Abstract
Sun-Induced fluorescence at 760 nm (F760) is increasingly being used to predict gross primary production (GPP) through light use efficiency (LUE) modeling, even though the mechanistic processes that link the two are not well understood. We analyzed the effect of nitrogen (N) and phosphorous (P) availability on the processes that link GPP and F760 in a Mediterranean grassland manipulated with nutrient addition. To do so, we used a combination of process-based modeling with Soil-Canopy Observation of Photosynthesis and Energy (SCOPE), and statistical analyses such as path modeling. With this study, we uncover the mechanisms that link the fertilization-driven changes in canopy nitrogen concentration (N%) to the observed changes in F760 and GPP. N addition changed plant community structure and increased canopy chlorophyll content, which jointly led to changes in photosynthetic active radiation (APAR), ultimately affecting both GPP and F760. Changes in the abundance of graminoids, (%graminoids) driven by N addition led to changes in structural properties of the canopy such as leaf angle distribution, that ultimately influenced observed F760 by controlling the escape probability of F760 (Fesc). In particular, we found a change in GPP–F760 relationship between the first and the second year of the experiment that was largely driven by the effect of plant type composition on Fesc, whose best predictor is %graminoids. The P addition led to a statistically significant increase on light use efficiency of fluorescence emission (LUEf), in particular in plots also with N addition, consistent with leaf level studies. The N addition induced changes in the biophysical properties of the canopy that led to a trade-off between surface temperature (Ts), which decreased, and F760 at leaf scale (F760leaf,fw), which increased. We found that Ts is an important predictor of the light use efficiency of photosynthesis, indicating the importance of Ts in LUE modeling approaches to predict GPP.
Highlights
An accurate estimation of gross primary production (GPP) by terrestrial ecosystems is crucial to understanding the variability of the global carbon (C) cycle [1]
The relative stability among treatments of light use efficiency of photosynthesis (LUEp), which is significantly different for the N treatment only in Campaign 6 and shows an increase of N and P together (NP) in Campaign 5 in 2015, suggests that our Mediterranean grasslands is quite constrained in its photosynthetic efficiency, and that any nutrient induced changes in GPP (Figure 2) are mostly modulated by changes in structural parameters such as fraction of photosynthetically active radiation (fAPAR)
Other works show that N addition strongly impacts canopy structural parameters such as leaf area index (LAI) and plant height in a short-grass prairie [58], there are no studies focused on the effect of N and NP on fluorescence at nm (Fesc)
Summary
An accurate estimation of gross primary production (GPP) by terrestrial ecosystems is crucial to understanding the variability of the global carbon (C) cycle [1]. In the LUE framework [2], estimates of GPP are based on three variables: (i) the fraction of photosynthetically active radiation (fAPAR) absorbed by the vegetation; (ii) the actual light use efficiency of photosynthesis (LUEp), i.e., the conversion efficiency of absorbed radiation to fixed carbon; and (iii) incident photosynthetically active radiation (PAR). Several studies have shown that sun-induced fluorescence at 760 nm retrieved from top-of-canopy (TOC) measurements (F760) can track changes in APAR and LUEp, and can be directly linked to GPP from leaves [6], ecosystem, [7,8,9,10] to regional and global scale [3,11,12,13]. The reason F760 and GPP correlate is that both processes start with the absorption of light by a chlorophyll molecule
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