Ammonium nitrate, soil moisture, temperature and oxygen level effects on soil methane flux across two growing seasons in a temperate forest

Methane (CH4) is an important greenhouse gas, which is produced and consumed in soil by two groups of microorganisms, the 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. An in situ nitrogen addition experiment was performed on temperate coastal pine forest soil. Ammonium nitrate in solution with water was applied at regular intervals during the 2008 and 2009 growing seasons at each of two different water table heights across an elevation transect away from a creek. The plots were divided evenly among two N levels and a control. The lower N 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 and ammonium and nitrate extracted at various intervals before and after nitrogen addition. Soil moisture, temperature and oxygen content were recorded continuously. In 2008, both the amount of nitrogen added and the water table height affected CH4 uptake, with low and high N level plots differing significantly from each other, but not from the control plots. Low N levels were found to stimulate CH4 uptake across plots, while high levels inhibited CH4 uptake or increased CH4 production. The trend differed in the 2009, with control plots consuming more CH4 than either of the N treatments in the wetter plots. There were correlations between CH4 release and higher nitrate and soil moisture content across sites, with inverse correlations between CH4 release and temperature. Further studies matching CH4 flux response to microbial community composition are being performed in order to more accurately test this phenomenon.