Effect of nitrogen fertilization on soil CH4 and N2O fluxes, and soil and bole respiration

The effect of nitrogen addition, through deposition or fertilization, on ecosystem processes responsible for storing and releasing greenhouse gasses (GHG) CO2, CH4 and N2O are poorly understood. We measured the effects of winter application of 200 kg N ha− 1 to a near-end-of-rotation coastal Douglas-fir stand on soil CH4 and N2O fluxes, soil and bole respiration (Rs and Rb, respectively), and soil CH4 and N2O concentrations. Fertilization resulted in a significant decrease in CH4 uptake and changed a small uptake of N2O to significant emissions. Based on the average reduction of 2 μmol m−2 h− 1 in CH4 uptake with N fertilization, from an average of 4 μmol m−2 h− 1 with no N fertilization, the approximate first-year annual reduction was estimated to be 2.8 kg CH4 ha− 1. In comparison, net annual increase in N2O emissions with N fertilization was 17.6 kg N2O ha− 1. Calculations showed that fertilizer-induced net GHG emissions due to the reduction in CH4 uptake and the increase in N2O emissions was 0.07 and 5.3 t CO2 ha− 1 equivalents (CO2_Eqv), respectively, indicating that the reduction in CH4 uptake was much less important than the increase in N2O emissions. Contrary to expectation, the annual loss of 17.6 kg N2O ha− 1 arising from this winter application of N was slightly higher than the 15.7 kg N2O ha− 1 loss following spring application of N in the previous year. N fertilization also resulted in small increase in soil respiration, confirming our previous results. Temporal dynamics of Rb matched those of soil respiration and N2O fluxes. Fertilization resulted in a small but persistent increase in Rb over the first 4–5 months. While soil temperature was the main control of soil N2O and CO2 fluxes as well as Rb, CH4 uptake was better correlated to soil water content, i.e., soil gas diffusivity and gas exchange with the atmosphere.

Research highlights
► Effects of winter application of 200 kg N ha- 1 to a Douglas-fir stand were studied.
► Fertilization decreased soil CH4 uptake but increased soil N2O and CO2 emissions.
► Fertilization resulted in small but persistent increase in bole respiration.
► GHG balance of CH4 uptake was smaller than that of soil N2O and CO2 emissions.
► Soil water controlled CH4 uptake and temperature controlled N2O and CO2 fluxes.