Interaction of Climate Change and Acid Deposition - Climate Change Impacts on Freshwater Ecosystems - page 160

N-NO 3 Tx-mm

NO 3 trend (eight rivers)
De-seasonalized Tx
De-seasonalized mm

0.30

4.0

0.20 2.0

0.10 0.0

-2.0

0.00

-0.10 -4.0

1977 1982 1987 1992 1997 2002 2007

Figure 7.6 Main common trend of NO 3 concentration in eight rivers (unbroken line),
and series of maximum temperature (Tx; dotted line) and precipitation amount (mm;
dashed line). The data were first de-seasonalized and normalized (mean zero; standard
deviation 1). Data are unitless owing to normalization. (From Rogora 2007.)

of maximum annual temperature, possibly reflecting higher rates of N
mineralization and thus the release of N to runoff (Rogora 2007) (Fig. 7.6). The
highest peaks of NO 3 concentration were usually detected after prolonged dry
and warm periods (Rogora & Mosello 2007).
One of the major uncertainties in all future projections of water chemistry
under changed climate is the long-term fate of the N from atmospheric deposition
that is retained each year in terrestrial ecosystems receiving chronic high levels of
N deposition. Neither the processes of N fixation nor those of denitrification are
sufficiently large in most catchment soils to account for the retention of N. The
soils are slowly accumulating N. These ecosystems cannot indefinitely retain
80% or more of the incoming N without eventually releasing a larger fraction of
the incoming N and even some of the stored N, most probably to runoff in the
form of NO 3 . The term 'nitrogen saturation' has been used to describe this
situation (Aber et al . 1989). Yet, the long-term records from many sites in Europe
and North America (Wright et al . 2001; Kaste et al . 2007) do not yet show major
increases in NO 3 concentrations.
On the contrary, some of the long-term records show declines in concentrations
of NO 3 in runoff. At the undisturbed W6 forested catchment at Hubbard Brook,
New Hampshire, the United States, for example, NO 3 concentrations in
streamwater reached peaks in the 1970s, and much of the subsequent decline is
ascribed to climatic factors (Aber et al . 2002). The long-term record from Lange
Bramke, Germany, shows a similar decline since peak years in the 1980s; there is
no explanation for this decline. In heavily acidified and damaged mountain
forests in the Czech Republic, NO 3 concentrations in streamwater have also
declined, but here the cause is ascribed to improved growth of the forests, which
had previously been damaged by high levels of air pollution (Majer et al . 2005;
Oulehle et al . 2008).