The impact of sky conditions on gross primary production and methane flux from different rice paddies

The increase of diffuse radiation fraction has been reported to greatly impact carbon uptake in agroecosystems. However, it is unclear how radiation components affected methane (CH4) emission. Based on eddy covariance measurement from six sites, the effects of sky conditions on gross primary productivity (GPP) and CH4 emission were investigated at half-hourly and daily scales. The results showed diurnal patterns of GPP were similar under all sky conditions, while CH4 emission displayed irregular unimodal curves with greater fluctuations under different sky conditions. GPP responded to the changing radiation more efficiently under overcast conditions than under sunny conditions, whereas CH4 emission under sunny conditions was higher at the same radiation levels. Parameters describing GPP and CH4 emission varied across sites and sky conditions. The maximum photosynthetic rate and CH4 rate at turning point under cloudy and overcast skies were lower than those under sunny conditions. The values of initial light use efficiency from GPP and CH4 emission were opposite with the increase of diffuse radiation fraction, respectively. Soil moisture, temperature (Ta), vapor pressure deficit, direct, and diffuse radiation (Rdif) were responsible for the variations of GPP and CH4 emission under different skies, CH4 emission depended heavily on GPP and Ta under the inhibition of Rdif. This study implies that the direct effects of sky conditions on GPP were greater than those on CH4 emission and should be paid more attention in ecosystem carbon cycle models.