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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 consequences are serious and long term. Based on our review of available scientific evidence, we are certain that human alterations of the nitrogen cycle have: approximately doubled the rate of nitrogen input into the terrestrial nitrogen cycle, with these rates still increasing; increased concentrations of the potent greenhouse gas N2O globally, and increased concentrations of other oxides of nitrogen that drive the formation of photochemical smog over large regions of Earth; caused losses of soil nutrients, such as calcium and potassium, that are essential for the long‐term maintenance of soil fertility; contributed substantially to the acidification of soils, streams, and lakes in several regions; and 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: increased the quantity of organic carbon stored within terrestrial ecosystems; 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 caused changes in the composition and functioning of estuarine and nearshore ecosystems, and contributed to long‐term declines in coastal marine fisheries.

Rangeland grazing management strategies have been developed in an effort to sustain efficient use of forage resources by livestock. However, the effects of grazing on the redistribution and cycling of carbon (C) and nitrogen (N) within the plant-soil system are not well understood. We examined the plant-soil C and N balances of a mixed-grass rangeland under three livestock stocking rates using an area that had not been grazed by domestic livestock for more than 40 years. We established nongrazed exclosures and pastures subjected to continuous season-long grazing at either a light stocking rate (20 steer-days/ha) or a heavy stocking rate (59 steer-days/ha, ∼50% utilization of annual production). Twelve years of grazing under these stocking rates did not change the total masses of C and N in the plant-soil (0-60 cm) system but did change the distribution of C and N among the system components, primarily via a significant increase in the masses of C and N in the root zone (0-30 cm) of the soil profile. The mass of soil C (0-60 cm) under heavy grazing was comparable to that of the light grazing treatment. Grazing at the heavy stocking rate resulted in a decrease in peak standing crop (PSC) of aboveground live phytomass, an increase in blue grama (Bouteloua gracilis [H.B.K.] Lag. Ex Steud.), and a decrease in western wheatgrass (Pascopyrum smithii [Rydb.] A. Love) compared to the light grazing treatment. The dominant species under light grazing was western wheatgrass, whereas in the nongrazed exclosures, forbs were dominant and appeared to have increased at the expense of western wheatgrass. The observed increase of soil C and N in the surface soil where roots dominate indicates a greater opportunity for nutrient availability and cycling, and hence enhanced grazing quality.

In a 12-year experimental study of nitrogen (N) deposition on Minnesota grasslands, plots dominated by native warm-season grasses shifted to low-diversity mixtures dominated by cool-season grasses at all but the lowest N addition rates. This shift was associated with decreased biomass carbon (C):N ratios, increased N mineralization, increased soil nitrate, high N losses, and low C storage. In addition, plots originally dominated by nonnative cool-season grasses retained little added N and stored little C, even at low N input rates. Thus, grasslands with high N retention and C storage rates were the most vulnerable to species losses and major shifts in C and N cycling.

Herbivores can often control plant dynamics by mediating positive feedbacks in plant species' influence on nutrient cycling. In a 7-yr field experiment in a nitrogen-limited Minnesota oak savanna, we tested whether herbivores accelerated or decelerated nitrogen (N) cycling through their effects on plants. We measured effects of excluding insect (primarily Orthoptera and Homoptera) and mammalian herbivores (primarily white-tailed deer, Odocoileus virginianus) on above- and belowground plant biomass, plant species composition, plant tissue nitrogen concentration, available soil N and light, and total N and carbon (C) in different pools (soil, roots, litter, etc.). Herbivore exclusion strongly increased cover and biomass of the legume Lathyrus venosus and a few species of woody plants. These effects were associated with greater aboveground standing crop, reduced belowground standing crop, and reduced light penetration to the ground surface. Herbivore exclusion also modified N cycling through greater N content of live, aboveground plant tissue early in the growing season and of litter and belowground tissue late in the growing season. Herbivore exclusion also increased soil nitrate and total available N concentrations but did not alter total soil or plant N. Our results support the hypothesis that herbivores indirectly decelerate N cycling by decreasing the abundance of plant species with nitrogen-rich tissues. However, other factors, such as disturbance from fire, may mediate herbivore effects on long-term changes in N and C pools. Herbivores may therefore indirectly control productivity, N cycling, and succession by consuming nitrogen-fixing and woody plants that have strong effects on plant resources (e.g., nitrogen and light).

Reported in this paper are foliar chemistry, tree growth (above- and below-ground), soil chemistry, nitrogen cycling (net mineralization and nitrification) and soil $\\mathrm{N}_2\\mathrm{O}$ 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$\\cdot$ ha$^{-1}\\cdot$ yr$^{-1})$, high nitrogen (150 kg$\\cdot$ ha$^{-1}\\cdot$ yr$^{-1})$, and low nitrogen plus sulfur treatments, with additions occurring in six equal doses over the growing season as NH$_4$NO$_3$ and Na$_2$SO$_4$. 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 substantially 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 $\\mathrm{NH}_4$ was consistently higher in treated plots than in controls in both stands, where extractable $\\mathrm{NO}_3$ was higher than controls only in the treated pine plots. Soil extracts yielded $<$1.5 kg/ha of $\\mathrm{NO}_3$-N for all plots in the hardwood stand throughout the experiment. Effluxes of $\\mathrm{N}_2$O were consistently greater in the pine high-N plot than in the pine control plot, but there were no observed large-scale increases in $\\mathrm{N}_2\\mathrm{O}$ 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.

Topography controls snowpack accumulation and hence growing-season length, soil water availability, and the distribution of plant communities in the Colorado Front Range alpine. Nutrient cycles in such an environment are likely to be regulated by interactions between topographically determined climate and plant species composition. We investigated variation in plant and soil components of internal N cycling across topographic gradients of dry, moist, and wet alpine tundra meadows at Niwot Ridge, Colorado. We expected that plant production and N cycling would increase from dry to wet alpine tundra meadows, but we hypothesized that variation in N turnover would span a proportionately greater range than productivity, because of feedbacks between plants and soil microbial processes that determine N availability. Plant production of foliage and roots increased over topographic sequences from 280 g· m-2· yr-1 in dry meadows to 600 g· m-2· yr-1 in wet meadows and was significantly correlated to soil moisture. Contrary to our expectation, plant N uptake for production increased to a lesser degree, from 3.9 g N· m-2· yr-1 in dry meadows to 6.8 g N· m-2· yr-1 in wet meadows. In all communities, the belowground component accounted for the majority of biomass, production, and N use for production. Allocation belowground also differed among communities, accounting for 70% of total production and 80% of N use for production in dry meadows compared to 55% of production and 65% of N use for production in moist meadows. Variation in microbial processes was highly related to soil moisture, and we found very consistent relationships among microbial respiration, gross N mineralization, and N immobilization among communities. These results indicate that the topographic soil moisture gradient is in fundamental control of the patterns of N turnover among communities and that differences in plant species do not appear to be as important.

The convergent evolution of polyphenol-rich plant communities has occurred on highly acidic and infertile soils throughout the world. The pygmy forest in coastal northern California is an example of an ecosystem on an extremely infertile soil that has exceptionally high concentrations of polyphenols. Many 'negative feedbacks' have been identified whereby plants degrade fertile soils through production of polyphenol-rich litter, sequestering soil nutrients into unavailable form and creating unfavorable conditions for seed germination, root growth, and nutrient uptake. But in the context of plant-litter-soil interactions in ecosystems adapted to soils that are inherently acidic and infertile (such as the pygmy forest), there are also many 'positive feedbacks' that result from polyphenol production. By inhibiting decomposition, polyphenols regulate the formation of a mor-humus litter layer, conserving nutrients and creating a more favorable medium for root growth. Polyphenols shift the dominant pathway of nitrogen cycling from mineral to organic forms to minimize potential N losses from the ecosystem and maximize litter-N recovery by mycorrhizal symbionts. Polyphenol complexation of Al, Mn and Fe reduce potential Al toxicity and P fixation in soil. Polyphenols regulate organic matter dynamics, leading to the accumulation of organic matter with cation exchange capacity to minimize leaching of nutrient cations. Humic substances derived from polyphenolic precursors coat rhizosphere soil surfaces, improving physical and chemical conditions for root growth and nutrient cycling. Although their long-accepted adaptive value for antiherbivore defense is now in doubt, polyphenol alteration of soil conditions and regulation of nutrient cycling illustrate how fitness can be influenced by the 'extended' phenotype in plant-litter-soil interactions.

We added 15N tracers to reference plots (receiving ambient N inputs) and to chronically fertilized plots (50 kg NH4NO3‐N·ha−1·yr−1 for 3 yr prior to and during tracer additions) in an oak‐dominated deciduous forest and a red pine plantation in order to quantify sinks for N inputs to these forests. Plots (30 × 30 m) were located at the Harvard Forest Long‐Term Ecological Research (LTER) site in central Massachusetts. Two forms of 15N tracer were applied, with 15NH4 and 15NO3 added to separate halves of reference and chronically fertilized plots in each forest. Tracers were applied monthly during two growing seasons in order to simulate movements of background (8 kg·ha−1·yr−1) and chronically elevated (58 kg·ha−1·yr−1) N deposition. Forest floors and soils were the dominant sinks for N deposition in these forests, but the relative importance of trees as sinks for N inputs increased with N loading rate. Accumulations of 15N in tree tissues after 2 yr of tracer additions showed that tree leaves, fine roots, bark, and recently formed wood assimilated <5% of the 15N added to reference plots and 20–24% of the 15N added to fertilized plots. The form of N input influenced its movement into ecosystem pools. Percent recoveries of 15N in trees were typically greater after 15NO3 additions than after 15NH4 additions. Woody tissues (wood ≤5 yr old plus bark) accumulated small fractions of N inputs after 2 yr of tracer additions, with 15N recoveries of <1% under ambient N deposition and <5% under chronic N fertilization. Regional and global assessments of the effects of N deposition on forest carbon balances should take into account observations suggesting that, although the proportion of N deposition assimilated by trees could increase with N input rate, most N deposition retained by temperate forests is likely to accumulate in soil pools with low C:N rather than in woody biomass with high C:N ratios.

This publication examines the risks associated with the release of excessive nitrogen into the environment (climate change, depletion of the ozone layer, air pollution, water pollution, loss of biodiversity, deterioration of soil quality). The report also examines the uncertainty associated with the ability of nitrogen to move from one ecosystem to another and cause \"cascading effects\". In addition to better management of nitrogen risks at the local level, there is a need to consider the global risks associated with the continued increase in nitrous oxide concentrations and to prevent excess nitrogen in all its forms by developing cost-effective strategies for all its sources. Other than the reduction of nitrogen pollution, this report provides guidance on the use of nitrogen policy instruments and how to ensure coherence with objectives such as food security, energy security and environmental objectives.