Nitrogen Saturation and Carbon Sequestration in a Coniferous Forest Catchment: 32 years of Nitrogen Additions at Gårdsjön, Sweden

At Gårdsjön, Sweden, a long-term experiment has been conducted in which a 5250-m2 coniferous-forested catchment (G2 NITREX) receiving 8 kg N ha−1 yr−1 ambient N deposition has been given an additional 40 kg N ha−1 yr−1 in 20 small portions yearly. Catchment inputs and outputs of N have been monitored. The soils were systematically sampled in 1990 and 2016. Over 26 years (1991—2016) cumulative inputs were 1290 kg N ha−1 (deposition 230, additions 1030, nitrogen fixation 30). Cumulative outputs were 231 kg N ha−1 (runoff dissolved inorganic N 89, runoff dissolved organic N 55, denitrification 87). The ecosystem retained about 80% (1060 kg N ha−1) of the incoming N in the biomass and soils. About 80% of the retained N was found in the soil, mostly in the mineral horizons. The C/N ratio of the organic soil decreased from 32 to 29. The remaining 20% of the retained N went to plant biomass. The enhanced N deposition has resulted in sequestration of 44 t C ha−1. The C sequestered in the ecosystem had a C/N ratio of 34. The ecosystem has moved towards N saturation; the fraction of N added that is leached to runoff has increased from about 1% in 1991 to nearly 30% in 2022, 32 years later. As the N leaching and denitrification increase, less of the incoming N is available for C sequestration.