Assessing the impact of acid rain and forest harvest intensity with the HD-MINTEQ model – soil chemistry of three Swedish conifer sites from 1880 to 2080
Forest soils are susceptible to anthropogenic acidification. In the past,
acid rain was a major contributor to soil acidification, but, now that
atmospheric levels of S have dramatically declined, concern has
shifted towards biomass-induced acidification, i.e. decreasing soil solution
pH due to tree growth and harvesting events that permanently remove base
cations (BCs) from forest stands. We use a novel dynamic model, HD-MINTEQ (Husby Dynamic MINTEQ), to
investigate possible long-term impacts of two theoretical future harvesting
scenarios in the year 2020, a conventional harvest (CH, which removes stems
only), and a whole-tree harvest (WTH, which removes 100 % of the
above-ground biomass except for stumps) on soil chemistry and weathering
rates at three different Swedish forest sites (Aneboda, Gårdsjön, and
Kindla). Furthermore, acidification following the harvesting events is
compared to the historical acidification that took place during the 20th century
due to acid rain. Our results show that historical acidification due
to acid rain had a larger impact on pore water chemistry and mineral
weathering than tree growth and harvesting, at least if nitrification
remained at a low level. However, compared to a no-harvest baseline, WTH and
CH significantly impacted soil chemistry. Directly after a harvesting event
(CH or WTH), the soil solution pH sharply increased for 5 to 10 years before
slowly declining over the remainder of the simulation (until year 2080). WTH
acidified soils slightly more than CH, but in certain soil horizons there
was practically no difference by the year 2080. Even though the pH in the WTH
and CH scenario decreased with time as compared to the no-harvest scenario
(NH), they did not drop to the levels observed around the peak of historic
acidification (1980–1990), indicating that the pH decrease due to tree growth
and harvesting would be less impactful than that of historic atmospheric
acidification. Weathering rates differed across locations and horizons in
response to historic acidification. In general, the predicted changes in
weathering rates were very small, which can be explained by the net effect of
decreased pH and increased Al3+, which affected the weathering rate in
opposite ways. Similarly, weathering rates after the harvesting scenarios in
2020 remained largely unchanged according to the model.</p