Carbon Flux Dynamics and Response to Extreme High Temperature in Rice Ecosystems Across the Lower Reaches of the Yangtze River, China

Under global climate warming, the impact of extreme high temperatures on carbon exchange in paddy rice ecosystems remains unclear, yet they exert a profound influence on the carbon cycle in agricultural ecosystems. The characteristics of carbon dioxide (CO2) fluxes and their response to temperature were explored at two sites (Jurong and Jiangdu) across the lower reaches of the Yangtze River in China using open-path eddy covariance observations in 2021–2024. During the rice-growing season, considerable inter-annual spatial variability in high temperature was observed, with a higher frequency and larger intensity in Jurong relative to Jiangdu and more severe heat stress in 2022 relative to 2023. The jointing–booting stage was identified as the hotspot exposed to the highest frequency and longest duration of high temperature across multiple years. There was obvious variation in net ecosystem CO2 exchange (NEE) throughout the rice-growing season, with the cumulative values being −462.2 ± 55.2 gC·m−2 in 2021–2023 at Jurong and −362.4 ± 43.0 gC·m−2 in 2022–2024 at Jiangdu. The period from jointing to flowering was identified as the most sensitive time slice for NEE variation, with a daily average value of −6.3 ± 0.2 gC·m−2·d−1 in jointing–booting and −5.2 ± 2.2 gC·m−2·d−1 in booting–flowering at Jurong, as well as −4.0 ± 0.7 gC·m−2·d−1 in jointing–booting and −5.7 ± 1.1 gC·m−2·d−1 in booting–flowering at Jiangdu. The respective correlation coefficients were −0.59 and −0.37 between periodical NEE and mean air temperature at Jurong and Jiangdu, meaning that NEE showed a decreasing trend as temperature increased, owing to the simultaneous but heterogeneous changes in gross ecosystem CO2 exchange and ecosystem respiration. When the temperature was lower than 38 °C, the corresponding correlation coefficient reached −0.85 at Jurong and −0.52 at Jiangdu, suggesting that extreme high temperature prevented a decline in NEE. The response of NEE to temperature highlighted that NEE ceased to decrease when temperature surpassed 38 °C, implying that a critical threshold existed for limiting the carbon sink under extreme high temperature. These findings could provide insight for understanding carbon cycling in agricultural systems under an extreme climate.