The influence of N load and harvest intensity on the risk of P limitation in Swedish forest soils

Nitrogen (N) is often considered to be the major factor limiting tree growth in northern forest
ecosystems. An increased N availability, however, increases the demand for other nutrients
such as base cations and phosphorous (P) which in turn may change which nutrient is the
limiting factor. If P or base cations become limiting, N will start to leach which means a risk
of increased eutrophication of surface waters. As many studies focus on base cations, this
study instead aims at estimating P budgets on a regional scale for different harvesting
scenarios relevant for Swedish conditions.
P budget calculations were carried out for 14,550 coniferous sites from the Swedish National
Forest Inventory, as weathering+deposition–harvesting–leaching. Three scenarios with
different harvest intensities were used: 1) no harvesting, 2) stem harvesting and 3) wholetree
harvesting. The input data were derived from measurements and model results.
The P budget estimates indicate that harvesting, especially whole-tree harvesting, result in
net losses of P in large parts of Sweden. The highest losses were found in southern Sweden
due to high growth rate in this area. In the whole-tree harvesting scenario the losses
exceeded 1 kg ha−1 y−1 on many sites. N budget calculations on the same sites indicate that
N generally accumulates in the whole country and especially in the southern parts.
Consequently, the N and P budget calculations indicate that the forests in southern Sweden
are in a transition phase from N-to P-limitation to growth. This transition will proceed as
long as the accumulation of N continues. These results are important in a sustainable
forestry context, as a basis for assessing the risk of future N leaching, and in designing
recommendations for abatement strategies of N deposition and for application of wood ash
recycling and N fertilization.