D2: Nutrient cycling and critical loads

Nutrient cycling is one of the key factors in the functioning of forest ecosystems. Many forest ecosystems in Europe, however, are being subjected to an excessively high nitrogen input and, in some areas, the forests are no longer able to utilize the excess nitrogen input. In the most severe cases this has led to so-called nitrogen saturation of the ecosystem, and leaching of nitrogen (primarily nitrate) into the groundwater and subsequently into the water courses. Another consequence of excess nitrogen is eutrophication, which can lead to e.g. a loss of biodiversity, especially of plants. The capacity of forest ecosystems to utilize and withstand nutrient and acidifying deposition is affected not only by the amount and composition of the deposition but also by, for example, the soil type, forest type, species composition and forest vitality. In the future many of these parameters will be affected by climate change, leading to a parallel change in the capacity of the forests to utilize nutrients and to neutralize acidifying deposition.

The critical loads concept provides a method for estimating the effects of acidification and eutrophication on specific forest ecosystems, for instance by studying nutrient cycling in different types of forest site type subjected to different deposition loads. When combined with scenarios of future climate and vegetation changes, critical loads can be used to identify those areas where eutrophication and acidification will potentially cause problems in ecosystem functioning, e.g. deterioration of water quality and biodiversity loss. This information, combined with scenario analyses of the future impacts of air pollution loads based on dynamic modelling and climate change scenarios, will allow the formulation of predictions and pinpoint potential future problem areas.

Implementation

The following surveys will be conducted on 195 intensive monitoring plots by 29 partners in 19 countries.

• Litterfall (mass and element concentrations) in accordance with the methods defined in ICP Forests Manual Chapter XI "Litterfall"
• Soil solution (chemical composition) in accordance with the methods defined in ICP Forests Manual Chapter IIIb "Soil Solution"
• More intensive foliar surveys (nutrient concentrations of all leaf age classes)
• Nutrient budget of the ground vegetation

Expected results

• Data on nutrient fluxes and deposition as a basis for the calculation of critical loads and as a basis for deriving information on nutrient fluxes through the soil:
  loss of base cations
  loss of nitrogen / nitrate flux to the ground water
  input / output balance of individual nutrients
• Estimations of nutrient budgets in vegetation (content and output)
• Estimations of critical loads on the plots
• Estimations of critical load exceedances on the plots
• Predictions on whether the critical loads will change if the vegetation cover on the plots