Effects of dolomite lime and clearcut on nitrate leaching from acid forest soils

Forest liming has received increased attention since the 1980s, when unusual needle yellowing and fall were observed in large parts of Europe and North America. This ‘new forest decline’ has been attributed to several causes acting individually or in synergy, amongst which were 'acid rain', nitrogen saturation and nutritional imbalances. Liming at moderate doses was suggested as a counteracting measure against soil acidification and to remove nutritional deficiencies. Another major forest management operation is whole tree harvesting. Both liming and clearcut may however affect the water quality leaving the treated areas. In particular, nitrate leaching is of major concern, as nitrate leaching may be linked to soil acidification, associated cation and/or aluminium release, and nitrate contamination of surface and groundwater.
In this paper, I summarise main results of studies performed in the Belgian Ardennes. In this area, the acid brown soils are naturally poor in magnesium and the observed forest dieback symptoms were supposed to be the consequence of increased pollution exacerbating magnesium deficiency. Liming was suggested for prevention and correction. However, possible side effects, in particular on soil solution and stream water chemistry needed to be evaluated. Another major concern was the effects of clearcut operations on the streamwater quality in this area important for drinking water collection.
The effects of dolomite lime were investigated at the watershed, plot and laboratory level. Paired watersheds of approximately 80 ha were mainly covered with Picea abies. One watershed of each pair was limed with 3 T/ha of fine ground dolomite and 200 kg/ha K2SO4. Runoff chemistry was analysed for major cations and anions and time series intervention analysis was used to detect statistically significant changes in stream water chemistry. Results showed increased magnesium concentrations immediately after liming, but a major part of the lime was retained by the soil system, and consequences on water chemistry were minor. However, in the plot study, where 5 T ha-1 of a dolomite lime suspension were applied to adjacent Quercus petraea and Picea abies stands, soil solution nitrate concentrations were significantly increased through liming in the Quercus plots. Increased net nitrification in the Quercus stand was confirmed by potential net nitrification measurements in the laboratory.
The effects of tree harvesting on streamwater ion concentrations were investigated by clear felling 22 ha (9650 m3 wood) of a 81 ha watershed. This area represented one third of the forested area, the remaining area being covered equally with young plantations and 70-100 year old Picea abies. First data indicate increased potassium, iron, lead and organic carbon concentrations, but no major change in streamwater nitrate concentrations.
Results will be discussed in relation to literature data and known controlling factors of the nitrification process. This analysis reveals that the speed, extent and duration of a perturbation in nitrate leaching after liming or felling vary at different sites. Risks assessment for nitrate leaching is therefore difficult and prediction of nitrate leaching in response to a perturbation remains an important challenge for further studies. In particular, basic controls of the nitrification process and relation nitrification/immobilisation need further investigation.