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This study investigated how nitrogen (N) fertilization with 200 kg N ha of urea affected ecosystem carbon (C) sequestration in the first-postfertilization year in a Pacific Northwest Douglas-fir ( Pseudotsuga menziesii) stand on the basis of multiyear eddy-covariance (EC) and soil-chamber measurements before and after fertilization in combination with ecosystem modeling. The approach uses a data-model fusion technique which encompasses both model parameter optimization and data assimilation and minimizes the effects of interannual climatic perturbations and focuses on the biotic and abiotic factors controlling seasonal C fluxes using a prefertilization 9-year-long time series of EC data (1998-2006). A process-based ecosystem model was optimized using the half-hourly data measured during 1998-2005, and the optimized model was validated using measurements made in 2006 and further applied to predict C fluxes for 2007 assuming the stand was not fertilized. The N fertilization effects on C sequestration were then obtained as differences between modeled (unfertilized stand) and EC or soil-chamber measured (fertilized stand) C component fluxes. Results indicate that annual net ecosystem productivity in the first-post-N fertilization year increased by∼83%, from 302 ± 19 to 552 ± 36 g m yr, which resulted primarily from an increase in annual gross primary productivity of∼8%, from 1938 ± 22 to 2095 ± 29 g m yr concurrent with a decrease in annual ecosystem respiration ( R) of∼5.7%, from 1636 ± 17 to 1543 ± 31 g m yr. Moreover, with respect to respiration, model results showed that the fertilizer-induced reduction in R (∼93 g m yr) principally resulted from the decrease in soil respiration R (∼62 g m yr). [ABSTRACT FROM AUTHOR]

Quantification of terrestrial CO2 sources and sinks at regional scales (~10²-106 km-2) is fundamental to improving our understanding of the terrestrial carbon cycle. Two independent methods to extract the gross primary productivity (GPP) from atmospheric CO2 concentration measurements were explored and compared in this study. The methods are (1) planetary boundary layer (PBL) carbon budget analysis that allows the estimation of regional GPP at daily time steps from hourly CO2 concentration measurements and (2) spatially explicit hourly carbon cycle modeling based on remote sensing and then integrating the daily flux field with a concentration footprint function depending on wind and stability. These methods have been applied to a 28-m tower at an old black spruce site near Candle Lake (~100 km NE of Prince Albert: 53.98717°N, 105.11779°W). The estimates of daily GPP by these two approaches agreed well for 2003 (slope = 0.99; r² = 0.89). In order to test these methods of inferring the regional GPP from mixing ratio measurements, we also compared the estimates of regional GPP with estimates made using eddy covariance (EC) flux measurements, although their respective source areas are different. They had similar seasonal patterns, but the regional estimates were consistently smaller than the local EC flux derived GPP throughout the growing season in 2003. These estimates of annual regional GPP were 649-664 g C m-2 for 2003 while the EC-derived annual GPP was 819-847 g C m-2. The annual difference was about 20-25%. The EC flux footprint of the tower was relatively homogeneous old black spruce while the concentration footprint, which was a few orders of magnitude larger than the flux footprint, covered boreal evergreen and deciduous broadleaf forests, grassland, cropland, and lakes. Nonforested land occupied about 10-50% of the concentration footprint depending on wind direction and speed and was less productive than the black spruce forest. The discrepancies between regional and local GPP estimates reflected the differences in underlying land surfaces represented by the different footprint areas. [ABSTRACT FROM AUTHOR]

Accurate estimation of temporal and spatial variations in photosynthetic discrimination of 13C is critical to carbon cycle research. In this study, a combined ecosystem–boundary layer isotope model, which was satisfactorily validated against intensive campaign data, was used to explore the temporal variability of carbon discrimination in response to environmental driving factors in a boreal ecosystem in the vicinity of Fraserdale Tower, Ontario, Canada (49°52′30″N, 81°34′12″W). A 14 year (1990–1996 and 1998–2004) hourly CO2 concentration and meteorological record measured on this tower was used for this purpose. The 14 year mean yearly diurnal amplitude of canopy-level discrimination Δcanopy was computed to be 2.8 ± 0.5‰, and the overall diurnal cycle showed that the greatest Δcanopy values occurred at dawn and dusk, while the minima generally appeared in mid-afternoon. The average annual Δcanopy varied from 18.3 to 19.7‰ with the 14 year average of 19 ± 0.4‰. The overall seasonality of Δcanopy showed a gradually increasing trend from leaf emergence in May–September and with a slight decrease at the end of the growing season in October. Δcanopy was negatively correlated to vapour pressure deficit and air temperature across hourly to decadal timescales. A strong climatic control on stomatal regulation of ecosystem isotope discrimination was found in this study. [ABSTRACT FROM AUTHOR]

Stable isotopes of CO2 contain unique information on the biological and physical processes that exchange CO2 between terrestrial ecosystems and the atmosphere. In this study, we developed an integrated modeling system to simulate dynamics of stable carbon isotope of CO2, as well as moisture, energy, and momentum, between a boreal forest ecosystem and the atmosphere, as well as their transport/mixing processes through the convective boundary layer (CBL), using remotely sensed surface parameters to characterize the surface heterogeneity. It has the following characteristics: (i) it accounts for the influences of the CBL turbulent mixing and entrainment of the air aloft; (ii) it scales individual leaf-level photosynthetic discrimination up to the whole canopy (Δcanopy) through the separation of sunlit and shaded leaf groups; (iii) it has the capacity to examine the detailed interrelationships among plant water-use efficiency, isotope discrimination, and vapor pressure deficit; and (iv) it has the potential to investigate how an ecosystem discriminates against 13C at various time and spatial scales. The monthly mean isotopic signatures of ecosystem respiration (i.e. δ13CR) used for isotope flux calculation are retrieved from the nighttime flask data from the intensive campaigns (1998–2000) at 20 m level on Fraserdale tower, and the data from the growing season in 1999 are used for model validation. Both the simulated CO2 mixing ratio and δ13C of CO2 at the 20 m level agreed with the measurements well in different phases of the growing season. On a diurnal basis, the greatest photosynthetic discrimination at canopy level (i.e. Δcanopy) occurred early morning and late afternoon with a varying range of 10–26‰. The diurnal variability of Δcanopy was also associated with the phases of growing season and meteorological variables. The annual mean Δcanopy in 1999 was computed to be 19.58‰. The monthly averages of Δcanopy varied between 18.55‰ and 20.84‰ with a seasonal peak during the middle growing season. Because of the strong opposing influences of respired and photosynthetic fluxes on forest air (both CO2 and 13CO2) on both the diurnal and seasonal time scales, CO2 was consistently enriched with the heavier 13C isotope (less negative δ13C) from July to October and depleted during the remaining months, whereas on a diurnal basis, CO2 was enriched with the heavier 13C in the late afternoon and depleted in early morning. For the year 1999, the model results reveal that the boreal ecosystem in the vicinity of Fraserdale tower was a small sink with net uptake of 29.07 g 12C m−2 yr−1 and 0.34 g 13C m−2 yr−1. [ABSTRACT FROM AUTHOR]