Seasonal variation of carbon dioxide fluxes over irrigated soybean (Glycine max L.)

In this study, variations in carbon dioxide (CO₂) fluxes resulting from gross primary production (GPP), net ecosystem exchange (NEE), and respiration (R_e) of soybean (Glycine max L.) were investigated by the Eddy Covariance method during the growing period from June to November 2005 on an irrigated sand field at the Arid Land Research Center, Tottori University in Tottori, Japan. Although climatic conditions were humid and temperate, the soybeans required frequent irrigation because of the low water holding capacity of the sandy soil at the field site. Finally, it has been found that the accumulated NEE, GPP, and R_e fluxes of soybean over 126 days amount to -93, 319, and 226 gC m^-2, respectively. Furthermore, the average ratio of GPP to R_e was 1.4 and the average ratio of NEE to GPP was about -0.29 for the growth period of soybean. Daily maximum NEE of -3.8 gC m^-2 occurred when LAI was 1.1.