The magnitude, temporal, and spatial patterns of soil-atmospheric greenhouse gas (hereafter referred to as GHG) exchanges in forests near the Tropic of Cancer are still highly uncertain. To contribute towards an improvement of actual estimates, soil-atmospheric CO 2 , CH 4 , and N 2 O fluxes were measured in three successional subtropical forests at the Dinghushan Nature Reserve (hereafter referred to as DNR) in southern China. Soils in DNR forests behaved as N 2 O sources and CH 4 sinks. Annual mean CO 2 , N 2 O, and CH 4 fluxes (mean AE SD) were 7.7 AE 4.6 Mg CO 2 -C ha À1 yr À1 , 3.2 AE 1.2 kg N 2 O-N ha À1 yr À1 , and 3.4 AE 0.9 kg CH 4 -C ha À1 yr À1 , respectively. The climate was warm and wet from April through September 2003 (the hot-humid season) and became cool and dry from October 2003 through March 2004 (the cool-dry season). The seasonality of soil CO 2 emission coincided with the seasonal climate pattern, with high CO 2 emission rates in the hot-humid season and low rates in the cool-dry season. In contrast, seasonal patterns of CH 4 and N 2 O fluxes were not clear, although higher CH 4 uptake rates were often observed in the cool-dry season and higher N 2 O emission rates were often observed in the hot-humid season. GHG fluxes measured at these three sites showed a clear increasing trend with the progressive succession. If this trend is representative at the regional scale, CO 2 and N 2 O emissions and CH 4 uptake in southern China may increase in the future in light of the projected change in forest age structure. Removal of surface litter reduced soil CO 2 effluxes by 17-44% in the three forests but had no significant effect on CH 4 absorption and N 2 O emission rates. This suggests that microbial CH 4 uptake and N 2 O production was mainly related to the mineral soil rather than in the surface litter layer.

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The spatial and temporal variations in soil respiration and its relationship with biophysical factors in forests near the Tropic of Cancer remain highly uncertain. To contribute towards an improvement of actual estimates, soil respiration rates, soil temperature, and soil moisture were measured in three successional subtropical forests at the Dinghushan Nature Reserve (DNR) in southern China from March 2003 to February 2005. The overall objective of the present study was to analyze the temporal variations of soil respiration and its biophysical dependence in these forests. The relationships between biophysical factors and soil respiration rates were compared in successional forests to test the hypothesis that these forests responded similarly to biophysical factors. The seasonality of soil respiration coincided with the seasonal climate pattern, with high respiration rates in the hot humid season (April-September) and with low rates in the cool dry season (October-March). Soil respiration measured at these forests showed a clear increasing trend with the progressive succession. Annual mean (± SD) soil respiration rate in the DNR forests was (9.0 ± 4.6) Mg CO 2 -C/hm 2 per year, ranging from (6.1 ± 3.2) Mg CO 2 -C/hm 2 per year in early successional forests to (10.7 ± 4.9) Mg CO 2 -C/hm 2 per year in advanced successional forests. Soil respiration was correlated with both soil temperature and moisture. The T/M model, where the two biophysical variables are driving factors, accounted for 74%-82% of soil respiration variation in DNR forests. Temperature sensitivity decreased along progressive succession stages, suggesting that advanced-successional forests have a good ability to adjust to temperature. In contrast, moisture increased with progressive succession processes. This increase is caused, in part, by abundant respirators in advanced-successional forest, where more soil moisture is needed to maintain their activities.