Remotely sensed top-of-the-canopy (TOC) SIF is highly impacted by non-physiological structural and environmental factors that are confounding the photosystems' emitted SIF signal. Our proposed method for scaling TOC SIF down to photosystems' (PSI and PSII) level uses a three-dimensional (3D) modeling approach, capable of accounting physically for the main confounding factors, i.e., SIF scattering and reabsorption within a leaf, by canopy structures, and by the soil beneath. Here, we propose a novel SIF downscaling method that separates the structural component from the functional physiological component of TOC SIF signal by using the 3D Discrete Anisotropic Radiative Transfer (DART) model coupled with the leaf-level fluorescence model Fluspect-CX, and estimates the Fluorescence Quantum Efficiency (FQE) at photosystem level. The method was first applied on in- situ diurnal measurements acquired at the top of the canopy of an alfalfa crop with a near-distance point- measuring FloX system. The retrieved photosystem-level FQE diurnal courses correlated significantly with photosynthetic yield of PSII measured by an active leaf florescence instrument MiniPAM (R = 0.87, R2 = 0.76 before and R =-0.82, R2 = 0.67 after 2.00 pm local time). Diurnal FQE trends of both photosystems jointly were descending from late morning 9.00 am till afternoon 4.00 pm. A slight late-afternoon increase, observed for three days between 4.00 and 7.00 pm, could be attributed to an increase in FQE of PSI that was retrieved separately from PSII. The method was subsequently extended and applied to airborne SIF images acquired with the HyPlant imaging spectrometer over the same alfalfa field. While the input canopy SIF radiance computed by two different methods, i) a spectral fitting method (SFM) and ii) a spectral fitting method neural network (SFMNN), produce broad and irregularly shaped (skewed) histograms (spatial coefficients of variation: CV = 29-35 % and 14-20 %, respectively), the retrieved HyPlant per-pixel FQE estimates formed significantly narrower and regularly bell- shaped near-Gaussian histograms (CV = 27-34 % and 14-17 %, respectively). The achieved spatial homogeneity of resulting FQE maps confirms successful removal of the TOC SIF radiance confounding impacts. Since our method is based on direct matching of measured and physically modelled canopy SIF radiance, simulated by 3D radiative transfer, it is versatile and transferable to other canopy architectures, including structurally complex canopies such as forest stands.

Seagrass meadows are globally recognized as critical natural carbon sinks, commonly known as 'blue carbon'. However, seagrass decline attributed to escalating human activities and climate change, significantly influences their carbon sequestration capacity. A key aspect in comprehending the impact of seagrass decline on carbon sequestration is understanding how degradation affects the stored blue carbon, primarily consisting of sediment organic carbon (SOC). While it is widely acknowledged that seagrass decline affects the input of organic carbon, little is known about its impact on SOC pool stability. To address this knowledge, we examined variations in total SOC and recalcitrant SOC (RSOC) at a depth of 15 cm in nine seagrass meadows located on the coast of Southern China. Our findings revealed that the ratio of RSOC to SOC (RSOC/SOC) ranged from 27 % to 91 % in the seagrass meadows, and the RSOC/SOC increased slightly with depth. Comparing different seagrass species, we observed that SOC and RSOC stocks were 1.94 and 3.19-fold higher under Halophila beccarii and Halophila ovalis meadows compared to Thalassia hemprichii and Enhalus acoroides meadows. Redundancy and correlation analyses indicated that SOC and RSOC content and stock, as well as the RSOC/SOC ratio, decreased with declining seagrass shoot density, biomass, and coverage. This implies that the loss of seagrass, caused by human activities and climate change, results in a reduction in carbon sequestration stability. Further, the RSOC decreased by 15 %, 29 %, and 40 % under unvegetated areas compared to adjacent Halophila spp., T. hemprichii and E. acoroides meadows, respectively. Given the anticipated acceleration of seagrass decline due to climate change and increasing coastal development, our study provides timely information for developing coastal carbon protection strategies. These strategies should focus on preserving seagrass and restoring damaged seagrass meadows, to maximize their carbon sequestration capacity.