Forest N cycling and acidification

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 

N deposition can stimulate N mineralization and nitrification, at least initially, in soils (McNulty and Aber 1993 McNulty et al. 1996b Kjpnaas et al. 1998). However, long-term studies suggest that N mineralization rates can eventually decline in response to N deposition, whereas nitrification rates can continue to increase or remain elevated above initial conditions (Magill et al. 2004). Increases in nitrification have important implications for soil acid-base relations, the availability of other nutrient ions to forest plants, and nitrogen outputs to the drainage water and the atmosphere (above). 

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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Figure 5.3. Forest N cycling and acidification. Atmospheric N inputs and N outputs to drainage waters are shown in italics. Soil processes (left) and plant processes (right) are clustered within ovals. Dashed lines indicate soil-plant exchanges (plant N uptake or organic N return to soil). Solid lines show processes within soils or plants. Dotted lines show fluxes into or out of forests. Values in brackets refer to net consumption [-] or production [+] of 1 mol associated with the transformation of 1 mol N. When forest N cycles are closed (small N inputs and outputs), the sum of consumed and produced by soil and plant processes is zero and no acidity is generated. When 1 mol of organic N is mineralized (1 mol consumed) and subsequently nitrified (2 mol produced), 1 mol remains to acidify soil or drainage water if nitrate is not removed from soil and converted to organic form by plants. Denitrification to any of three gaseous products consumes 1 mol H. Direct inputs of acidity can also result from ammonium and nitrate deposition. Reprinted from Nadelhoffer (2001) with permission from Elsevier...

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