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• Non-growing season ecosystem respiration negated the CO 2 sink during the growing season. • Carbon allocation to rice components varied substantially over the seasons. • The ratio of net to gross primary productivity varied seasonally and interannually. • Mid-season drainage substantially reduced CH 4 emission. • The proportion of ebullitive CH 4 flux was 10 to 17% of the total during the cultivation period. Rice is a major crop that feeds more than half of the global population. Rice paddies play a complex role in the carbon cycle by emitting CH 4 to the atmosphere while either sequestering or releasing CO 2 to the atmosphere. Here, we present 3.5 years of eddy covariance measurements of CO 2 and CH 4 fluxes over an intermittently irrigated, single crop rice paddy in South Korea in tandem with carbon stock measurements for leaves, stems, grains, and roots. The rice paddy acted as a slight sink or neutral in CO 2 (−47 ± 51 g C m −2 y −1; mean ± 95% confidence interval) with 941 ± 130 g C m −2 y −1 and 894 ± 108 g C m −2 y −1 as the gross primary production (GPP) and ecosystem respiration (Reco), respectively. The mean annual net primary productivity (NPP) was 666 ± 31 g C m −2 y −1, which was partitioned to leaves (9%), stems (30%), grains (51%), and roots (10%), for a harvest index of 57%. The paddy emitted 20.6 ± 1.5 g C m −2 y −1 in the form of CH 4 , of which between 82 and 96% was emitted during the cultivation period (~132 days). The CH 4 flux showed a mid-season decline caused by mid-season drainage, which produced aerobic soil conditions that inhibited CH 4 emissions. The proportion of estimated ebullitive flux to total CH 4 flux ranged from 10 to 17% during the cultivation period. GPP, Reco, and CH 4 fluxes varied by 12%, 11%, and 7%, respectively, over the 3-year period. When considering the harvest and the global warming potential of CH 4 , the paddy shifted from a greenhouse gas sink to a strong source (2,741 ± 468 g CO 2 eq m −2 y −1). These findings will improve our understanding of carbon budgets and cycles, and help develop greenhouse gas and water management strategies for rice paddies. [ABSTRACT FROM AUTHOR]

Sun-induced chlorophyll fluorescence (SiF) is increasingly used as a proxy for vegetation canopy photosynthesis. While ground-based, airborne, and satellite observations have demonstrated a strong linear relationship between SiF and gross primary production (GPP) at seasonal scales, their relationships at high temporal resolution across diurnal to seasonal scales remain unclear. In this study, far-red canopy SiF, GPP, and absorbed photosynthetically active radiation (APAR) were continuously monitored using automated spectral systems and an eddy flux tower over an entire growing season in a rice paddy. At half-hourly resolution, strong linear relationships between SiF and GPP (R 2  = 0.76) and APAR and GPP (R 2  = 0.76) for the whole growing season were observed. We found that relative humidity, diffuse PAR fraction, and growth stage influenced the relationships between SiF and GPP, and APAR and GPP, and incorporating those factors into multiple regression analysis led to improvements up to R 2  = 0.83 and R 2  = 0.88, respectively. Relationships between LUE p (=GPP/APAR) and LUE f (=SiF/APAR) were inconsistent at half-hourly and weak at daily resolutions (R 2  = 0.24). Both at diurnal and seasonal time scales with half-hourly resolution, we found considerably stronger linear relationships between SiF and APAR than between either SiF and GPP or APAR and GPP. Overall, our results indicate that for subdiurnal temporal resolution, canopy SiF in the rice paddy is above all a very good proxy for APAR at diurnal and seasonal time scales and that therefore SiF-based GPP estimation needs to take into account relevant environmental information to model LUE p . These findings can help develop mechanistic links between canopy SiF and GPP across multiple temporal scales. [ABSTRACT FROM AUTHOR]