Forest fertilization transiently increases soil CO2 efflux in young Norway spruce stands in Sweden

Late-rotation fertilization of Norway spruce stands is a frequently used management tool in Fennoscandia to increase timber yields. Meanwhile, the growing demand for renewable resources has sparked great interest in earlier and repeated fertilizer application but it remains unclear how this affects carbon dioxide (CO₂) fluxes in the understory, especially forest floor respiration (R_ff). This study investigated the effects of forest fertilization on R_ff and net forest floor exchange (NFFE) in young, nitrogen (N) limited Norway spruce stands in southern Sweden. In a short-term dose experiment, R_ff and NFFE were recorded during 2016 after varying doses of N (0, 150, 300, or 450 kg ha^-1 of N, hereafter N0, N150, N300, N450) were added to circular, 3-m-diameter plots in April. In a second, long-term experiment, two stand-level fertilizer applications with 150 kg ha^-1 of N on each occasion were performed in 2014 and 2016 and R_ff was measured at semi-regular intervals from mid-2013 to the end of 2017. In the dose experiment, fertilization increased R_ff by 23 %, 81 % and 55 % in the N150, N300 and N450 treatments, respectively. Under well-lit conditions, the N300 and N450 treatments significantly enhanced photosynthetic CO₂ uptake of the forest floor vegetation by 97 % and 66 %, respectively, while the N150 treatment had no significant effect. The results of the long-term experiment indicate an initial stimulation of R_ff, but this effect was transient. Our findings imply that fertilization in young Norway spruce stands, using the N150 dose (the typical dose used in Swedish forestry), may cause a transient burst in R_ff that is far outweighed by nutrient-driven increases in forest floor photosynthesis under favourable light conditions prior to canopy closure.