Seasonal changes and yearly budgets of CH4 and CO2 fluxes at lotus field in Japan (Formerly Paper 9.14)

The lotus field is a kind of agricultural field to product lotus tuber which is used as vegetable in eastern Asian countries. The lotus field is maintained flooded condition throughout the year and much organic matter was applied into the field that resulted in the highest levels of CH4 emission among various types of vegetation. The field was located beside lake Kasumigaura at central Japan (36 05ÕN, 140 15ÕE) and the area was about 3km from north to south and 1km from east to west. Dominant wind direction was south in summer and north in winter.

CH4 flux was determined by aerodynamic method using NDIR-CH4 analyser and micrometeorology sensors, and CO2 flux was determined by eddy correlation method with an open path IRGA and 3D sonic anemometer. The field measurements were carried out continuously around a week every month intervals from October in 1996 to April in 1999.

Lotus sprouts in mid may and plant height increased up to 1.7 m in late August (LAI 3.5), then its senescence begins and died in late October. The tuber harvest starts in mid October and continued to next March. CO2 flux changed from 2 gCO2 m-2 day-1 of source in March to 28 gCO2 m-2 day-1 of sink in late August. Hourly CO2 flux was evaluated an empirical model and the flux levels in unmeasured days were complimented. Yearly budgets of CO2 at the lotus field in 1997 and 1998 were sinks of 2178 and 1988 gCO2 m-2, respectively.

CH4 flux was upward throughout the year that level changed from 0.08 gCH4 m-2 day-1 in early June to 0.58 gCH4 m-2 day-1 in early August. The levels in late autumn to late spring were less than 0.05 gCH4 m-2 day-1. The level was about two times greater then that of typical rice paddy in Japan. Daily amounts of CH4 flux in unmeasured days were evaluated by the empirical model using measured daily average temperature. The annual CH4 efflux levels in 1997 and 1998 were 65.4 and 73.4 gCH4 m-2, and about 85 % of the total efflux was emitted in the vegetation period (May to October. The yearly difference was caused by vegetation condition.