Twenty years of nitrogen deposition to Norway spruce forests in Sweden

The yearly, total (dry+wet) deposition of inorganic nitrogen (inorg-N) to Norway spruce forests was estimated with a full spatial coverage over Sweden for a twenty-year period, 2001-2020, based on combined measurements with Teflon string samplers, throughfall deposition and bulk deposition to the open field. The results were based on a novel method to apply estimates of the dry deposition based on measurements at a limited number of sites, to a larger number of sites with only bulk deposition measurements, in turn based on the existence of a strong geographical gradient in the dry deposition of inorg-N from southwest to northeast Sweden. The method should be applicable for other geographical regions where gaseous NH3, NO2 and HNO3 are not main drivers of N dry deposition and where geographical gradients in dry deposition could be defined. It was shown that Norway spruce forests in south Sweden receive more N from deposition than has been previously estimated, based on modelling. Clear time trends were demonstrated for decreased deposition of inorg-N to Norway spruce forests in all parts of Sweden. The decreases were somewhat larger than what could be expected from the decrease in the reported emissions of inorg-N from Europe. The results emphasize that estimates of the total deposition are necessary in order to map levels and follow the development of N deposition in forests.

Automated summary

This study estimated the yearly, total (dry+wet) deposition of inorganic nitrogen to Norway spruce forests in Sweden for a twenty-year period using a novel method. The results showed that spruce forests in south Sweden receive more nitrogen deposition than previously estimated, and there is a clear time trend of decreased deposition of inorganic nitrogen in all parts of Sweden. The findings emphasize the importance of estimating total deposition in order to map levels and follow the development of nitrogen deposition in forests.