Since the 1970s, when anthropogenic acidification was recognized as a wide spread phenomenon in many European and North American areas, tremendous progress has occurred in the documentation, understanding, and predictions of acid rain effects on element cycling in waters and soils and whole ecosystem functioning. In parallel with significantly reduced emissions of S (and also N in some regions), numerous terrestrial and aquatic ecosystems have been significantly recovering from acidic stress (especially in central Europe) since the late 1980s.
While the chemical recovery, manifested by decreasing terrestrial export of H+, strong acid anions, base cations, and ionic Al, had been expected, the rapid increase in DOC leaching was a surprise, not predicted by available models and previous experience. Nitrate has become the dominant strong acid anion in terrestrial export in some areas, which increased scientific interest in N-saturation. The increased DOC leaching, N-saturation, combined effects of climate change and atmospheric pollution on nutrient cycling in terrestrial ecosystems, and possible trajectories of their recovery have become new targets of the present environmental research in the acidified areas.
The new environmental models combine more individual cycles, like gears in an “ecosystem clock” and use them to explain observed changes in ecosystem functioning. The future research should focus on deeper understanding of relative roles of these cycles in ecosystems. For example, elevated deposition of N and S compounds caused changes in soil fungi to bacteria ratio, decline in soil water pH, and elevated availability of electron acceptors (SO42- and NO3-) in anoxic soil microsites. All these changes may individually have only relatively small effect on soil DOC concentrations. But their combined effect can reduce DOC availability to soil microorganisms to such an extent that some microbial process shift from N to DOC limitation, resulting in elevated NO3- leaching. The elevated terrestrial export of NO3- is accompanied by Al leaching, which affects P availability in aquatic ecosystems. One change can thus affect cycles of many elements in the whole system.
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