Tracking the Breathing of the Biosphere Using Tall Tower Eddy Covariance Coupled with Remotely Sensed Solar Induced Fluorescence

Advances in satellite-based measurements of solar induced chlorophyll fluorescence (SIF) show great promise for examining spatiotemporal variations in terrestrial gross primary production (GPP) at regional to global scales. The potential for estimating net ecosystem production (NEP) has, however, received less attention, presumably because respiratory fluxes are not directly related to SIF. We hypothesize that remotely sensed SIF calibrated against tall tower observations can provide robust constraints on growing season NEP in an agricultural landscape, due to the high correlations between GPP and ecosystem respiration. We examined the carbon balance of Iowa and southern Minnesota in 2015 (May–Nov), synthesizing tall tower eddy covariance measurements with SIF observations from the OCO-2 satellite. The goodness-of-fit for a linear NEP fit against SIF (R² = 0.75) was poorer than that for GPP (R² = 0.91), reflecting the contribution of ecosystem respiration to the net flux. The substantial improvement in the goodness-of-fit upon separating the photosynthetic and respiration components is evidence of a robust partitioning approach, but it does not preclude the presence of bias that may be imparted by conventional partitioning. Monthly mean GPP and NEP fluxes estimated from gridded SIF data for the larger domain deviated from the tall tower by -30 to 15% (r = 0.98, p < 0.01) and -30 to 170% (r = 0.94, p < 0.01), respectively. However, upon integrating over time, seasonal GPP and NEP budgets for the two methods converged to within 3%. A clear limitation of this approach for constructing annual NEP budgets is that it does not directly resolve respiratory fluxes that are important in the absence of photosynthesis. Nevertheless, a method for obtaining simultaneous growing season GPP and NEP budgets is a powerful tool for improving constraints on regional carbon balances. Whether the SIF-based approach for regional NEP is valid in other biomes is an open question deserving attention.