R.L. Desjardins

The fate of nitrogen in agroecosystems: An illustration using Canadian estimates

Agroecosystems rely on inputs of nitrogen (N) to sustain productivity. But added N can leak into adjacent environments, affecting the health of other ecosystems and their inhabitants. Worries about global warming have cast further attention on the N cycle in farmlands because farms are a main source of N2O, and because carbon sequestration, proposed to help reduce CO2 loads, requires a build-up of N. Our objective was to estimate, as an illustrative example, the net N balance of Canadian agroecosystems in 1996 and then infer some hypotheses about the routes of N loss, their magnitude, and ways of reducing them. We defined agroecosystems as all agricultural lands in Canada including soil to 1 m depth and all biota, except humans. Only net flows of N across those boundaries were counted in our balance – all others represent internal cycling. Based on our estimates, about 2.35 Tg N entered Canadian agroecosystems from biological fixation, fertilizers, and atmospheric deposition (excluding re-deposited NH3). In the same year, about 1.03 Tg N were exported in crop products and 0.19 Tg were exported in animals and animal products. Consequently, N inputs exceed exports in products by about 1.13 Tg, a surplus that is either accumulating in agroecosystems or lost to the environment. Because potential soil organic matter gains can account for only a small part of the surplus N, most is probably lost to air or groundwater. Our finding, that N losses amount to almost half of N added, concurs with field experiments that show crop recovery of added N in a given year is often not more than 60%. Better management may reduce the fraction lost somewhat but, because N in ecosystems eventually cycles back to N2, substantive gains in efficiency may not come easily. As well as trying to reduce losses, research might also focus on steering losses directly to N2, away from more harmful intermediates. If some of the `missing N can be assimilated into organic matter, agricultural soils in Canada may need little added N to achieve C sequestration targets.

ESTIMATED N2O AND CO2 EMISSIONS AS INFLUENCED BY AGRICULTURAL PRACTICES IN CANADA

The Denitrification-Decompostion (DNDC) model was used to estimate the impact of change in management practices on N2O emissions in seven major soil regions in Canada, for the period 1970 to 2029. Conversion of cultivated land to permanent grassland would result in the greatest reduction in N2O emissions, particularly in eastern Canada wherethe model estimated about 60% less N2O emissions for thisconversion. About 33% less N2O emissions were predicted for a changefrom conventional tillage to no-tillage in western Canada, however, a slight increase in N2O emissions was predicted for eastern Canada. GreaterN2O emissions in eastern Canada associated with the adoption of no-tillage were attributed to higher soil moisture causing denitrification, whereas the lower emissions in western Canada were attributed to less decomposition of soil organic matter in no-till versus conventional tilled soil. Elimination of summer fallow in a crop rotation resulted in a 9% decrease in N2O emissions, with substantial emissions occurringduring the wetter fallow years when N had accumulated. Increasing N-fertilizer application rates by 50% increased average emissions by 32%,while a 50% decrease of N-fertilizer application decreased emissions by16%. In general, a small increase in N2O emissions was predicted when N-fertilizer was applied in the fall rather than in the spring. Previous research on CO2 emissions with the CENTURY model (Smith et al.,2001) allowed the quantification of the combined change in N2O andCO2 emissions in CO2 equivalents for a wide range of managementpractices in the seven major soil regions in Canada. The management practices that have the greatest potential to reduce the combined N2O andCO2 emissions are conversion from conventional tillage to permanent grassland, reduced tillage, and reduction of summer fallow. The estimated net greenhouse gas (GHG) emission reduction when changing from cultivated land to permanent grassland ranged from 0.97 (Brown Chernozem) to 4.24 MgCO2 equiv. ha−1 y−1 (BlackChernozem) for the seven soil regions examined. When changing from conventional tillage to no-tillage the net GHG emission reduction ranged from 0.33 (Brown Chernozem) to 0.80 Mg CO2 equiv. ha−1 y−1 (Dark GrayLuvisol). Elimination of fallow in the crop rotation lead to an estimated net GHG emission reduction of 0.43 (Brown Chernozem) to 0.80 Mg CO2 equiv.ha−1 y−1 (Dark Brown Chernozem). The addition of 50% more or 50% less N-fertilizer both resulted in slight increases in combined CO2 and N2O emissions. There was a tradeoff in GHG flux with greaterN2O emissions and a comparable increase in carbon storage when 50% more N-fertilizer was added. The results from this work indicate that conversion of cultivated land to grassland, the conversion from conventional tillage to no-tillage, and the reduction of summerallow in crop rotations could substantially increase C sequestration and decrease net GHG emissions. Based on these results a simple scaling-up scenario to derive the possible impacts on Canadas Kyoto commitment has been calculated.

Quantifying the Reduction of Greenhouse Gas Emissions as a Resultof Composting Dairy and Beef Cattle Manure

Greenhouse gas emissions from the agricultural sector can be reduced through implementation of improved management practices. For example, the choice of manure storage method should be based on environmental decision criteria, as well as production capacity. In this study, greenhouse gas emissions from three methods of storing dairy and beef cattle manure were compared during the summer period. The emissions of CH4, N2O and CO2 from manure stored as slurry, stockpile, and compost were measured using a flow-through closed chamber. The largest combined N2O–CH4 emissions in CO2 equivalent were observed from the slurry storage, followed by the stockpile and lastly the passively aerated compost. This ranking was governed by CH4 emissions in relation to the degree of aerobic conditions within the manure. The radiative forcing in CO2 equivalent from the stockpiled manure was 1.46 times higher than from the compost for both types of cattle manure. It was almost twice as high from the dairy cattle manure slurry and four to seven times higher from the beef cattle manure slurry than from the compost. The potential reduction of GHG was estimated, by extrapolating the results of the study to all of Canada. By composting all the cattle manure stored as slurry and stockpile, a reduction of 0.70xa0Tg CO2-eqxa0year−1 would be achieved. Similarly, by collecting and burning CH4 emissions from existing slurry facilities, a reduction of 0.76xa0Tg CO2-eqxa0year−1 would be achieved. New CH4 emission factors were estimated based on these results and incorporated into the IPCC methodology. For North-America under cool conditions, the CH4 emission factors would be 45xa0kg CH4 hd−1xa0year−1 for dairy cattle manure rather than 36xa0kg CH4 hd−1xa0year−1, and 3xa0kg CH4 hd−1xa0year−1 for beef cattle manure rather than 1xa0kg CH4 hd−1xa0year−1.

Post-fire carbon dioxide fluxes in the western Canadian boreal forest: evidence from towers, aircraft and remote sensing

Abstract Recent CO 2 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 CO 2 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 CO 2 flux relative to 10 years following fire=0.11+0.92xa0log 10 (years since fire) ( r 2 =0.5). The CO 2 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.

Spatial and temporal variations of the fluxes of carbon dioxide and sensible and latent heat over the FIFE site

Airborne measurements of flux densities of carbon dioxide CO2, sensible heat, and latent heat (H2O) obtained over the First ISLSCP Field Experiment (FIFE) site during three intensive field campaigns in 1987 and one in 1989 are examined to characterize the spatial and temporal variations of CO2 and energy transfer processes. These data were collected by the National Research Council Twin Otter using low-level flight patterns, all flown at constant pressure altitude during relatively clear days. The spatial variations are larger in 1989 than in 1987 and a higher correlation is observed between the fluxes and the surface features. The temporal patterns are easier to characterize with the relatively homogeneous situation of 1987. Functional relationships obtained between fluxes of CO2 and latent heat, CO2 fluxes and greenness index, latent heat fluxes and greenness index, and between sensible heat fluxes and surface air temperature differences are presented for one day in 1987 and one in 1989 as an example of the kind of information that can be obtained from grid flights at constant pressure altitude. 20 refs.

Measuring nighttime CO2 flux over terrestrial ecosystems using eddy covariance and nocturnal boundary layer methods

The respiration from plants and soil is an important component of the carbon balance of ecosystems. Its measurement is challenging due to the relatively small size of the carbon dioxide fluxes and because these fluxes occur under environmental conditions that are frequently unfavorable for flux measurements. Micrometeorological techniques based on turbulent transfer frequently underestimate CO2 fluxes during nighttime conditions. An approach based on calculating the CO 2 budget in the nocturnal boundary layer (NBL) might be an alternative during light wind conditions to estimate nighttime CO2 fluxes. This study presents typical net CO2 efflux observations measured at night over agricultural crops for several years near Ottawa, Ont. and over an old black spruce stand near Candle Lake, Sask. during the intensive field campaigns of the Boreal Ecosystem Atmosphere Study (BOREAS). We used the eddy covariance technique for windy nights and the NBL budget approach for calm nights. Criteria for screening data into either windy or calm conditions were made using the friction velocity ( u∗) and the standard deviation of the vertical wind speed (σ W). The threshold at which the 30 min turbulent CO2 flux observations were independent of u∗ or σ W and had limited scatter were determined to be in the range 0.075–0.1 m s −1 for u∗ and σ W, and about 1.5 m s −1 for horizontal wind speed (U) for multiple years of corn and soybean data. σW ≥ 0. 4ms −1 was shown to be a good screening threshold over the black spruce canopy. Current methods for handling nocturnal CO2 data involve systematically replacing data during calm conditions where eddy covariance is deficient with those from windy conditions. This can lead to an overestimation of the nocturnal CO2 flux. We suggest a variation in this procedure which, through the screening of entire nights, allows the retaining of an acceptable proportion of calm periods within predominantly windy nights. The NBL budget method requires calm nights for measuring the respiration unless other budget terms are quantified. A good agreement was found between CO2 flux measured using the NBL approach when the NBL was well developed and the eddy covariance technique, when restricted to windy nights. The NBL profiles integrate a larger area than eddy covariance, which means that high-emission spots can be included with this kind of approach. Crown Copyright

Estimation of maize (Zea mays L.) canopy conductance by scaling up leaf stomatal conductance

Abstract Transpiration is partially controlled by the plant at leaf level through the degree of aperture of the stomata. Mathematical models estimating the transpiration of plant stands using a conductance network approach to water vapor transfer thus need a plant surface control term ( g s ). Techniques involving different degrees of simplification of canopy structure have been proposed to estimate g s , from measurements or estimates of leaf stomatal conductance ( g s ). This study compares the performance of some of these techniques by examining the patterns of diurnal and seasonal variation for a maize crop grown at Ottawa, Canada. Measurements of g s , were made for sunlit and shaded leaves at three levels in the plant canopy and the response of g s , to photosynthetic photon flux density ( Q p ) has been parameterized. Values of g s , obtained by different scaling-up methods were compared among themselves and with those obtained from the Penman-Monteith equation ( g c ). The main results and conclusions were: daily maximum g s , values of sunlit leaves generally occurred on or slightly before maximum radiation; the response of g s to low Q p was a function of leaf levels in the canopy and growth stage; the response of g s to low Q p increased with leaf area index for shaded leaves; modelling of g s , based on Q p , leaf temperature and leaf water potential failed to give good estimates under overcast afternoon conditions; measurement of g s , on horizontal portions of leaves led to an overestimation of g s ; spherical leaf angle distribution assumption gave the best estimates of g s ; the shelter factor (ratio of scaled-up g s , over g c ) tended to increase as the ratio of the canopy aerodynamic conductance to top leaf stomatal conductance increased.

Estimates of the interannual variations of N2O emissions from agricultural soils in Canada

The DNDC model was used to estimate direct N2O emissions from agricultural soils in Canada from 1970 to 1999. Simulations were carried out for three soil textures in seven soil groups, with two to four crop rotations within each soil group. Over the 30-year period, the average annual N2O emission from agricultural soils in Canada was found to be 39.9 Gg N2O–N, with a range from 20.0 to 77.0 Gg N2O–N, and a general trend towards increasing N2O emissions over time. The larger emissions are attributed to an increase in N-fertilizer application and perhaps to a trend in higher daily minimum temperatures. Annual estimates of N2O emissions were variable, depending on timing of rainfall events and timing and duration of spring thaw events. We estimate, using DNDC, that emissions of N2O in eastern Canada (Atlantic Provinces, Quebec, Ontario) were approximately 36% of the total emissions in Canada, though the area cropped represents 19% of the total. Over the 30-year period, the eastern Gleysolic soils had the largest average annual emissions of 2.47 kg N2O–N ha−1 y−1 and soils of the dryer western Brown Chernozem had the smallest average emission of 0.54 kg N2O–N ha−1 y−1. On average, for the seven soil groups, N2O emissions during spring thaw were approximately 30% of total annual emissions. The average N2O emissions estimates from 1990 to 1999 compared well with estimates for 1996 using the IPCC methodology, but unlike the IPCC methodology our modeling approach provides annual variations in N2O emissions based on climatic differences.

Management Strategies to Sequester Carbon in Agricultural Soils and to Mitigate Greenhouse Gas Emissions

Carbon sequestration in agricultural soils is frequently promoted as a practical solution for slowing down the rate of increase of CO2 in the atmosphere. Consequently, there is a need to improve our understanding of how land management practices may affect the net removal of greenhouse gases (GHG) from the atmosphere. In this paper we examine the role of agriculture in influencing the GHG budget and briefly discuss the potential for carbon mitigation by agriculture. We also examine the opportunities that exist for increasing soil C sequestration using management practices such as reduced tillage, reduced frequency of summer fallowing, introduction of forage crops into crop rotations, conversion of cropland to grassland and nutrient addition via fertilization. In order to provide information on the impact of such management practices on the net GHG budget we ran simulations using CENTURY (a C model) and DNDC (a N model) for five locations across Canada, for a 30-yr time period. These simulations provide information on the potential trade-off between C sequestration and increased N2O emissions. Our model output suggests that conversion of cropland to grassland will result in the largest reduction in net GHG emissions, while nutrient additions via fertilizers will result in a small increase in GHG emissions. Simulations with the CENTURY model also indicated that favorable growing conditions during the last 15 yr could account for an increase of 6% in the soil C at a site in Lethbridge, Alberta.

Footprint considerations in BOREAS

Comparisons of observations of concentration or flux from platforms at various heights, such as tower and aircraft, must take into account differences in the location and extent of upwind surface source or sink areas which affect the individual observations, with their physical and biological characteristics. Such “footprint” estimates are based on solutions of the diffusion/advection equation which have not previously been evaluated over a boreal ecosystem. In order to adjust an analytical footprint model within the surface layer above forest canopies typical for the Boreal Ecosystem-Atmosphere Study (BOREAS) sites, 29 tracer gas release experiments were carried out between August 30 and September 9, 1994, at three tower sites in the northern study area (NSA). Sulphur hexafluoride (SF6) was released from point sources at various upwind distances from the towers under various meteorological, terrain, and release conditions. Wind, temperature, and stability parameters, during each release period, were used as input into calculations of vertical concentration profiles sampled at the towers, based on a three-dimensional diffusion model. Predictions of concentration profiles, or back calculation of source strength from observed profiles, were in good agreement with observed concentrations or actual release rates, respectively. The diffusion model was then used to compute footprint estimates for stable to unstable conditions, for tower and aircraft-based observation platforms. They show spatially constrained footprints in the surface layer, due to effective vertical coupling, so observations from towers and low-flying aircraft must be expected to be very site specific, and scaling up to larger areas will have to be done with careful consideration of surface mosaics. Our study also included footprint estimates made for airborne observations above the surface layer, based on large-eddy simulations over “BOREAS-like” terrain, for boundary layer structures comparable to those observed in BOREAS. They document the progressive decoupling of airborne observations from the surface features at these heights.