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Photosynthesis is the process by which plants harvest sunlight to produce sugars from carbon dioxide and water. It is the primary source of energy for all life on Earth; hence it is important to understand how this process responds to climate change and human impact. However, model-based estimates of gross primary production (GPP, output from photosynthesis) are highly uncertain, in particular over heavily managed agricultural areas. Recent advances in spectroscopy enable the space-based monitoring of sun-induced chlorophyll fluorescence (SIF) from terrestrial plants. Here we demonstrate that spaceborne SIF retrievals provide a direct measure of the GPP of cropland and grassland ecosystems. Such a strong link with crop photosynthesis is not evident for traditional remotely sensed vegetation indices, nor for more complex carbon cycle models. We use SIF observations to provide a global perspective on agricultural productivity. Our SIF-based crop GPP estimates are 50-75% higher than results from state-of-the-art carbon cycle models over, for example, the US Corn Belt and the Indo-Gangetic Plain, implying that current models severely underestimate the role of management. Our results indicate that SIF data can help us improve our global models for more accurate projections of agricultural productivity and climate impact on crop yields. Extension of our approach to other ecosystems, along with increased observational capabilities for SIF in the near future, holds the prospect of reducing uncertainties in the modeling of the current and future carbon cycle.

Ecosystem-scale estimation of photosynthesis and respiration using micrometeorological techniques remains an important, yet difficult, challenge. In this study, we combined micrometeorological and stable isotope methods to partition net ecosystem CO2 exchange (FN) into photosynthesis (F(A)) and respiration (F(R)) in a corn-soybean rotation ecosystem during the summer 2003 corn phase. Mixing ratios of (12)CO2 and (13)CO2 were measured continuously using tunable diode laser (TDL) absorption spectroscopy. The dynamics of the isotope ratio of ecosystem respiration (R), net ecosystem CO2 exchange (deltaN) and photosynthetic discrimination at the canopy scale (delta canopy) were examined. During the period of full canopy closure, F(N) was partitioned into photosynthesis and respiration using both the isotopic approach and the conventional night-time-derived regression methodology. Results showed that deltaR had significant seasonal variation (-32 to -11% per hundred) corresponding closely with canopy phenology. Daytime deltaN typically varied from -12 to -4% per hundred, while delta canopy remained relatively constant in the vicinity of 3% per hundred. Compared with the regression approach, the isotopic flux partitioning showed more short-term variations and was considerably more symmetric about F(N). In this experiment, the isotopic partitioning resulted in larger uncertainties, most of which were caused by the uncertainties in deltaN. and the daytime estimate of deltaR. By sufficiently reducing these uncertainties, the tunable diode laser (TDL)-micrometeorological technique should yield a better understanding of the processes controlling photosynthesis, respiration and ecosystem-scale discrimination.

Net CO2 exchange data of legume crops at 17 flux tower sites in North America and three sites in Europe representing 29 site-years of measurements were partitioned into gross photosynthesis and ecosystem respiration by using the nonrectangular hyperbolic light-response function method. The analyses produced net CO2 exchange data and new ecosystem-scale ecophysiological parameter estimates for legume crops determined at diurnal and weekly time steps. Dynamics and annual totals ofgross photosynthesis, ecosystem respiration, and net ecosystem production were calculated by gap filling with multivariate nonlinear regression. Comparison with the data from grain crops obtained with the same method demonstrated that CO2 exchange rates and ecophysiological parameters of legumes were lowerthan those of maize (ZeamaysL.) but higherthanforwheat (TriticumaestipumL.) crops. Year-round annuallegume crops demonstrated a broad range of net ecosystem production, from sinks of 760 g CO2 m-2 yr-1 to sources of -2100 g CO2 m-2 yr-1, with an average of -330 g CO2 m-2 yr-1, indicating overall moderate CO2-source activity related to a shorter period of photosynthetic uptake and metabolic costs of N2 fixation. Perennial legumes (alfalfa, Medicago sativa L.) were strong sinks for atmospheric CO2, with an average net ecosystem production of 980 (range 550-1200) g CO2 m-2 yr-1. [ABSTRACT FROM AUTHOR]

Abstract: Development of optical isotope techniques over the last several years has provided scientists a set of tools for tracing the transport and cycling of CO2 and water vapor between the biosphere and atmosphere. Here, I take a micrometeorological perspective and review these technological advances, assess key instrument performance characteristics, examine how these techniques have been used in the field to improve our understanding of the processes governing the exchange of CO2 and water vapor, and discuss future research directions. Review of the recent literature indicates that: (1) optical techniques have been used to quantify the isotope composition of biosphere–atmosphere exchange using the traditional Keeling mixing line, flux–gradient, eddy covariance, and chamber approaches under a variety of field conditions with near-continuous data records now extending to more than 5-years; (2) high frequency and near continuous isotope measurements at the canopy scale have demonstrated important new insights regarding the behaviour of kinetic fractionation at the leaf versus canopy scales and the controls on ecosystem respiration that could not have been observed previously using traditional methods; (3) based on the assessment of instrument performance, carbon isotope disequilibrium (the difference between the isotope composition of photosynthesis and respiration), carbon turnover rates, and measurement uncertainties, 13C-CO2 investigations are best suited for examining the contributions and changes to ecosystem respiration with a need for more innovative 13C-isotope labeling experiments and compound specific isotope analyses under field conditions; (4) significant progress has been made in measuring the oxygen isotope composition of water vapor fluxes and canopy leaf water enrichment. These new data have provided an opportunity to evaluate models of leaf water enrichment and their application to the canopy scale; and (5) the use of 18O-H2O and 18O-CO2 as tracers of the coupled carbon–water cycle has matured significantly in recent years. Evidence from a range of ecosystems indicates that 18O-CO2 disequilibrium (ranging from 0 to 17‰) is much larger than for 13C-CO2 (typical less than 3‰ in natural C3 ecosystems) making it a useful tracer of coupled carbon and water cycle processes. However, a better understanding of the role of carbonic anhydrase in photosynthetic and respiratory processes under field conditions is now needed in order to make further progress. Finally, the increasing use and development of isotope-enabled land surface schemes along with the acquisition of high temporal resolution isotope data is providing a new opportunity to constrain the carbon and water cycle processes represented in these models. [Copyright &y& Elsevier]

Abstract: The stable carbon isotope ratio, , is a valuable tracer for studying the processes controlling the autotrophic (F Ra ) and heterotrophic (F Rh ) contributions to ecosystem respiration (F R ) and the influence of photosynthesis on F R . There is increasing interest in quantifying the temporal variability of the carbon isotope composition of ecosystem respiration (δ R ) because it contains information about the sources contributing to respiration and is an important parameter used for partitioning net ecosystem CO2 exchange using stable isotope methods. In this study, eddy covariance, flux gradient, automated chambers, and stable carbon isotope techniques were used to quantify and improve our understanding of the temporal variability in F R and δ R in a C3/C4 agricultural ecosystem. Six years (2004–2009) of isotope flux-gradient measurements indicated that δ R had a very consistent annual pattern during both C3 (soybean) and C4 (corn) growing seasons due to significant contributions from F Ra , which was strongly influenced by the isotope composition of the recent photosynthate. However, in the spring, δ R exhibited a C3 signal regardless of the crop grown in the previous season. One hypothesis for this anomaly is that at these low soil temperatures microbial activity relied predominantly on C3 substrates. Automated chamber measurements of soil respiration () and its isotope composition () were initiated in the early corn growing season of 2009 to help interpret the variability in δ R . These measurements showed good agreement with EC measurements of F R (within 0.5μmolm2 s−1) and isotope flux gradient measurements of δ R (within 2‰) at nighttime for near-bare soil conditions (LAI<0.1). At peak growth, nighttime δ R above the corn canopy was consistently 1–6‰ more enriched than . The relatively enriched signal above the canopy indicates that δ R was strongly influenced by aboveground plant respiration (F R,ag ), which accounted for about 40% of F R . The automated chamber data and analyses also revealed a strong diel pattern in . In the early growth period, showed a sharp morning enrichment of up to 4‰ followed by a gradual depletion throughout the afternoon and evening. Daytime enrichment in was most pronounced during dry conditions and was not observed when the upper soil was near saturation. We provide anecdotal evidence that the diel variability during early growth may have been influenced by turbulence (advection/non-diffusive transport), which reduced the kinetic fractionation effect. At peak growth, there is evidence that the sheltering effect of the corn plants diminished the influence of turbulence on the chamber measurement of . Further research is needed to evaluate and separate the contributions of biotic and abiotic (advection and non-steady state effects) influences on chamber observations. [Copyright &y& Elsevier]

Abstract: Separation of the photosynthetic (F P ) and respiratory (F R ) fluxes of net CO2 exchange (F N ) remains a necessary step toward understanding the biological and physical controls on carbon cycling between the soil, biomass, and atmosphere. Despite recent advancements in stable carbon isotope partitioning methodology, several potential limitations can cause uncertainty in the partitioned results. Here, we combined an automated chamber system with a tunable diode laser (TDL) to evaluate carbon isotope partitioning under controlled environmental conditions. Experiments were conducted in a climate controlled greenhouse utilizing both soybean (C3 pathway) and corn (C4 pathway) treatments. Under these conditions, net exchange of and was obtained with an improved signal to noise ratio. Further, the chamber system was used to estimate soil evaporation (E) and plant transpiration (T), allowing for an improved estimate of the total conductance to CO2 (g c ). This study found that the incorporation of short-term and diel variability in the isotope composition of respiration (δ R ) caused F P to nearly double in the corn system while only slightly increasing in the soybean system. Variability in both g c and the CO2 bundle sheath leakage factor for C4 plants (ϕ) also had a significant influence on F P . In addition, chamber measurements of F N and its isotope composition (δ N ) indicated that post-illumination processes caused a decrease in plant respiration for up to 3h following light termination. Finally, this study found systematic differences between the isotope and temperature-regression partitioning methods on the diel time scale. [Copyright &y& Elsevier]

Ecosystem-scale estimation of photosynthesis and respiration using micrometeorological techniques remains an important, yet difficult, challenge. In this study, we combined micrometeorological and stable isotope methods to partition net ecosystem CO2 exchange ( FN) into photosynthesis ( FA) and respiration ( FR) in a corn–soybean rotation ecosystem during the summer 2003 corn phase. Mixing ratios of 12CO2 and 13CO2 were measured continuously using tunable diode laser (TDL) absorption spectroscopy. The dynamics of the isotope ratio of ecosystem respiration ( δR), net ecosystem CO2 exchange ( δN) and photosynthetic discrimination at the canopy scale (Δcanopy) were examined. During the period of full canopy closure, FN was partitioned into photosynthesis and respiration using both the isotopic approach and the conventional night-time-derived regression methodology. Results showed that δR had significant seasonal variation (−32 to −11‰) corresponding closely with canopy phenology. Daytime δN typically varied from −12 to −4‰, while Δcanopy remained relatively constant in the vicinity of 3‰. Compared with the regression approach, the isotopic flux partitioning showed more short-term variations and was considerably more symmetric about FN. In this experiment, the isotopic partitioning resulted in larger uncertainties, most of which were caused by the uncertainties in δN and the daytime estimate of δR. By sufficiently reducing these uncertainties, the tunable diode laser (TDL)–micrometeorological technique should yield a better understanding of the processes controlling photosynthesis, respiration and ecosystem-scale discrimination. [ABSTRACT FROM AUTHOR]