Effect of nitrogen fertilization on soil CH 4 and N 2O fluxes, and soil and bole respiration

Abstract

The effect of nitrogen addition, through deposition or fertilization, on ecosystem processes responsible for storing and releasing greenhouse gasses (GHG) CO 2, CH 4 and N 2O 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 CH 4 and N 2O fluxes, soil and bole respiration ( R s and R b, respectively), and soil CH 4 and N 2O concentrations. Fertilization resulted in a significant decrease in CH 4 uptake and changed a small uptake of N 2O to significant emissions. Based on the average reduction of 2 μmol m −2 h − 1 in CH 4 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 CH 4 ha − 1 . In comparison, net annual increase in N 2O emissions with N fertilization was 17.6 kg N 2O ha − 1 . Calculations showed that fertilizer-induced net GHG emissions due to the reduction in CH 4 uptake and the increase in N 2O emissions was 0.07 and 5.3 t CO 2 ha − 1 equivalents (CO 2_Eqv), respectively, indicating that the reduction in CH 4 uptake was much less important than the increase in N 2O emissions. Contrary to expectation, the annual loss of 17.6 kg N 2O ha − 1 arising from this winter application of N was slightly higher than the 15.7 kg N 2O 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 R b matched those of soil respiration and N 2O fluxes. Fertilization resulted in a small but persistent increase in R b over the first 4–5 months. While soil temperature was the main control of soil N 2O and CO 2 fluxes as well as R b, CH 4 uptake was better correlated to soil water content, i.e., soil gas diffusivity and gas exchange with the atmosphere.