Methane uptake responses to nitrogen deposition in three tropical forests in southern China

Methane (CH4) uptake responses to simulated nitrogen (N) deposition in a mature forest, a rehabilitated forest and a disturbed forest in tropical China were studied. The experiment was designed with four N treatment levels (three replicates) (0, 50, 100, 150 kg N ha(-1) a(-1) for Control, Low-N, Medium-N, and High-N treatment, respectively) in the mature forest, but only three levels (Control, Low-N, and Medium-N) in the disturbed and rehabilitated forests. Between October 2005 to September 2006, soil CH4 flux was measured once a week from April to September and once every other week in the other time using static chamber and gas chromatography techniques. Monthly ammonium-nitrate (NH4NO3) application had been applied previously to the plots since July 2003 and continued during the CH4 flux measurement period. The average CH4 uptake rates in control plots were -41.1 +/- 1.8, -28.6 +/- 2.2, and -17.8 +/- 1.6 mu g CH4-C m(-2) h(-1) in the mature, rehabilitated, and disturbed forest, respectively. For the mature forest, average CH4 uptake rates decreased by 6, 14, and 32% when compared to the control plots for the Low-N, Medium-N, and High-N plots, respectively. These decreases in soil CH4 uptake mainly occurred in the fall (October and November). Nitrogen additions had no significant effect on CH4 uptake in the rehabilitated and disturbed forests. Our results suggest that the response of soil CH4 uptake to N deposition in tropical forests may vary depending on the soil N status directly, and on land-use history of the forest indirectly.