Effect of nitrogen deposition on CH4 uptake in forest soils in Hokkaido, Japan

It has been well documented by short-term artificial experiments that the CH, uptake is inhibited by N input, especially NH4+-N input. To investigate the effect of the natural N input by throughfall and other factors on the CH, uptake in forest soils, we measured the CH4 uptake rates for 6 months during the snow-free period of the year and N input by throughfall throughout the year at 10 sites in Hokkaido, Japan, from 1997 to 2002. Water-filled pore space (WTPS) and pH values in the soils varied widely among the sites (38-93% and 3.9-6.2, respectively). The rates of NH4+-N and NO4+-N inputs ranged from 1.3 to 6.9 kg N ha(-1) year(-1) and from 0.8 to 2.9 kg N ha(-1) year(-1), respectively. The NH4+-N input was generally higher than the NO3--N input. Total N input by throughfall amounted to 2.3-9.4 kg N ha(-1) year(-1). The highest CH4 uptake rate occurred within the period from July to September (41-215 mug CH4 m(-1) h(-1)) each year at most sites. CH4 uptake rate was relatively low (similar to50 mug CH4 m(-1) h(-1)) at northern sites, while a high CH4 uptake rate was observed throughout the year (-- 100 mug CH4 m(-2) h(-1)) at southern sites. The mean CH4 uptake rates were significantly different among the sites. Cumulative CH4 uptake ranged from 1.4 to 6.6 kg CH4 ha(-1) [184 d](-1) with a mean values of 3.22 +/- 1.36 kg CH4 hat [184 d](-1). Cumulative CH4 uptake increased with increasing temperature and decreased with an increase in precipitation (Rain), NH4+-N input (TFNH4), WFPS, soil total C (TC), and total N (TN). There was a quadratic relationship between the CH4 uptake and NO3--N input (TFNO3), soil pH, and C / N ratio in soil. A regression equation was obtained as follows to predict the CH4 uptake in forest soils: Cumulative CH4 uptake = 0.47 / Rain + 0.38 / TFNH4 + 0.34 / TC - 0.30 / TFNO3 (R-2 = 0.74, p = 0.0001). This equation indicates that atmospheric N input into forest soils is one of the main factors that control cumulative CH4 uptake with precipitation, total carbon content in soil in Hokkaido, Japan.