status of forest ecosystems in this region? Surface water data suggest a significant increase in nitrate losses with N deposition Soil data show an increase in nitrification with decreasing ratio of soil carbon to nitrogen (C:N) but weaker relationships between N deposition and soil C:N ratio or nitrification Relationships between foliar chemistry and N deposition are no stronger than with gradients of climate and elevation The differences in patterns for these three groups of indicators are explained by the degree of spatial and temporal integration represented by each sample type The surface water data integrate more effectively over space than the foliar or soil data and therefore allow a more comprehensive view of N saturation We conclude from these data that N deposition is altering N status in northeastern forests

https://udel.edu/~inamdar/nps2007/n_discussion.pdf

Is Nitrogen Deposition Altering the Nitrogen Status of Northeastern Forests

https://harvardforest.fas.harvard.edu/sites/harvardforest.fas.harvard.edu/files/publications/pdfs/Magill_ForestEco&Manag_2004.pdf

Ecosystem response to 15 years of chronic nitrogen additions at the Harvard Forest LTER, Massachusetts, USA

Reported in this paper are foliar chemistry, tree growth (above- and below- ground), soil chemistry, nitrogen cycling (net mineralization and nitrification) and soil N20 flux responses to the first 6 yr of chronic nitrogen amendments at the Harvard Forest (Massachusetts, USA). A 70-yr-old red pine (Pinus resinosa Ait.) stand and a 50-yr-old mixed hardwood stand received control, low nitrogen (50 kg.ha-'-yr-'), high nitrogen (150 kg.ha-'-yr-'), and low nitrogen plus sulfur treatments, with additions occurring in six equal doses over the growing season as NH4NO3 and Na2SO4. Foliar N concentrations increased up to 25% in the hardwood stand and 67% in the pines, and there was no apparent decrease of N retranslocation due to fertilization. Wood production increased in the hardwood stand in response to fertilization but decreased in the pine stand. Fine-root nitrogen concentrations increased with N additions, and fine roots were a significant sink for added nitrogen. Nitrate leaching losses increased continuously over the 6-yr period in the treated pine stands but remained insignificant in the hardwoods. Annual net N mineralization increased substan- tially in response to treatments in both stands but declined in the pine high-N plot by the end of year six. Net nitrification increased from 17% of net mineralization in 1988 to 51% in 1993 for the pine high-N plot. Only a slight increase in net nitrification was measured in the hardwood stand, and only in 1993. Extractable NH4 was consistently higher in treated plots than in controls in both stands, where extractable NO3 was higher than controls only in the treated pine plots. Soil extracts yielded <1.5 kg/ha of NO3-N for all plots in the hardwood stand throughout the experiment. Effluxes of N2O were consistently greater in the pine high-N plot than in the pine control plot, but there were no observed large-scale increases in N20 emissions immediately following fertilizer application. Calculated nitrogen budgets for the first 6 yr showed extremely high N retention (85-99%). Of the retained N, 50-83% appears to be in the long-term, recalcitrant soil pool. The relative importance of biotic and abiotic mechanisms of N incorporation into soils remains uncertain. Size, kinetics, and uptake capacity of this soil pool are critical and largely unknown factors determining ecosystem response to increased N loading and may be related to land-use history.

Biogeochemical response of forest ecosystems to simulated chronic nitrogen deposition

At the Harvard Forest, Massachusetts, a long-term effort is under way to study responses in ecosystem biogeochemistry to chronic inputs of N in atmospheric deposition in the region. Since 1988, experimental additions of NH4NO3 (0, 5 and 15 g N m-2 yr-) have been made in two forest stands: Pinus resinosa (red pine) and mixed hardwood. In the seventh year of the study, we measured solute concentrations and estimated solute fluxes in throughfall and at two soil depths, beneath the forest floors (Oa) and beneath the B horizons. Beneath the Oa, concentrations and fluxes of dissolved organic C and N (DOC and DON) were higher in the coniferous stand than in the hardwood stand. The mineral soil exerted a strong homogenizing effect on concentrations beneath the B horizons. In reference plots (no N additions), DON composed 56% (pine) and 67% (hardwood) of the total dissolved nitrogen (TDN) transported downward from the forest floor to the mineral soil, and 98% of the TDN exported from the solums. Under N amendments, fluxes of DON from the forest floor correlated positively with rates of N addition, but fluxes of inorganic N from the Oa exceeded those of DON. Export of DON from the solums appeared unaffected by 7 years of N amendments, but as in the Oa, DON composed smaller fractions of TDN exports under N amendments. DOC fluxes were not strongly related to N amendment rates, but ratios of DOC:DON often decreased. The hardwood forest floor exhibited a much stronger sink for inorganic N than did the pine forest floor, making the inputs of dissolved N to mineral soil much greater in the pine stand. Under the high-N treatment, exports of inorganic N from the solum of the pine stand were increased >500-fold over reference (5.2 vs. 0.01 g N m-2 yr-V), consistent with other manifestations of nitrogen saturation. Exports of N from the solum in the pine forest decreased in the order NO3-N > NH4-N > DON, with exports of inorganic N 14-fold higher than exports of DON. In the hardwood forest, in contrast, increased sinks for inorganic N under N amendments resulted in exports of inorganic N that remained lower than DON exports in N-amended plots as well as the reference plot.

Alison H. Magill

Papers

Nitrogen Saturation in Temperate Forest Ecosystems

Is Nitrogen Deposition Altering the Nitrogen Status of Northeastern Forests

Ecosystem response to 15 years of chronic nitrogen additions at the Harvard Forest LTER, Massachusetts, USA

Biogeochemical response of forest ecosystems to simulated chronic nitrogen deposition

Vertical transport of dissolved organic C and N under long-term N amendments in pine and hardwood forests