Greenhouse Gas Flux From Fertilized Switchgrass.

Switchgrass (Panicum virgatum L.) is being evaluated as a bioenergy crop for the United States.  Field measurements of CO₂, CH₄, and N₂O flux from switchgrass are needed to estimate the net greenhouse gas (GHG) balance of this biofeedstock.  The study objective was to determine effects of N fertilization on growing-season soil-atmosphere CO₂, CH₄, and N₂O flux from established switchgrass stands.  The study was conducted in 2010 near Mandan, ND on a Parshall fine sandy loam.  Static chamber methodology and gas chromatography were used to measure CO₂, CH₄, and N₂O flux between 24 May and 14 Sep 2010 from unfertilized switchgrass and switchgrass fertilized with 67 kg N ha^-1.  Soil temperature and water-filled pore space (WFPS) affected growing season CO₂, CH₄, and N₂O flux (P<0.01).  Carbon dioxide flux was greatest during periods of active switchgrass growth, but differed rarely between N fertilizer treatments.  In contrast, N fertilization tended to inhibit CH₄ uptake when soil conditions were warm and dry, while N₂O flux was consistently greater under N fertilization than without N throughout the growing season.  Overall, N fertilization of switchgrass affected cumulative growing-season N₂O flux (9.2±1.3 vs. 29.0±4.8 mg N₂O-N ha^-1 for 0 and 67 kg N ha^-1, respectively; P<0.01), but not cumulative CO₂ or CH₄ flux (P=0.08 and 0.51, respectively).  Aboveground biomass production was greater with N application (6.8±0.5 Mg ha^-1) than without N (3.2±0.5 Mg ha^-1) (P<0.05).  Conversely, net greenhouse gas intensity (GHGI) for switchgrass production was similar between N application (2.4 Mg CO_2e Mg^-1) and without N (4.2 Mg CO_2e Mg^-1) (P=0.18).  Results from this study suggest greater N₂O emissions will occur when switchgrass is fertilized with N, but increased biomass production from N fertilizer results in similar GHGI.