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The long-term understanding of the carbon budget of cropping systems needs to consider varying meteorological conditions, site/soil conditions, crop selection, and management decisions. On the Canadian Prairies, a variety of crops are used in yearly rotation, often not repeating the same crop within a 3-year period. We measured net ecosystem production (NEP) in two cropping systems in southern Manitoba, Canada, for the 2006–2016 period. One system had continuous annual crops, whereas the other had a perennial phase for 4 years. NEP was calculated using the flux-gradient method, where the carbon dioxide gradient was measured with a tunable-diode laser analyser and the transfer coefficient was based on momentum similarity theory with inputs from sonic anemometer-thermometers. NEP varied from −100 (source) to +300 (sink) g C m −2 y −1 among four adjacent 4-ha fields and 10 years, depending on annual crop (corn, faba, spring wheat, barley, rapeseed, soybean) and environmental conditions. There was no period of 3 years or greater when NEP in the continuous annual crop rotation was different from carbon neutral. Inserting a 4-year perennial phase of an alfalfa/grass mix created a statistically significant carbon sink for NEP (P = 0.04), but the perennial period itself was carbon neutral. Enhanced ecosystem respiration was observed following termination of the perennial crop. Harvest practices varied from no harvest (in the establishment year of the perennial crop), grain removal only, grain + straw removal, and straight cutting of green biomass for silage. Once harvest removals were included, 3 of the 4 fields were carbon neutral, but one annual field had net carbon losses for some periods. Ecosystem respiration averaged about twice as large as harvest removals. Net carbon (mean ±S.E.) over the 11-year period for the annual cropping system (−78 ± 51 g C m −2 y −1 ) was not different from the annual-perennial system (−48 ± 47 g C m −2 y −1 ). [ABSTRACT FROM AUTHOR]

Old-growth (>100 years old) boreal forests are recognized as having accumulated large inventories of terrestrial carbon, but the magnitude of annual net exchange is not well established. Eddy covariance measurements of carbon dioxide exchange were made at a flux tower at the Northern Old Black Spruce forest in Manitoba, Canada, from 1994 to 2008. We evaluated the uncertainty in estimates of annual net ecosystem production by comparing four methods of filling missing measurement periods. Three of the methods were previously developed by groups at Harvard University, the Fluxnet Canada Research Network, and the Max Planck Institute of Biogeochemistry; and the fourth method used the mean of measurements from the dataset selected for the same time and day. Two methods showed carbon losses near the beginning of the period but all four methods estimated carbon gains in the later years. Individual years and methods ranged ±90 g C m −2 y −1 , demonstrating a wide range of estimates if data had been available from only a single year and a single method. When averaged over multiple years, the mean and standard error among methods gave a net forest carbon sink of 29 ± 10 g C m −2 y −1 , ranging from 4 to 48 g C m −2 y −1 . Only the upper (Mean Data and Max Planck Institute of Biogeochemistry methods) and lower (Harvard method) estimates were significantly different, with the Fluxnet Canada Research Network method being the same as all other methods. There was no difference among methods for the January to August period, or for daytime periods. All methods were statistically different from a carbon-neutral forest suggesting that the Northern Old Black Spruce forest was likely a small carbon sink. Methods that estimated high ecosystem respiration also estimated high gross ecosystem production, and the small net differences could not be easily attributed to differences in flux partitioning. For forests that are close to carbon neutral, the use of several gap-filling methods: increases confidence in conclusions regarding the ecosystem carbon balance. [ABSTRACT FROM AUTHOR]

Highlights: [•] NEP, harvest removals, and manure additions of carbon were measured. [•] Conversion of perennial grasslands to annual crops loses soil carbon. [•] Carbon was lost from fields with annual cropping systems. [•] Net ecosystem production was highly variable among years for a hay field. [ABSTRACT FROM AUTHOR]

Recent CO2 flux measurements from towers and aircraft (net ecosystem exchange by eddy covariance) and remote sensing/modeling (net primary productivity—NPP) following fire show that the regenerating boreal forest in western Canada has a low initial flux that increases with time since fire. Daytime CO2 fluxes are downward, even after 2 years following fire, although fluxes were upward during the first year after the fire. In summer, the forest is a net carbon sink a few years following fire. A regression of all data gives a relationship where the CO2 flux relative to 10 years following fire=0.11+0.92 log10 (years since fire) (r2=0.5). The CO2 flux reaches the same rate as that of a mature site between 10 and 30 years following fire, depending on the site and the data set. Many studies in the literature indicate that soil respiration decreases following fire, although several models assume that heterotrophic respiration increases. If fire reduces respiration and our growing season measurements showing a net sink in early years are widely applicable, it is likely that some models may have overestimated the impact of fire on the carbon balance of the boreal landscape. [Copyright &y& Elsevier]

• Flux-gradient N 2 O emissions were monitored over 4 agricultural fields for 11 years. • 50% of emissions followed spring fertilizer application and 15–20% occurred during spring thaw. • N application rate and time integrated nitrate concentration were more important than environmental factors to N 2 O. • Emissions were less during inclusion of a 4-year perennial legume phase in an annual crop rotation. • Over 11-years, emissions were not reduced with inclusion of the perennial phase because of large emissions following its termination. • Net CO 2 equivalent emissions based on partial life cycle analysis showed both treatments to be a similar source. Nitrous oxide (N 2 O) emissions were measured over 4 fields in Manitoba, Canada, continuously from 2006 to 2016 using the micrometeorological flux-gradient technique. The fields were divided into 2 treatments: 2 fields with a rotation of annual crops (corn, faba bean, spring wheat, rapeseed, barley, spring wheat, corn, soybean, spring wheat, soybean, soybean); while 2 fields had a four-year period with a leguminous perennial crop in the rotation (corn, faba bean, alfalfa/grass, alfalfa/grass, alfalfa/grass, alfalfa/grass, corn, soybean, spring wheat, soybean, soybean). Nitrogen fertilizer was applied according to local practice, and was typically not applied in years when a legume was planted. N 2 O was emitted from the field in episodes during spring thaw and following nitrogen fertilizer application. Emissions were less during the perennial phase of the comparison (mean 1 kg N ha−1 y−1 from perennial fields; 4.7 kg N ha−1 y−1 from the annual fields; P < 0.001). However, over the 11-year period, the treatments were not different (P = 0.3), averaging 5 kg N ha−1 y−1 because of large emissions of about 20 kg N ha−1 y−1 in the year following termination of the perennial crop. Nitrogen fertilizer application rate explained 69% of the variability in emissions (P < 0.001), with only a marginal advantage including other environmental variables such as soil nitrate exposure or soil ammonium concentrations. The N 2 O flux data were combined with carbon dioxide flux measurements, fertilizer N manufacturing, and potential product end uses to estimate the net greenhouse gas budget. Net emissions were a source of 1.7 Mg CO 2 equivalents ha−1 y−1, driven by 3 Mg CO 2 equivalents ha-1 y-1 source from N 2 O emissions, net carbon exchange of 3 to 5 Mg CO 2 equivalents ha-1 y-1 sink, and a consumption source of 6 to 7 Mg CO 2 equivalents ha-1 y-1. High inter-annual variability in fluxes resulted in no significant difference (P = 0.87) in overall greenhouse gas emissions between treatments over the full period including N fertilizer inputs and end uses of crop products. [ABSTRACT FROM AUTHOR]

Abstract: Agricultural soils are a significant anthropogenic source of nitrous oxide (N2O) to the atmosphere. Despite likely having large emissions of N2O, there are no continuous multi-year studies of emissions from poorly drained floodplain soil. In the present study, the micrometeorological flux of N2O (F N ) was measured over three years (2006–2008) in a maize (Zea mays L.)/faba (Vicia faba minor L.)/spring-wheat (Triticum aestivum L.) rotation in the Red River Valley, Manitoba, Canada on a gleyed humic verticol soil. Comparison of newly established reduced and intensive tillage treatments showed no difference in F N within the constraints of the high variability between duplicate plots. The annual gap-filled ΣF N across tillage treatments was 5.5, 1.4, and 4.3kgNha−1 in the maize, faba, and spring-wheat crop years, respectively. Emissions from fertilizer N addition and soil thaw the following spring was responsible for the greater ΣF N in the maize and spring-wheat years. Using four approaches to approximate background ΣF N resulted in estimates of 3.5–3.8% and 1.4–1.8% of applied fertilizer N emitted as N2O for the maize and spring-wheat crops, respectively. The CO2 global warming potential equivalent of ΣF N over the three study years was an emission of 5.4MgCO2-equiv.ha−1 which adds to the previously determined C balance emission of 11.6MgCO2-equiv.ha−1. [Copyright &y& Elsevier]

Abstract: Light use efficiency (LUE) is used widely in scaling and modeling contexts. However, the variation and biophysical controls on LUE remain poorly documented. Networks of eddy covariance (EC) towers offer an opportunity to quantify ɛ g, the ratio of P, gross primary productivity, to Q a, absorbed photosynthetically active radiation (PAR), across climate zones and vegetation types. Using data from the Fluxnet Canada Research Network (n =24 sites) in 2004, we examined the relationship between daily and yearly ɛ g, driving variables, and site characteristics on a site-specific and plant functional type (PFT) basis using tree regression and linear regression. Data were available for three biomes: grassland, forest, and wetland. Yearly ɛ g values ranged from 0.1 to 3.6gCMJ−1 Q a overall. Daily ɛ g was highest in the grassland (daily median±interquartile range: 3.68±1.98gCMJ−1 Q a), intermediate in the forested biome (0.84±0.82gCMJ−1 Q a), and lowest for the wetlands (0.65±0.54gCMJ−1 Q a). The most important biophysical controls were light and temperature, to the exclusion of water-related variables: a homogeneity of slopes model explained c. 75% of the variation in daily ɛ g. For a subset of sites with diffuse PAR data, the ratio of diffuse to total PAR, a proxy for cloudiness, was a key predictor. On the yearly time scale, ɛ g was related to leaf area index and mean annual temperature. Aggregating to PFTs did not show functional convergence within any PFT except for the three wetland sites and the Picea mariana toposequence at the daily time step, and when using the Köppen climate classification on a yearly time step. The general lack of conservative daily ɛ g behavior within PFTs suggests that PFT-based parameterizations are inappropriate, especially when applied on shorter temporal scales. [Copyright &y& Elsevier]

Abstract: The effects of climatic factors and vegetation type on evapotranspiration (E) and water use efficiency (WUE) were analyzed using tower-based eddy-covariance (EC) data for nine mature forest sites, two peatland sites and one grassland site across an east–west continental-scale transect in Canada during the period 2003–2006. The seasonal pattern of E was closely linked to growing-season length and rainfall distribution. Although annual precipitation (P) during the observation period was highly variable among sites (250−1450mm), minimum annual E was not less than 200mm and was limited to 400−500mm where annual P exceeded 700mm. Site-specific interannual variability in E could be explained by either changes in total P or variations in solar irradiance. A highly positive linear correlation was found between monthly mean values of E and net radiation (R n ) at the grassland site (AB-GRL), the two peatland sites (AB-WPL and ON-EPL), and only one of the forest sites (coastal Douglas-fir, BC-DF49) whereas a hysteretic relationship at the other forest sites indicated that E lagged behind the typical seasonal progression of R n . Results of a cross-correlation analysis between daily (24-h) E and R n revealed that site-specific lag times were between 10 and 40 days depending on the lag of vapour pressure deficit (D) behind R n and the decoupling coefficient, Ω. There was significant seasonal variation in daytime mean dry-foliage Priestley–Taylor α with maxima occurring in the growing season at all sites except BC-DF49 where it was relatively constant (∼0.55) throughout all years. Annual means of daytime dry-foliage α mostly ranging between 0.5 and 0.7 implied stomatal limitation to transpiration. Increasing D significantly decreased canopy conductance (g c ) at the forest sites but had little effect at the peatland and grassland sites, while variation in soil water content caused only minor changes in g c . At all sites, a strong linear correlation between monthly mean values of gross primary production (GPP) and E resulted in water use efficiency being relatively constant. While at most sites, WUE was in the range of 2.6–3.6gCkg−1 H2O, the BC-DF49 site had the highest WUE of the twelve sites with values near 6.0gCkg−1 H2O. Of the two peatland sites, AB-WPL, a western treed fen, had a significantly higher WUE (∼3.0gCkg−1 H2O) than ON-EPL, an eastern ombrotrophic bog (∼1.8gCkg−1 H2O), which was related to peatland productivity and plant functional type. [Copyright &y& Elsevier]

Abstract: We describe an approach for evaluating the representativeness of eddy covariance flux measurements and assessing sensor location bias (SLB) based on footprint modelling and remote sensing. This approach was applied to the 12 main sites of the Fluxnet-Canada Research Network (FCRN)/Canadian Carbon Program (CCP) located along an east-west continental-scale transect, covering grassland, forest, and wetland biomes. For each site, monthly and annual footprint climatologies (i.e. monthly or annual cumulative footprints) were calculated using the Simple Analytical Footprint model on Eulerian coordinates (SAFE). The resulting footprint climatologies were then overlaid on to images of the Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) derived from LANDSAT Thematic Mapper (TM) imagery, which were used as surrogates of land surface fluxes to estimate SLB. Results indicate that (i) the sizes of annual footprint climatology increased exponentially with increasing cumulative footprint percentages and, for a given percentage of footprint climatology, the footprint areas were significantly different among the sites. Typically, the 90% annual footprint climatology areas varied from 1.1km2 to 5.0km2; (ii) using either NDVI or EVI as the flux surrogate, the SLB was less than 5% for most sites with respect to the reference area of interest (A r ) at 90% annual footprint climatology (scenario A) and a circular area with radius of 1km centred at the individual tower (scenario B), with several exceptions; (iii) the SLB decreased with increasing size of footprint climatology for all sites for both scenarios A and B; (iv) out of 12, eight flux towers represented most of the ecosystem surrounding the towers for an area of 0.3km2 up to 10km2 with a satisfactorily low bias of <5%, whereas four towers represented areas ranging from only 0.75 to 4km2; and (v) the seasonal differences in monthly SLB using NDVI as a flux surrogate were about 1–4% for most sites for both scenarios A and B. [Copyright &y& Elsevier]