Methane flux response to nitrogen amendment in an upland pine forest soil and riparian zone

Methane (CH 4 ) is an important anthropogenic greenhouse gas, up to 15% of which is consumed by terrestrial soils. In this field study of the CH 4 cycle of a pine forest, 18 plots were established at each of two sites, located 40 m apart. The upper site was well‐drained and the lower site was poorly drained, but they shared similar overstory tree composition. Nitrogen was added as NH 4 NO 3 incrementally across the 2009 growing season in a high (67 kg NH 4 NO 3 ha −1 yr −1 ) and a low (5 kg NH 4 NO 3 ha −1 yr −1 ) concentration. The sites were monitored for soil and air temperature, soil moisture, precipitation, air pressure, and NH 4 and NO 2 +NO 3 concentrations throughout the growing season. Across all treatments for the duration of the field season, average CH 4 flux showed consumption of −0.84 kg CH 4 ha −1 yr −1 , but CH 4 flux differed between the upper and lower sites. Across all treatments, upper site CH 4 flux averaged −5.38 kg CH 4 ha −1 yr −1 , while lower site flux averaged 3.72 kg CH 4 ha −1 yr −1 , with greater variance than was observed at the upper site. High N treatments caused greater CH 4 release than the control in the lower, but not the upper, site. The main correlated variable with CH 4 flux was soil moisture; however, it accounted for <14% of the variation. Statistics were run several different ways, resulting in multiple environmental factors contributing up to 84% of the variation in CH 4 flux. Long‐term drainage differences between the sites likely drove the differences in CH 4 flux.