Agricultural sources, nitrogen deposition

Similarly, deposition of ammonium is not considered a new input, as this is largely recycled nitrogen volatilized from animal wastes within the same region or large watershed (Howarth et al., 1996 Boyer et al., 2002). Approximately 90% of ammonia in the global atmosphere comes from agricultural sources (Dentener Crutzen, 1994), with major emissions coming from animal wastes (manure) and lesser contributions from volatilization of fertilizers. Because ammonia is short-lived... 

Given that NOy deposition is a much smaller input than agricultural sources in most regions, and that nitrogen limitation of production is prevalent in the natural ecosystems throughout most of these regions, we find it especially intriguing that... 

The nitrogen depositions were calculated by two models, HEMISPHERE (Sofiev, 1998) and MOGUNTIA (Dentener Crutzen, 1994), and showed similar values (10.7-11.0 kg/ha/yr.). Application of mineral fertilizers averaged 226kg/ha/yr in South Korean agriculture being the maximum input source in mass balance of nitrogen. 

The inputs of nitrogen to wetlands include biological N2 fixation, and point and nonpoint loads from external sources (Eigure 8.2). Examples include atmospheric nitrogen deposition, agricultural and urban runoff, application of fertilizers to rice paddies, stormwater runoff carrying nutrients and... 

Because nitrification occurs only under aerobic (or microaerobic) conditions and denitrification under anaerobic conditions, the two processes are spatially separated. However, if the sites where these processes occur are sufficiently close together, NO transport and consumption are very rapid and the overall process is considered to be coupled nitrification-denitrification. On the basis of a literature review, Seitzinger (1988) concluded that nitrification is generally the major source of NOj" for denitrification in river, lake, and coastal sediments. The same is likely to be true of non agricultural soils that are largely dependent on mineralization and atmospheric deposition for fixed nitrogen. 

Nutrients enter Lake Victoria mainly through two diffuse pathways, or nonpoint sources land runoff and atmospheric deposition, together accounting for —90% of phosphorus and —94% of nitrogen input into the lake. About one-third of the nitrogen inputs to Lake Victoria are from atmospheric deposition and over half of the phosphorus comes from particles associated with runoff from agricultural lands (Scheren et al.,... 

During the past twenty five years acid rain, formally referred to as acid deposition, has been the focus of much political debate and scholarly research. Acid deposition occurs when important precursor pollutants, such as sulfur dioxide (SOj) and nitrogen oxides (NOx), chemically mix with water vapor and oxidants in the atmosphere and fall back to earth in wet or dry form. Wet deposition comes in the form of dew, fog, snow or rain, while dry deposition occurs as either gasses or dry particulates. Research has shown that acid deposition adversely affects freshwater lakes and streams, coastal habitats, agricultural production, forests, soils, human health and building materials. Fossil-fuel power plants, refineries, and paper and pulp mills are the major sources of SOj emissions, while automobiles and other vehicles are the primary emitters of NO. ... 

Both fossil fuel combustion and agricultural practices contribute significantly to atmospheric fluxes of nitrogen but not phosphorus. The magnitude of the contribution of these atmospheric fluxes to coastal nutrient pollution remains uncertain, and understudied. Nonetheless, atmospheric deposition is clearly an important contributor to coastal nutrient pollution. This source... 

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