Initial effects of forest N, Ca, Mg and B large-scale fertilization on surface water chemistry and leaching from a catchment in central Sweden

• Large scale forest fertilization resulted in considerable nitrogen and calcium leaching.
• High nitrate concentrations were reached in the first months after fertilization.
• Total nitrogen outflow in the first year was 14% of applied fertilizer.
• Small pH decline with short-term duration occurred after the fertilization.
• Small amounts of boron (6%) were leached after application.

Highly increased use of biomass production is placing great demands on Swedish forests. Several silvicultural measures can be implemented to increase forest production and fertilization being addressed in this paper is one. Forest companies are now increasingly applying fertilizer, with the main nutrient needed for high forest growth being nitrogen (N).This study investigated how commercial N fertilization (150 kg N/ha, including also Ca, Mg and B) of the 45 ha forest catchment Risfallet in central Sweden affected chemical composition and runoff export in stream water during one year after the fertilization. This well-defined and long-term monitored catchment proved very suitable for studies of water quality and nutrient losses. The fertilizer consisted of ammonium nitrate (50/50 ammonium and nitrate, respectively), plus calcium (Ca, 22 kg ha−1) and magnesium (Mg, 12 kg ha−1) to mitigate acidification and boron (B 1.1 kg ha−1) to compensate for decreased boron availability. The study was carried out according to the paired catchment method using a control area and a calibration period. Data from the after treatment period were compared with previous 25-year monitoring data for the catchment and also data for a similar 83 ha control catchment, Gusseltjärn, in the same region. During the first year after treatment, the nitrate concentration in stream water increased from 0.05 mg L−1 to 3.3 mg L−1 on average. Other elements showing increased concentrations were ammonium (300%), B (3-fold), Mg (80%), Ca (60%), potassium (K) (50%) and sodium (Na) (40%). The pH decreased in the first half-year by 0.2 pH-units. One year after treatment, 14% of the N applied had leached from the catchment.