Abstract. Soil and water acidification was internationally
recognised as a severe environmental problem in the late 1960s. The
interest in establishing “critical loads” led to a peak in weathering
research in the 1980s and 1990s, since base cation weathering is the
long-term counterbalance to acidification pressure. Assessments of
weathering rates and associated uncertainties have recently become an area
of renewed research interest, this time due to demand for forest residues to
provide renewable bioenergy. Increased demand for forest fuels increases the
risk of depleting the soils of base cations produced in situ by weathering.
This is the background to the research programme Quantifying Weathering
Rates for Sustainable Forestry (QWARTS), which ran from 2012 to 2019. The
programme involved research groups working at different scales, from
laboratory experiments to modelling. The aims of this study were to (1) investigate the variation in published weathering rates of base cations from
different approaches in Sweden, with consideration of the key uncertainties
for each method; (2) assess the robustness of the results in relation to
sustainable forestry; and (3) discuss the results in relation to new insights
from the QWARTS programme and propose ways to further reduce uncertainties.
In the study we found that the variation in estimated weathering rates at
single-site level was large, but still most sites could be placed reliably
in broader classes of weathering rates. At the regional level, the results
from the different approaches were in general agreement. Comparisons with
base cation losses after stem-only and whole-tree harvesting showed sites
where whole-tree harvesting was clearly not sustainable and other sites
where variation in weathering rates from different approaches obscured the
overall balance. Clear imbalances appeared mainly after whole-tree
harvesting in spruce forests in southern and central Sweden. Based on the
research findings in the QWARTS programme, it was concluded that the
PROFILE/ForSAFE family of models provides the most important fundamental
understanding of the contribution of weathering to long-term availability of
base cations to support forest growth. However, these approaches should be
continually assessed against other approaches. Uncertainties in the model
approaches can be further reduced, mainly by finding ways to reduce
uncertainties in input data on soil texture and associated hydrological
parameters but also by developing the models, e.g. to better represent
biological feedbacks under the influence of climate change.
Forest biomass accumulation is an important source of acidity to forest soils: Data from Swedish inventories of forests and soils 1955 to 2010
