Estimates of vegetation carbon uptake by solar-induced chlorophyll fluorescence (SIF) are modulated by photosynthetic pathway (C3 vs. C4)

Agricultural systems consistently show the largest peak CO₂ fluxes globally, contributing significantly to global terrestrial carbon uptake. However, without accurate observational constraints on the timing and magnitude of carbon uptake by crops, these strong CO₂ drawdowns can bias global flux inversions. A promising metric for estimating photosynthesis in croplands is through the remote sensing of solar-induced chlorophyll fluorescence (SIF). While a consistently linear relationship between SIF and gross primary productivity (GPP) has been shown across a wide range of ecosystems, the translation from retrieved SIF at the top of a vegetation canopy to GPP is not straightforward. At the leaf scale, we show that the slope and magnitude of the non-linear relationship between photosynthesis and SIF is substantially different for C3 and C4 crops. These results are confirmed at the canopy scale using data from a scanning spectrometer system, PhotoSpec, at both a corn (C4) and soybean (C3) field during the summer of 2017. While the saturation of the SIF/photosynthesis relationship differs based on photosynthetic pathway (C3 vs. C4), a mutual dynamic response of SIF and vegetation carbon uptake to environmental conditions (soil moisture, VPD) was observed throughout the growing season. This suggests that SIF responds to instantaneous changes in plant physiology, which should be accounted for when scaling SIF data collected during a single satellite overpass time to canopy integrated GPP. Averaging the PhotoSpeca data at different time-scales (ranging from 30 min - 1 week) helps to explain the linear relationship observed from space between GPP and SIF for both C3 and C4 vegetation canopies. We will discuss the discrepancies between the C3 vs. C4 SIF/photosynthesis relationship, and explain how this may contribute to the utility of SIF for monitoring agricultural productivity into the future.