A key issue in understanding the nitrogen (N) cycle is to determine human-induced perturbations to the natural N cycle. Such determination involves first understanding, describing, and quantifying the natural N cycle at global, regional, and local scales. The concentration of N species in the environment varies greatly over short distances. This natural variability can make it difficult to evaluate human influences on the N cycle. Ideally, all the components of the natural N cycle should be measured with known precision and accuracy. In practice, this is impossible. In reality, estimates are made of the magnitude of the major components of the N cycle based on fragmentary measurements. There are large uncertainties on the magnitude of the estimated natural fluxes of N and the natural storage of N in the N reservoirs. For example, the estimates of marine N fixation range from 40-200 million metric tons/yr, atmospheric deposition of organic-N 10-100 million metric tons/yr and, terrestrial ammonia/ammonium (NH3/NH4+) emissions 91-186 million metric tons N/yr. One reason for such large uncertainties is that the natural world is large and heterogeneous and it has been widely modified by a variety of human influences. This makes it exceedingly difficult to measure the natural N cycle.
Once estimates of the natural N cycle are made, then estimates of human perturbations to the natural N cycle can be made. Human activities influence the N cycle through interacting physical, chemical and biological processes. In estimating N fixation by fossil-fuel combustion, N-fixation rates are estimated based on data on, for example, combustion temperature, combustion process, and the N content of the fuel. N fixation by legumes is estimated based on extrapolation of N-fixation measurements at a small number of specific locations.
In some situations, it is easy to determine with confidence that N cycling has been increased due to human activities. For example, direct discharges of human waste to a river can be measured and concentrations can be so high that it is irrefutable that nearly all the N in the water is due to human inputs. Conversely, measured human inputs of N can be low, in which case it may be more difficult to state categorically that human inputs of N have increased the natural N content of the river, because the human inputs of N are within the range of uncertainty of the estimated natural N content of the river. It can be even more difficult to determine the extent to which human activities have perturbed the natural N cycle when both the natural N cycle and the human inputs have large uncertainties.
The challenge is to assess the relative contributions of natural-and human-induced processes to N cycling under conditions of uncertainty. To do this, a full N-accounting system must be established with specified levels of accuracy. And the accounting system must include human activities that deplete N reservoirs and reduce N cycling, as well as those that increase the magnitude of the N cycle
The evolution of human influences on the N cycle.
As discussed in Biosphere, virtually everywhere prehistoric people lived they modified the environment for their benefit by the use of fire. By physically removing the land's cover of vegetation, fire enhanced the transfer of N from the land by wind and water erosion and by leaching of dissolved N. Reduced shade and a blackened surface made the surface warmer (and by reducing evapotranspiration) often made the land wetter. This stimulated biological nitrification and denitrification, which, in turn, further enhanced losses of N from the land to the hydrosphere and atmosphere, respectively. Similarly, fire chemically stimulated mineralization of soil organic matter to NH4+, much of which the highly alkaline ash left behind by fire converted to NH3 gas, further enhancing loss of soil N to the atmosphere. Additionally, fire itself chemically converted much of the N contained in the burnt vegetation to N2 gas in a process called pyrodenitrification.
Conversely, fire preferentially stimulated the growth of plants which supported symbiotic and mutualistic N fixation as well as asymbiotic populations of N-fixing bacteria. In spite of fire-induced enhanced losses of soil N to the hydrosphere and atmosphere, the effect of fire in enhancing N fixation is so great, the fired landscape generally gained in N as a result of fire. In Illinois, the prehistoric use of fire by Native Americans aided to generally double the content of soil N by maintaining most of the land in prairie rather than in forest. These prairies (and the burned forest remnants) supported an animal population whose effect on the Native American-shaped ecology was to further enhance transfers of N to the atmosphere and hydrosphere. But, as with fire, the animal kingdom enhanced N fixation more than it enhanced N transfers.
As more conventional civilizations developed in historic times, human influences on the N cycle shifted with the changing land-use practices. Fire was still used, however, it was used in combination with other techniques. Fired grasslands and woodlands supported domesticated animals. The N-rich manure and urine of these animals were mixed and composted with relatively N-poor plant materials to produce a slow release organic fertilizer of remarkably-consistent biochemical properties. This organic fertilizer was applied to tilled and hoed cropland. The physical disturbance of tillage hastened the mineralization of N in organic matter to NH4+ and the aeration produced by tillage hastened the conversion of this mineralized N to NO3-, both forms of inorganic N being readily used by crops. In humid temperate climates, such as in Illinois, about two acres of pasturage was required to support every acre of plowed cropland; some of this cropland was used to feed farm animals used for labor whose manures were recycled back to the cropland. Nevertheless, cropland lost fertility relative to its so-called virgin state. For example, in the Midwest Cornbelt, tilled croplands lost almost half of their N content in the first 60 years. Wetland agricultures were developed by civilizations in both the New and Old World. The inherently high N-fixing capacity of wetland ecosystems meant that these wetland agricultures required little or no augmentation to sustain fertility.
As civilizations became industrialized, human influences on the N cycle shifted yet again. Animal power was replaced by machine power; manure was replaced by chemical fertilizers and use of improved N-fixing legumes. Plant breeding and reduced losses of crops to weeds, insects and disease increased per acre yields of major crops (e.g., corn, wheat, rice) 10-fold, or more, in the developed world. For example, U.S. crop production has increased more than 5-fold while area of cropland has decreased from 162 million ha around 1900 to 113 million ha in the 1990s. However, the area of Illinois row crops has increased from 4 million ha in 1900 to about 9.3 million ha by the late 1990s.
Today's intensive crop production requires application of N fertilizer greater than that applied in the mid-1900s. The more N fertilizer applied the greater is the potential for losses of N to the atmosphere and the hydrosphere. Recognizing these fertilizer-related problems, agriculture is developing improved best management practices (BMPs) such as slow-release chemical fertilizers and water table management (and other) techniques to decrease rates of nitrification/denitrification during periods of low plant N demand. Also agriculture is shifting to a combination of rebuilding the soil organic N reservoir and enhanced plant rhizosphere N fixation and N uptake to decrease dependency on chemical-N fertilizers.
Energy generation releases geologic N stored in fossil fuel and also fixes atmospheric O2 and N2 under conditions of high temperature and pressure. Engineering solutions have and continue to reduce the amount of fixed N produced per unit of energy produced.
Current major concerns over human modifications to the N cycle.
The major human activities that today influence the global N cycle are fossil fuel combustion,
the production and use of chemical fertilizer, and the growing of N-fixing crops. These activities are
reported to have doubled the magnitude of N fixation over continents. The major environmental
issues associated with this enhancement are global climate change, stratospheric ozone depletion,
regional smog, visibility degradation, acid rain, water-use impairment, and eutrophication.
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