Observations have shown that the dominant reactive odd-nitrogen emission from soils to the atmosphere is NO, with lesser emissions of N02 and HONO. Studies indicate that a wide range of factors influence the net soil emission of NOx to the atmosphere. These include climate (through temperature and rainfall), plant growth and decay, the clearing of forests, biomass burning, and fertilization. The three most important variables influencing NOx emissions are soil temperature, soil moisture content, and soil vegetation cover. NO emission rates have been found to vary almost exponentially with soil temperature, whereas a more linear relationship has been observed with respect to soil nitrate levels. The dependence on soil moisture content appears to be a complex one. Below approximately 15% soil moisture, microbial activity has been found to be primarily water limited and strongly favors nitrification, whereas at higher moisture contents denitrification eventually becomes predominant and NO emissions decrease rapidly. Order of magnitude differences in emission rates also occur between heavily fertilized soils, grasslands, and forested ecosystems (Williams et al., 1992). Large increases in the rates of soil emission have been observed after rain events following long periods of drought and in areas where biomass burning had recently occurred (Neff et al., 1995). Canopy cover has also been shown to be a key factor controlling the net flux of NOx into the atmosphere, particularly, tropical rain forest canopies, which have been shown to be an effective sink for N02 (Jacob and Wofsy, 1990). Agriculture and grass lands account for the bulk of net emissions (41 and 35%, respectively) (Yienger and Levy, 1995). Future changes in soil emissions are expected to be linked to increased use of nitrogen fertilizers and agricultural production.
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