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

Eric McGivney,Jon Petter Gustafsson,Salim Belyazid,Stefan Löfgren,Therese Zetterberg

doi:10.5194/soil-5-63-2019

Abstract

Abstract. 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.

Highlights

Anthropogenic acidification has an impact on soils, streams, organisms, agriculture, and forestry
The HD-MINTEQ simulations for three sites indicate that forest harvesting contributes to long-term soil acidification, with marginally larger effects for whole-tree harvesting compared with stem-only harvesting scenarios
Acidification due to harvesting events had much less impact on soil pH compared to historical acid rain, provided that harvesting did not cause substantial nitrification

Summary

Introduction

Anthropogenic acidification has an impact on soils, streams, organisms, agriculture, and forestry. That S deposition has dropped to around the 1930s level throughout Western Europe (Bertills et al, 2007; Engardt et al, 2017), focus has shifted towards understanding forest soil dynamics in response to forest biomass production and different harvesting scenarios (Akselsson et al, 2007; Iwald et al, 2013; de Jong et al, 2017). Tree growth acidifies the soil through the net uptake of cations over anions, which results in an accumulation of H+ in the form of organic acids (Nilsson et al, 1982). Mass balance calculations show considerable losses of base cations Ca2+, Mg2+, Na+, and K+ (BCs) due to forest management practices, which may have strong acidifying effects on soils of base-poor mineralogy (Akselsson et al, 2007; Iwald et al, 2013). There is a need to develop sustainable forestry practices in which the net losses of BCs are minimized to avoid acidification and long-term depletion of BCs (Vadeboncoeur et al, 2014)

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