Losses of nitrate from gaps of different sizes in a managed beech (Fagus sylvatica) forest

In the ongoing discussion about sustainable forestry, gap regeneration is suggested to reduce nitrate (NO3-) losses from forest ecosystems. The effect of gap formation and gap size on soil moisture and NO3- leaching was studied in two managed beech (Fagus sylvatica L.) stands in Denmark for about 2 years after formation of four gaps (approx. 20 and 30 m in diameter). Soil moisture content, soil solution NO3-N concentrations, and nitrogen (N) concentrations in throughfall and precipitation were measured along transects from the gaps into the surrounding forests. Losses of NO3-N were estimated using the water balance model WATBAL. Soil moisture content in gaps remained close to field capacity throughout the year, while it decreased to 50%-70% of field capacity under the closed canopy during the growing season. Drainage water fluxes, soil solution NO3-N concentrations, and NO3-N losses were increased in the gaps as compared to under the canopy. For the whole study period, losses of NO3-N were 3- to 13-fold higher in the gaps than in the surrounding forests. However, a significant effect of gap size was not found within the range of the investigated gap diameters and canopy heights. Presumably, not only the aboveground canopy gaps, but also the belowground root gaps affected soil moisture and thus drainage water fluxes and NO3- losses.