Atmospheric N deposition

In the Front Range of the Rocky Mountains, the N enrichment of alpine catchments from increased atmospheric N deposition from agricultural and urban development on the plains is an important environmental and resource issue (Williams et al., 1996). The difference in N content (about 0.5-1.0% for terrestrially derived DOM vs. about 2-3% for microbially derived DOM) could be significant in terms of estimating the contribution of dissolved fulvic acid flux to the yield of N from alpine and subalpine catchments in the Rocky Mountains. The example for this alpine catchment illustrates the potential usefulness of the fluorescence index in field studies addressing applied issues related to environmental management. 

Watershed types are based on the primary sources of N to each estuary. Watersheds dominated by urban N sources (e.g., point, septic, and non-point source runoff) were classified as urban, watersheds dominated by agricultural N sources (e.g., fertilization, fixation, and manure) were classified as agriculture, and watersheds dominated by atmospheric N deposition were classified as atmospheric. Effluent from sewage treatment plants in the Barnegat Bay watershed is discharged offshore sewage inputs to the Barnegat Bay estuary are from septic systems in the watershed. Modified from Castro et al. (2003). 

Atmospheric N deposition can be an important source of N to coastal and open ocean ecosystems. The potential importance of N in atmospheric deposition has been recognized for over twenty years (e.g., CorreU and Ford, 1982 Duce, 1986 Paerl, 1985). Recognition of the importance of atmospheric deposition as a source of N to coastal waters increased rapidly following the analysis by Fisher and Oppenheimer (1991) for a number of coastal systems, including Chesapeake Bay. Atmospheric deposition to watersheds contributes to diffuse N loads in rivers as discussed previously in this chapter. In this section we are specifically referring to N deposited (wet and dry) direcdy to the surface of coastal and open ocean waters. 

Figure 36-6 Net reactive N inputs (Tg N year ) to continental world regions during the mid-1990s, from anthropogenic and natural sources. Anthropogenic sources include N fertilizer use, N fixation in cultivated lands, net N imports in food and feed, and atmospheric N deposition from fossil-fuel combustion. The natural sources include biological N fixation in non-cultivated vegetated lands and N fixation by lightning. Modified from Boyer et al. (2004).

Modeling experiments allowed us to control for factors that might cause variation in field-based estimates of woody plant age-SOC relationships. Model estimates of SOC accumulation were comparable to field estimates for upland patch types and substantially lower than field estimates for lowland patch types (Table 4). Model estimates of soil N accumulation were substantially lower than field estimates, especially in lowlands. Given that woody patch age explained only 26-68% of the variance in soil C and N content, our field estimates of accumulation rates cannot be taken as definitive. Model results underestimated field observations, especially for N. Reliability of model estimates of soil carbon could likely be improved with a better understanding of how turnover of the substantial root mass (Table 2) might differ among patch types. Model estimates of soil N are likely constrained by lack of information on inputs associated with N, fixation, atmospheric N deposition, translocation between uplands and lowlands, and root turnover. 

Elevated atmospheric N deposition can modify ecosystem N cycling by increasing ratios of N inputs to internal N cycling and by changing the amounts and forms of N made available to plants and microbes. In addition to these direct effects, N deposition can alter feedbacks between plants and soils. For example, increases in leaf or fine root N concentration occurring... 

Responses of temperate forests in the northeastern United States and Europe to variations in Nitrogen deposition, both along gradients of atmospheric N deposition and in response to long term N addition experiments, have led to a conceptual model of ecosystem response to chronically elevated N deposition referred to as the Nitrogen Saturation Hypothesis" (Aber et al. 1989 Aber et al. 1998 Aber et al. 2003). This model and various component... 

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