Seasonal variation of carbon dioxide exchange in rice paddy field in Japan

Based on the results of continuous flux measurements at the Mase paddy flux site in central Japan, carbon dioxide (CO2) exchange between a customarily cultivated single rice-cropping paddy field and the atmosphere during the 2002 growing season was analyzed. The daily Net Ecosystem CO2 Exchange (NEE) showed a seasonal variation with a maximum uptake rate of 39 g CO2 m−2 day−1 in late July when the leaf area index of the rice reached its peak. Daily NEE turned positive, indicating emission from the paddy, about 2 weeks before harvest, because of the declining photosynthesis rate of rice in its mature stage and enhanced ecosystem respiration from the drained paddy. Quantum yield of the primary crop showed a declining seasonal trend from 0.11 mol CO2 (mol quantum)−1 in the early growth stage to below 0.05 mol CO2 (mol quantum)−1 after heading. The ratoon crop had little effect on the seasonal CO2 budget of 2002, because its growing period was restricted to 1 month. The paddy field acted as a CO2 sink for about 100 days from late May until the end of August, indicating that the paddy field functioned as a source of CO2 for more than 70% of the year. The ratio of gross primary production (GPP) to ecosystem respiration (Re) was 1.53 for the primary crop period and 0.43 for the ratoon crop period. Drained days accounted for as many as a half of the primary crop period days and the ratoon crop period was restricted in length, thus we obtained small ratios of GPP to Re in the two periods. During the post-heading period, the half-hourly CO2 storage below the flux measurement height was occasionally comparable in magnitude to the eddy CO2 flux over the canopy. When the friction velocity was less than 0.1 m s−1, the storage term accounted for 6% of NEE on average even after influence of temperature on nighttime NEE was removed. Estimating the storage term from a single height measurement underestimated it by 22%.