Reduced microbial respiration sensitivity to soil moisture following long-term N fertilization enhances soil C retention in a boreal Scots pine forest

Abstract. Nutrient availability effects microbial respiration kinetics and their sensitivities to environmental conditions, thus the soil organic C (SOC) stocks. We examined long-term nitrogen (N) addition effects on soil heterotrophic respiration (R_h), methane (CH₄) oxidation, and nitrous oxide (N₂O) emissions in an N-limited boreal Scots pine (Pinus sylvestris) forest. Measurements included long term 1960–2020 tree biomass monitoring, 2023 SOC, 2021–2023 monthly aboveground litterfall, 2021–2023 growing seasons biweekly CO₂, CH₄, and N₂O fluxes, and quarter-hourly soil temperature (T), and soil water content (SWC) in both control and N-fertilized plots. We assessed mean greenhouse gas (GHG) flux differences and R_h dependence on T and SWC using polynomial and parametric non-linear regression models.

Tree biomass, litterfall and SOC increased with long-term N fertilization. However, N fertilization significantly increased mean R_h, reduced CH₄ oxidation slightly, and modestly raised N₂O emissions. SOC-normalized R_h (R_h/SOC) did not significantly differ between treatments, yet relationships between R_h/SOC and T and SWC diverged with fertilization. In control plots, R_h/SOC peaked at 15 °C but increased monotonically with T in N-fertilized plots. Under N fertilization, R_h/SOC was weakly SWC-dependent, contrasting with a distinct humped SWC response in control plots, enhancing annual R_h/SOC. Annually, N-fertilized plots respired 11.2 % of SOC, compared to 12.6 % in controls, suggesting N fertilization promoted SOC retention. Consequently, N fertilization reduced net CO₂ emissions by 262.5 g CO₂ m⁻² year⁻¹, while combined effects on CH₄ and N₂O fluxes and the production energy of N fertilizer contributed a minor CO₂-equivalent increase of 15.8 g CO₂-eq m⁻² year⁻¹.

In conclusion, long-term N fertilization in boreal forests could mitigate climate warming by reducing soil GHG emissions, slowing R_h/SOC, and altering its responses to T and SWC, thereby enhancing SOC sequestration in addition to the increased tree biomass carbon sink.