Interaction of Climate Change and Acid Deposition - Climate Change Impacts on Freshwater Ecosystems - page 156

to evaluate the effects of various future scenarios for acid deposition and climate.
These three approaches are interrelated: trends seen in analyses of empirical
data give rise to hypotheses of cause-effect relationships that can be tested by
experiments. The results can then be used to develop and calibrate models.
Modelling can reveal shortcomings and gaps in the empirical data, which can
then form the basis for new monitoring or measurements.
Effects of climate change on nitrate leaching
The role of nitrate (NO 3 ) in climate change effects on recovery of surface waters
from acidification is of special interest. While S concentrations in deposition
have decreased by 60%-80% since the peak years in the 1980s, N concentrations
continue to be high. As many terrestrial ecosystems in acid-sensitive regions are
growth-limited by nitrogen, typically most of the N deposited is retained, largely
in soil organic matter. If climate change entails mobilization and release of N
stored in soil organic matter, the NO 3 levels in streams and lakes could increase
and delay recovery or even cause re-acidification.
Climate effects on NO 3 concentrations can be revealed by analysing long data
records of climate, N deposition and streamwater NO 3 concentrations. de Wit et al .
(2008) analysed 20-year records of NO 3 in four small streams in Norway, three
of which (Birkenes, Langtjern, Storgama) are highly acidified with pH 4.5-5.5.
Empirical models explained between 45% and 61% of the variation in weekly
concentrations of NO 3 , and described both upward and downward seasonal
trends tolerably well (de Wit et al . 2008). Key explanatory variables were snow
depth, discharge, temperature and N deposition. All catchments showed
reductions over time in snow depth and increases in winter discharge due to
warmer winters in the years since about 1990. In two inland catchments, located
in moderate N deposition areas, these climatic changes appeared to drive distinct
decreases in winter and spring concentrations and fluxes of NO 3 .
At one of the sites, Storgama, a series of experiments was conducted in 2003-7
to test the role of snowpack in regulating NO 3 concentrations and fluxes in runoff
(Kaste et al . 2008). Here, whole-catchment manipulations in mini-catchments
included extra insulation of soils in two catchments (by means of rock wool
mats) to prevent sub-zero temperatures during winter and removal of snow in
two other catchments to promote soil frost (Fig. 7.3). The main results from this
study show that increased soil temperatures during winter (due to heavy snow
pack and/or extra insulation) increased the springtime concentrations and fluxes
of NH 4 and NO 3 in runoff (Kaste et al . 2008). The experiments thus support the
statistical analysis of the long-term record from Storgama which indicate that less
snow gives colder soils and lower flux of N from the soil.
de Wit and Wright (2008) then used the statistical analyses of de Wit et al .
(2008) (as supported by the experiments of Kaste et al . (2008)) to project
future NO 3 concentrations and fluxes at Storgama, given several future
scenarios of N deposition and climate. Two N deposition scenarios were used,
CLe and maximum feasible reduction (MFR), together with four climate
scenarios, two greenhouse gas emission scenarios A2 and B2 (IPCC 2007) run