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The human induced input of reactive N into the global biosphere has increased to approximately 150 Tg N each year and is expected to continue to increase for the foreseeable future. The need to feed (?125 Tg N) and to provide energy (?25 Tg N) for the growing world population drives this trend. This increase in reactive N comes at, in some instances, significant costs to society through increased emissions of NO_x, NH3, N2O and NO3^- and deposition of NO_y and NH_x.In the atmosphere, increases in tropospheric ozone and acid deposition (NO_y and NH_x) have led to acidification of aquatic and soil systems and to reductions in forest and crop system production. Changes in aquatic systems as a result of nitrate leachinghave led to decreased drinking water quality, eutrophication, hypoxia and decreases in aquatic plant diversity, for example. On the otherhand, increased deposition of biologically available N may have increased forest biomass production and may have contributed to increasedstorage of atmospheric CO2 in plant and soils. Most importantly, synthetic production of fertilizer N has contributed greatly to the remarkable increase in food production that has taken place during the past 50 years. The development of policy to control unwantedreactive N release is difficult because much of the reactive N release is related to food and energy production and reactive N species can be transported great distances in the atmosphere and in aquatic systems. There are many possibilities for limiting reactive N emissions from fuel combustion, and in fact, great strides have been made during the past decades. Reducing the introduction of new reactive N and in curtailing the movement of this N in food production is even more difficult. The particular problem comes from the fact that most of theN that is introduced into the global food production system is not converted into usable product, but rather reenters the biosphere as a surpl [ABSTRACT FROM AUTHOR]

Plant community structure is expected to regulate the microbial processes of nitrification and denitrification by controlling the availability of inorganic N substrates. Thus it could also be a factor in the concomitant release of NO and N2O from soils as a resultof these processes. C-3 and C-4 plants differ in several attributes related to the cycling of nitrogen and were hypothesized to yield differences in trace gas exchange between soil and atmosphere. In this study we estimated fluxes of NO, N2O and CH4 from soils of shortgrass steppe communities dominated by either C-3 plants, C-4 plants or mixtures of the two types. We collected gas samplesweekly from two sites, a sandy clay loam and a clay, throughout the growing seasons of 1995 and 1996. Plant functional type effects on gas fluxes at the clay site were not apparent, however we found severaldifferences among plant communities on the sandy clay loam. CH4 uptake from atmosphere to soil was significantly greater on C-4 plots than C-3 plots in both years. NO fluxes were significantly greater from C-4 plots than from C-3 plots in 1995. NO fluxes from C-3 and mixed plots were not significantly different between 1995 and 1996, however fluxes from C-4 plots were significantly greater in 1995 compared to 1996. Results indicate that under certain environmental conditions, particularly when factors such as moisture and temperature an not limiting, plant community composition can play an important role in regulating trace gas exchange. [ABSTRACT FROM AUTHOR]