Duration of forest fertilization effects on streamwater chemistry in a catchment in central Sweden

Abstract

Demands for forest biomass production for energy, construction and carbon storage purposes are increasing, and therefore measures to increase tree growth are required. One potential measure is nitrogen (N) fertilization, as N is usually the most growth-limiting nutrient in boreal forests and partly due to decreasing atmospheric N deposition in northern Europe in recent decades. However, N fertilization can have adverse effects, such as soil acidification and N leaching, particularly nitrate leaching via streamwater flow. To mitigate the acidification risk, dolomite (CaMg(CO 3 ) 2 ) is added to N fertilizer boron (B) as increased tree growth hamper tree uptake of this essential micronutrient.This study examined the effects of forest fertilization on streamwater chemistry in the 45 ha Swedish catchment Risfallet (RF), around 80% of which was treated with fertilizer. That was rather exceptional, as most previous catchments studied have had <50% treated area, which may give weaker treatment signals. A paired catchment method combined with the control area and calibration period technique was applied to evaluate leaching effects from forest fertilization. Effects over 7.5 years were compared with previously reported initial effects in the first year, in order to assess the duration of fertilization effects on surface water. High excess outflow of N over five years was detected, with 20% of the applied amount leached and with nitrate dominating total nitrogen. Excess outflow of Ca and Mg was highest in the first year. Effects on pH were limited, with calculated untreated pH on 5.9 being on average 0.4 units lower during the first six months and then remaining at 0.2 units lower.Recommended could be to mainly fertilize well-drained soil, avoid wet areas and open streams. Consider the hydrological conditions while weather would be more hazardous to foresee.