Remote Estimation of Net Ecosystem CO2 Exchange in Crops: Principles, Technique Calibration and Validation

Accurate and synoptic estimation of spatially distributed CO2 fluxes is of great importance for regional and global studies of carbon balance. A technique solely based on remotely sensed data was developed and tested for estimating net ecosystem CO2 exchange (NEE) in maize and soybean. The model is based on the reflectance in two spectral channels: the near-infrared and either the green or red-edge around 700 nm. The technique provides accurate estimations of mid-day NEE in both crops under either rainfed or irrigated conditions, explaining more than 85% of NEE variation in maize and more than 81% in soybean, and shows great potential for remotely tracking crop NEE

Earlier snowmelt may lead to late season declines in plant productivity and carbon sequestration in Arctic tundra ecosystems

Arctic warming is affecting snow cover and soil hydrology, with consequences for carbon sequestration in tundra ecosystems. The scarcity of observations in the Arctic has limited our understanding of the impact of covarying environmental drivers on the carbon balance of tundra ecosystems. In this study, we address some of these uncertainties through a novel record of 119 site-years of summer data from eddy covariance towers representing dominant tundra vegetation types located on continuous permafrost in the Arctic. Here we found that earlier snowmelt was associated with more tundra net CO2 sequestration and higher gross primary productivity (GPP) only in June and July, but with lower net carbon sequestration and lower GPP in August. Although higher evapotranspiration (ET) can result in soil drying with the progression of the summer, we did not find significantly lower soil moisture with earlier snowmelt, nor evidence that water stress affected GPP in the late growing season. Our results suggest that the expected increased CO2 sequestration arising from Arctic warming and the associated increase in growing season length may not materialize if tundra ecosystems are not able to continue sequestering CO2 later in the season

Earlier snowmelt may lead to late season declines in plant productivity and carbon sequestration in Arctic tundra ecosystems

Arctic warming is affecting snow cover and soil hydrology, with consequences for carbon sequestration in tundra ecosystems. The scarcity of observations in the Arctic has limited our understanding of the impact of covarying environmental drivers on the carbon balance of tundra ecosystems. In this study, we address some of these uncertainties through a novel record of 119 site-years of summer data from eddy covariance towers representing dominant tundra vegetation types located on continuous permafrost in the Arctic. Here we found that earlier snowmelt was associated with more tundra net CO2 sequestration and higher gross primary productivity (GPP) only in June and July, but with lower net carbon sequestration and lower GPP in August. Although higher evapotranspiration (ET) can result in soil drying with the progression of the summer, we did not find significantly lower soil moisture with earlier snowmelt, nor evidence that water stress affected GPP in the late growing season. Our results suggest that the expected increased CO2 sequestration arising from Arctic warming and the associated increase in growing season length may not materialize if tundra ecosystems are not able to continue sequestering CO2 later in the season.Peer reviewe

Towards long-term standardised carbon and greenhouse gas observations for monitoring Europe's terrestrial ecosystems : a review

Research infrastructures play a key role in launching a new generation of integrated long-term, geographically distributed observation programmes designed to monitor climate change, better understand its impacts on global ecosystems, and evaluate possible mitigation and adaptation strategies. The pan-European Integrated Carbon Observation System combines carbon and greenhouse gas (GHG; CO2, CH4, N2O, H2O) observations within the atmosphere, terrestrial ecosystems and oceans. High-precision measurements are obtained using standardised methodologies, are centrally processed and openly available in a traceable and verifiable fashion in combination with detailed metadata. The Integrated Carbon Observation System ecosystem station network aims to sample climate and land-cover variability across Europe. In addition to GHG flux measurements, a large set of complementary data (including management practices, vegetation and soil characteristics) is collected to support the interpretation, spatial upscaling and modelling of observed ecosystem carbon and GHG dynamics. The applied sampling design was developed and formulated in protocols by the scientific community, representing a trade-off between an ideal dataset and practical feasibility. The use of open-access, high-quality and multi-level data products by different user communities is crucial for the Integrated Carbon Observation System in order to achieve its scientific potential and societal value.Peer reviewe

Annual patterns and budget of CO\u3csub\u3e2\u3c/sub\u3e flux in an Arctic tussock tundra ecosystem

The functioning of Arctic ecosystems is not only critically affected by climate change, but it also has the potential for major positive feedback on climate. There is, however, relatively little information on the role, patterns, and vulnerabilities of CO2 fluxes during the nonsummer seasons in Arctic ecosystems. Presented here is a year-round study of CO2 fluxes in an Alaskan Arctic tussock tundra ecosystem, and key environmental controls on these fluxes. Important controls on fluxes vary by season. This paper also presents a new empirical quantification of seasons in the Arctic based on net radiation. The fluxes were computed using standard FluxNet methodology and corrected using standard Webb-Pearman-Leuning density terms adjusted for influences of open-path instrument surface heating. The results showed that the nonsummer season comprises a significant source of carbon to the atmosphere. The summer period was a net sink of 24.3 g Cm-2, while the nonsummer seasons released 37.9 g Cm-2. This release is 1.6 times the summer uptake, resulting in a net annual source of +13.6 g Cm-2 to the atmosphere. These findings support early observations of a change in this particular region of the Arctic from a long-term annual sink of CO2 to an annual source from the terrestrial ecosystem and soils to the atmosphere. The results presented here demonstrate that nearly continuous observations may be required in order to accurately calculate the annual net ecosystem CO2 exchange of Arctic ecosystems and to build predictive understanding that can be used to estimate, with confidence, Arctic fluxes under future conditions

Novel technique for remote estimation of CO\u3csub\u3e2\u3c/sub\u3e flux in maize

There is considerable interest in assessing the magnitude of carbon sources and sinks for agricultural lands, grasslands, and forests. In this paper, we propose a novel technique to remotely assess CO2 fluxes in maize using reflectances (ρ) in two spectral channels either in the green around 550 nm or in the red edge near 700 nm and the NIR (beyond 750 nm). Differences of reciprocal reflectances [(ρGreen)-1 - (ρNIR)-1] and [(ρRedEdge)-1- (ρNIR)-1] accounted for more than 90 percent of the variability in mid-day canopy photosynthesis of irrigated maize. The technique was validated by an independent data set; root mean square error in predicting mid-day canopy photosynthesis by [(ρRedEdge)-1 - (ρNIR)-1 was 0.17 mg/ m2/s and 0.2 mg/m2/s by [(ρGreen)-1 - (ρNIR)-1]

Remote Estimation of Net Ecosystem CO2 Exchange in Crops: Principles, Technique Calibration and Validation

Accurate and synoptic estimation of spatially distributed CO2 fluxes is of great importance for regional and global studies of carbon balance. A technique solely based on remotely sensed data was developed and tested for estimating net ecosystem CO2 exchange (NEE) in maize and soybean. The model is based on the reflectance in two spectral channels: the near-infrared and either the green or red-edge around 700 nm. The technique provides accurate estimations of mid-day NEE in both crops under either rainfed or irrigated conditions, explaining more than 85% of NEE variation in maize and more than 81% in soybean, and shows great potential for remotely tracking crop NEE

Relationship between gross primary production and chlorophyll content in crops: Implications for the synoptic monitoring of vegetation productivity

Accurate estimation of spatially distributed CO2 fluxes is of great importance for regional and global studies of carbon balance. We applied a recently developed technique for remote estimation of crop chlorophyll content to assess gross primary production (GPP). The technique is based on reflectance in two spectral channels: the near-infrared and either the green or the red-edge. We have found that in irrigated and rainfed crops (maize and soybean), midday GPP is closely related to total crop chlorophyll content. The technique provided accurate estimations of midday GPP in both crops under rainfed and irrigated conditions with root mean square error of GPP estimation of less than 0.3 mg CO2/m2s in maize (GPP ranged from 0 to 3.1 mg CO2/m2s) and less than 0.2 mg CO2/m2s in soybean (GPP ranged from 0 to 1.8 mg CO2/m2s). Validation using an independent data set for irrigated and rainfed maize showed robustness of the technique; RMSE of GPP prediction was less than 0.27 mg CO2/m2s

Annual carbon dioxide exchange in irrigated and rainfed maize-based agroecosystems

Carbon dioxide exchange was quantified in maize–soybean agroecosystems employing year-round tower eddy covariance flux systems and measurements of soil C stocks, CO2 fluxes from the soil surface, plant biomass, and litter decomposition. Measurements were made in three cropping systems: (a) irrigated continuous maize, (b) irrigated maize–soybean rotation, and (c) rainfed maize–soybean rotation during 2001–2004. Because of a variable cropping history, all three sites were uniformly tilled by disking prior to initiation of the study. Since then, all sites are under no-till, and crop and soil management follow best management practices prescribed for production-scale systems. Cumulative daily gain of C by the crops (from planting to physiological maturity), determined from the measured eddy covariance CO2 fluxes and estimated heterotrophic respiration, compared well with the measured total above and belowground biomass. Two contrasting features of maize and soybean CO2 exchange are notable. The value of integrated GPP (gross primary productivity) for both irrigated and rainfed maize over the growing season was substantially larger (ca. 2:1 ratio) than that for soybean. Also, soybean lost a larger portion (0.80–0.85) of GPP as ecosystem respiration (due, in part, to the large amount of maize residue from the previous year), as compared to maize (0.55– 0.65). Therefore, the seasonally integrated NEP (net ecosystem production) in maize was larger by a 4:1 ratio (approximately), as compared to soybean. Enhanced soil moisture conditions in the irrigated maize and soybean fields caused an increase in ecosystem respiration, thus eliminating any advantage of increased GPP and giving about the same values for the growing season NEP as the rainfed fields. On an annual basis, the NEP of irrigated continuous maize was 517, 424, and 381 g C m-2 year-1 , respectively, during the 3 years of our study. In rainfed maize the annual NEP was 510 and 397 g C m-2 year-1 in years 1 and 3, respectively. The annual NEP in the irrigated and rainfed soybean fields were in the range of −18 to −48 g C m-2. Accounting for the grain C removed during harvest and the CO2 released from irrigation water, our tower eddy covariance flux data over the first 3 years suggest that, at this time: (a) the rainfed maize–soybean rotation system is C neutral, (b) the irrigated continuous maize is nearly C neutral or a slight source of C, and (c) the irrigated maize–soybean rotation is a moderate source of C. Direct measurement of soil C stocks could not detect a statistically significant change in soil organic carbon during the first 3 years of no-till farming in these three cropping systems

Annual carbon dioxide exchange in irrigated and rainfed maize-based agroecosystems

Carbon dioxide exchange was quantified in maize–soybean agroecosystems employing year-round tower eddy covariance flux systems and measurements of soil C stocks, CO2 fluxes from the soil surface, plant biomass, and litter decomposition. Measurements were made in three cropping systems: (a) irrigated continuous maize, (b) irrigated maize–soybean rotation, and (c) rainfed maize–soybean rotation during 2001–2004. Because of a variable cropping history, all three sites were uniformly tilled by disking prior to initiation of the study. Since then, all sites are under no-till, and crop and soil management follow best management practices prescribed for production-scale systems. Cumulative daily gain of C by the crops (from planting to physiological maturity), determined from the measured eddy covariance CO2 fluxes and estimated heterotrophic respiration, compared well with the measured total above and belowground biomass. Two contrasting features of maize and soybean CO2 exchange are notable. The value of integrated GPP (gross primary productivity) for both irrigated and rainfed maize over the growing season was substantially larger (ca. 2:1 ratio) than that for soybean. Also, soybean lost a larger portion (0.80–0.85) of GPP as ecosystem respiration (due, in part, to the large amount of maize residue from the previous year), as compared to maize (0.55– 0.65). Therefore, the seasonally integrated NEP (net ecosystem production) in maize was larger by a 4:1 ratio (approximately), as compared to soybean. Enhanced soil moisture conditions in the irrigated maize and soybean fields caused an increase in ecosystem respiration, thus eliminating any advantage of increased GPP and giving about the same values for the growing season NEP as the rainfed fields. On an annual basis, the NEP of irrigated continuous maize was 517, 424, and 381 g C m-2 year-1 , respectively, during the 3 years of our study. In rainfed maize the annual NEP was 510 and 397 g C m-2 year-1 in years 1 and 3, respectively. The annual NEP in the irrigated and rainfed soybean fields were in the range of −18 to −48 g C m-2. Accounting for the grain C removed during harvest and the CO2 released from irrigation water, our tower eddy covariance flux data over the first 3 years suggest that, at this time: (a) the rainfed maize–soybean rotation system is C neutral, (b) the irrigated continuous maize is nearly C neutral or a slight source of C, and (c) the irrigated maize–soybean rotation is a moderate source of C. Direct measurement of soil C stocks could not detect a statistically significant change in soil organic carbon during the first 3 years of no-till farming in these three cropping systems