Preliminary studies on methane flux from the ornithogenic soils on Xi-sha atoll, South China Sea

Methane flux from the ornithogenic soils was preliminarily measured by closed chamber method on Xi-sha atoll, South China Sea during March 10 to April 11, 2003 for the first time. The CH_4 flux ranged from -226.7 μg/(m~2·h) to 226.3 μg/(m~2·h) at the observation sites on Dong Island. High atmospheric CH_4 consumption was observed from the ornithogenic soils on sunny days. CH_4 uptake rates showed the highest value after the midday and they had a strong positive correlation with soil temperatures. Under the same weather conditions, the CH_4 fluxes were also observed from the intact and disturbed soils on Yongxing Island. Results showed that the intact soils with natural vegetation also showed high atmospheric CH_4 consumption and the average flux was -141.8 μg/(m~2·h). However, disturbed soils via anthropogenic reclamation showed CH_4 emissions and the average flux was 441.7 μg/(m~2·h). Therefore land use changes may have an important effect on the CH_4 fluxes from the tropical ornithogenic soils. In addition, different observation sites show a high spatial variation in CH_4 fluxes. The wetland in salt marsh showed the CH_4 emission on Dong Island, and the dry soil sites all showed high atmospheric CH_4 consumption, suggesting that CH_4 fluxes were predominantly controlled by soil water regime. The effects of soil chemical properties on CH_4 fluxes were also analyzed and discussed in this paper.