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 NH, NO and HNO 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.