Nutrient budgets in forest soils at increased biomass removal – do Swedish and Finnish studies provide the same conclusion?

Nutrient budget calculations in forest soil are an important tool to assess the environmental effect of increased biomass harvesting and can be applied as a tool for policy makers to assess the environmental effect of different biomass scenarios. Environmental effects include losses of nutrients (phosphorous, nitrogen, base cations), which can reduce soil fertility and the soil buffering capacity against acidification.

Nutrient budget calculations are performed by means of a simple mass balance model where nutrient inputs to the forest ecosystem (weathering, deposition, nitrogen fixation) are compared with the outputs (leaching and harvesting). Nutrient budget calculations have been calculated both at site level and at regional level in Sweden and Finland. This study aims at comparing nutrient budget calculations focusing on conclusions and indata applied.

Despite the application of different inputs to the regional base cation budget calculations in Sweden and Finland, the result is similar, i.e. showing that the weathering rates is of the same magnitude or larger than the removal of nutrients at stem harvesting (particularly for Ca and Mg) except for some areas of the country. However, at whole tree harvesting, the output exceeds the input in many areas. When calculating the base cation balance also including leaching and deposition (only for Sweden), the output exceeds input in most of Sweden for Ca, Mg and K even at conventional stem harvesting.

In Finland, nutrient budgets for N and P have been calculated at site level, but not yet at the regional level. In Sweden, the regional mass balance calculation shows losses of P throughout the country, even at conventional stem harvesting. The N budget varies within the country, with nitrogen accumulation in southern Sweden and nitrogen losses in the north.

The similar conclusions from the Swedish and Finnish studies have strengthened the view of nutrient budget calculations as a useful tool to assess effects of increased biomass harvesting. Further work should concentrate on reducing uncertainties further, e.g. regarding nitrogen fixation and leaching of base cations. It is suggested that recommendations for nutrient compensation should be based on a combination of results from experiments (on intensive harvesting effects and fertilisation), nutrient budget calculations and dynamic modelling. Furthermore, it is important to maintain long-term experiments to assess long-term effects of intensive biomass harvesting on forest ecosystems.

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