Fertilizer Management, Parent Material, and Stand Age Influence Forest Soil Greenhouse Gas Fluxes

Impacts of forest management on greenhouse gas (GHG) fluxes have not been well documented. Therefore, we examined GHG fluxes from Pacific Northwest Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] forest soils as affected by fertilizer type (no fertilizer or 224 kg N ha(-1) as either urea or coated urea fertilizer [CUF]), stand age (younger vs. older), and parent material (sedimentary vs. volcanic). Following spring fertilization, soil GHG fluxes were measured for four seasons. Daily N2O (0.17 mg N2O-N m(-2) d(-1)) and CO2 (2.32 g CO2-C m(-2) d(-1)) emissions increased with urea application compared with the control (N2O: 0.09 mg N2O-N m(-2) d(-1); CO2: 1.87 g CO2-C m(-2) d(-1)); however, CUF did not. Daily CH4 uptake was inhibited with both urea (0.95 mg CH4-C m(-2) d(-1)) and CUF (0.91 mg CH4-C m(-2) d(-1)) compared with the control (1.12 mg CH4-C m(-2) d(-1)). Nitrous oxide fluxes and CH4 uptake were greater in older stands. Sedimentary parent material emitted more N2O and inhibited CH4 uptake relative to volcanic parent material. Urea increased annual N2O flux by 0.48 kg N2O-N ha(-1) yr(-1) and CO2 flux by 1.6 Mg CO2-C ha(-1) yr(-1) and decreased CH4 uptake by 0.7 kg CH4-C ha(-1) yr(-1). The global-warming potential (GWP) after urea and CUF application was 1.7 and 1.1 Mg CO2 equivalent ha(-1) yr(-1), respectively, greater than the unfertilized control for the first year after fertilization. Nitrogen fertilization had little or no effect on GWP when considered the added growth benefit from fertilization.