Long-term impact of nitrogen addition on the carbon balance of a boreal pine forest in Northern Sweden

Nitrogen (N) added through atmospheric deposition or as fertilizer in boreal forests may alter their carbon (C) sequestration potential and sensitivity to climatic changes. While previous studies have primarily explored the responses of individual ecosystem components such as stem biomass production and soil carbon changes following N addition, the long-term impacts of N addition on the ecosystem-scale C balance of boreal forests still remain unclear. Here, we use data from eddy-covariance measurements in a fertilized Scots pine (Pinus sylvestris L.) forest (i.e. 16 ha receiving 100 and 50 kg N ha^-1 yr^-1 since 2006 and 2012, respectively) and an adjacent unfertilized control stand in boreal Sweden to investigate how one decade of N addition affected the net ecosystem productivity (NEP), gross primary production (GPP) and ecosystem respiration (ER) over five fertilization years (2015-2019). Results showed that N fertilization increased GPP in all five years with by 18% at average to 1183±41 g C m^-2 yr^-1 in the N-fertilized stand compared to 1003±56 g C m^-2 yr^-1 in the control stand. ER was also increased from 744±29 g C m^-2 yr^-1 in the control stand to 875±37 g C m^-2 yr^-1 in the fertilized stand. As a result, fertilization increased NEP from 259±28 g C m^-2 yr^-1 in the control stand to 308±20 g C m^-2 yr^-1 in the N-fertilized stand. Our results further suggested that the annual NEP was similar between stands during years with normal weather conditions (2015-2016) while NEP diverged due to a larger reduction in the control stand in years with environmental constraints (i.e. a cool summer in 2017 and droughts in 2018 and 2019). These findings indicate that enhanced N input to boreal forests increases and stabilizes their C sequestration potential under future climate conditions.