Uncertainties in the assessment of critical loads for acidic deposition are
caused by the choice of biological indicators (BI), critical chemical valu
es (CCV), the current methods used to determine critical loads for an ecosy
stem, and deficient field data. This paper focuses on the present steady-st
ate mass balance (SSMB) approach, dynamic models and the importance of chan
ges in atmospheric base-cation deposition (BCD), particularly in China. It
is argued that 1) for the SSMB approach much uncertainty may come from the
choice of BI and the related CCV, and long-term and large-scale monitoring
data on weathering rates and growth uptake are urgently needed, especially
in China, 2) significant uncertainty may be caused by changes in BCD during
SO2 emission controls, particularly in China, 3) constructing a mechanisti
c Al submodel may be a promising direction for dynamic models, and 4) the n
utrient cycle in the vegetation through biogeochemical processes should be
incorporated into dynamic models but the input requirements should be moder
ate for broad application considerations. Generally higher BCD, different s
oil components and characteristics and different vegetation types in China
compared to Europe and North America suggest that more field investigations
on BI and their corresponding CCV be carried out before application of cur
rent approaches to specific areas, particularly in China.