Field observations and process-based model predictions of methane flux in a pine forest soil

Methane (CH4) is the second most important greenhouse gas. It is produced and consumed in soil by two groups of microorganisms, methanogens and methanotrophs, respectively. Environmental conditions, such as temperature, moisture, and substrate availability influence the function of both microbial groups, which together determine the overall amount and direction of flux in the soil. Labile nitrogen (N) availability is known to impact methane flux; in laboratory conditions, large amounts of ammonium are known to inhibit methane oxidation by methanotrophs by binding to their oxygenase enzyme receptors in place of CH4. Field measurements were taken in an in situ nitrogen addition experiment performed on temperate coastal pine forest soil. Ammonium nitrate in solution with water was applied at regular intervals during the 2009 growing season at two different water table heights across an elevation transect away from a creek. The lower N addition level replicated a doubling in atmospheric deposition (5Kg/hayr) and the high N treatment replicated fertilization levels used in agriculture (67Kg/hayr). Methane flux was measured, while ammonium and nitrate/nitrite were extracted at various intervals before and after nitrogen addition. Soil moisture, temperature and oxygen content were recorded continuously. The total N and C content were also measured at the start and end of the growing season, as well as Bulk density at the end of the growing season. It was found that water content and position relative to water table were the primary determinants of methane flux. In the higher water table site, high N and control treatments differed in methane flux, while in the lower water table site bulk density determined flux. Simple, process-based models were created to find the best relationships between all environmental characteristics and observed methane fluxes. These were used to successfully predict methane fluxes and the model was validated with a separate set of observations.