Signs of recovery in polluted Czech forests
Pollution has had a significant impact on European forests, where it disrupts many natural processes. Forests in Central Europe are among the most severely affected by pollution worldwide. New research1 from the Czech Republic suggests that attempts to cut sulfur-based pollution (such as acid rain) could also be important in a longer-term reduction of nitrate output from forest watersheds as these ecosystems begin to recover.
Human activity has altered the nitrogen cycle globally since the industrial revolution, with atmospheric pollution and fertiliser use two of the biggest human inputs into the cycle. Increased nitrogen entering forest ecosystems alters forests' productivity and nutrient cycles. Nitrogen can be stored in plants and soil, which can lead to increasingly acidic soil conditions. Minerals may then leach from the soil, with a negative effect on surrounding land and water-based ecosystems. When more nitrogen is available than the ecosystem can take in, nitrogen makes its way into surface and ground water and into the atmosphere.
The programme to reduce sulfur emissions adopted in the Czech Republic between 1993 and 1998 dramatically reduced airborne pollution, in particular sulfur dioxide (SO2) emissions, which killed large areas of Czech forest in the 1970s and 1980s. Unlike many other areas in Europe which have seen a reduction in nitrogen emissions, these have remained relatively static in the Czech Republic. Nitrogen emissions are caused by increased road traffic, and will continue to be a major concern in Central Europe for decades to come.
Since 1994, scientists have monitored fourteen small forest areas in the Czech Republic (known as the GEOMON network2), taking regular water and soil measurements. In these water and soil samples, nitrogen compounds have become the most important source of acidification since 2000, while sulfate levels have declined steadily, in line with reductions in sulfur emissions.
The amount of nitrates exiting the ecosystem dissolved in stream water is closely linked to the amount of sulfate present, and not just to the amount of nitrogen entering the system, the researchers suggest. This may be partly due to the impact of the sulfur deposition on tree health and the carbon: nitrogen ratio. As sulfur levels in the ecosystem are reduced over time, tree health improves and microbial activity in the forest floor increases. The scientists predict that this will lead to a decline in nitrate outputs during the next decade, despite the continuing problem of nitrogen emissions.
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