Red solar-induced chlorophyll fluorescence as a robust proxy for ecosystem-level photosynthesis in a rice field

Abstract

Terrestrial ecosystem photosynthesis plays a crucial role in global carbon cycling. Solar-induced chlorophyll fluorescence (SIF) is a state-of-the-art proxy for ecosystem photosynthesis. Although satellite-derived SIF correlates with gross primary productivity (GPP), it needs to be complemented with ground-based measurements to better understand the dynamics of carbon uptake by plants, because of the coarse spatiotemporal resolution of satellite-based data. We used an ultrafine-resolution spectrometer to detect both red SIF (SIFred, 687 nm) and far-red SIF (SIFfar-red, 760 nm) at ground level. Then, we investigated how well they represented eddy-tower-derived GPP in a rice field of central Japan during the mid to late growing season in 2019. Half-hourly and daily averaged SIFred showed stronger correlation with GPP (R2 = 0.61 and 0.83, respectively) than SIFfar-red (R2 = 0.42 and 0.57, respectively). Stronger seasonal correlation with GPP was observed for canopy-escaping SIF than for total SIF. The SIFred/SIFfar-red ratio was not significantly correlated with seasonal changes in the normalized difference vegetation index or in the pigment-specific simple ratios for chlorophyll-a and -b. There were no clear differences in the SIFred–GPP relationships between sunny and cloudy conditions. We concluded that SIFred could be an equivalent or better proxy than SIFfar-red for ecosystem-level photosynthesis. We further postulated that SIF emission is mainly controlled by plant physiology during the mid to late growing season, when temporal variations of the canopy structure in rice ecosystems are small.