8 Ammonium nitrate and soil moisture effects on methane flux in a temperate forest

Tuesday, August 4, 2009: 4:00 PM
La Cienega, Albuquerque Convention Center
Emma L. Aronson , Biology, Ecology & Evolution Group, University of Pennsylvania, Philadelphia, PA
Brent Helliker , Biology, University of Pennsylvania, Philadelphia, PA

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. Recent research on the effects of in situ environmental factors on methane has found that the interaction between nitrogen and CH4 flux is more complex, with small amounts of nitrogen often stimulating methane uptake by soil too quickly to be explained by microbial community shifts. One hypothesis for this unexpected, observed response is that the ratio of available N to CH4 in the soil, when given sufficient oxygen, will determine whether CH4 uptake is inhibited or augmented by additional N. When the ratio of available N to CH4 is low, added N may be used by methanotrophs to metabolize available methane, thus stimulating methane uptake. Once the ratio of available N to CH4 is high, any additional available N in ammonium form will begin binding to the methanotroph enzyme receptor in place of CH4, leading to decreased methane uptake.
This hypothesis was tested with an in situ nitrogen addition experiment, performed on temperate coastal pine forest soil. Ammonium nitrate in solution with water was applied twice to 9 plots at each of two different water table heights. The applied N levels replicated both increased atmospheric deposition (low) and fertilization levels (high). Methane flux was measured and ammonium and nitrate extracted at various intervals before and after nitrogen addition. Soil moisture and temperature were recorded continuously.

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. There were correlations between flux and ammonium levels, as well as with soil moisture content across sites. Further studies matching CH4 flux response to microbial community composition are being performed in order to more accurately test this phenomenon.