No-tillage (NT), a practice that has been shown to increase carbon sequestration in soils, has resulted in contradictory effects on nitrous oxide (N 2 O) emissions. Moreover, it is not clear how mitigation practices for N 2 O emission reduction, such as applying nitrogen (N) fertilizer according to soil N reserves and matching the time of application to crop uptake, interact with NT practices. N 2 O fluxes from two management systems [conventional (CP), and best management practices: NT 1reduced fertilizer (BMP)] applied to a corn (Zea mays L.), soybean (Glycine max L.), winter-wheat (Triticum aestivum L.) rotation in Ontario, Canada, were measured from January 2000 to April 2005, using a micrometeorological method. The superimposition of interannual variability of weather and management resulted in mean monthly N 2 O fluxes ranging from À1.9 to 61.3 g N ha À1 day À1 . Mean annual N 2 O emissions over the 5-year period decreased significantly by 0.79 from 2.19 kg N ha À1 for CP to 1.41 kg N ha À1 for BMP. Growing season (May-October) N 2 O emissions were reduced on average by 0.16 kg N ha À1 (20% of total reduction), and this decrease only occurred in the corn year of the rotation. Nongrowing season (November-April) emissions, comprised between 30% and 90% of the annual emissions, mostly due to increased N 2 O fluxes during soil thawing. These emissions were well correlated (r 2 5 0.90) to the accumulated degree-hours below 0 1C at 5 cm depth, a measure of duration and intensity of soil freezing. Soil management in BMP (NT) significantly reduced N 2 O emissions during thaw (80% of total reduction) by reducing soil freezing due to the insulating effects of the larger snow cover plus corn and wheat residue during winter. In conclusion, significant reductions in net greenhouse gas emissions can be obtained when NT is combined with a strategy that matches N application rate and timing to crop needs.

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Large N2O emissions from agricultural soils have been reported during winter and spring thaw. The objective of this study was to assess the ability of the DNDC model to simulate N2O emissions resulting from freeze–thaw cycles, particularly the timing of flux events. The DNDC model was tested against micrometeorological fluxes measured during 5 yr in Ontario, Canada. There was a very large discrepancy between simulated and observed fluxes in terms of magnitude and timing. The simulated event occurred, on average, 38 d later than observed, and N2O fluxes were up to 3.5 times larger than the highest measured flux. Examination of simulated soil conditions indicated that the mechanism underlying freeze–thaw‐induced N2O flux in the DNDC model, release of ice‐trapped N2O, was not correct. This misconception had not been identified before, possibly because cold conditions in previous studies were not as extreme as observed in our data set or because continuously measured N2O fluxes were not available for model assessment. As a result of this analysis, DNDC 9.1 was revised by removing the release of ice‐trapped N2O and adding N2O newly produced by denitrification in the surface layer as the main mechanism for N2O production (DNDC 9.3). Comparison between simulated N2O fluxes using DNDC 9.3 and our data indicated improved timing to within 1 d of observed events. The magnitude of simulated flux differed from measurements by more than a factor of two, however, suggesting that an improved algorithm for N2O production and diffusion under soil freezing and thawing is needed.

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. 2010. Nitrous oxide fluxes related to soil freeze and thaw periods identified using heat pulse probes. Can. J. Soil Sci. 90: 409Á418. Surface N 2 O fluxes have not been unequivocally linked to soil profile conditions, in particular the timing of water phase change. The heated needle probe is a sensor that has the potential to monitor in situ apparent volumetric heat capacity (C a ), which considers latent heat transfer, during freezing and thawing. The objective of this study was to relate the timing of N 2 O flux to the occurrence of soil water phase change between liquid and ice as determined by C a in no-tillage (NT) and conventional tillage (CT) plots monitored from fall to spring. Half-hourly micrometeorological N 2 O fluxes were measured using a tunable diode laser trace gas analyzer. Apparent heat capacity was measured at 5-cm depth using three 4-cm-long parallel needles, two equipped with thermistors and one with a heater. Two N 2 O flux events were observed for CT in January, followed by the main emission event in early March. For NT, only one emission event occurred, with lower magnitude than the CT event, and a later starting and ending date. The apparent heat capacity measured in situ with HPP showed a different temporal pattern between NT and CT, with CT presenting more phase change events. Two out of the three N 2 O emission events in CT that occurred during winter and early spring occurred immediately after phase change from ice to liquid water at 5-cm depth. The N 2 O flux associated with the phase change during the main thaw event in CT was an exponential function of the soil surface temperature increasing sharply when T!08C, but with smaller fluxes once T was!58C. The temperature response observed is consistent with the suggestion of a breakdown in the N 2 O reduction process in the 0 to 58C range, while the N 2 O production enzymes are less affected by low temperature.Key words: Nitrous oxide flux, freeze-thaw cycles, heat pulse probes, no-tillage, conventional tillage Wagner-Riddle, C., Rapai, J., Warland, J. et Furon, A. 2010. Usage de sondes a`choc thermique pour caracte´riser les flux d'oxyde nitreux associe´s au gel et au de´gel du sol. Can. J. Soil Sci. 90: 409Á418. On n'a pas relie´de fac¸on certaine les flux superficiels de N 2 O aux diffe´rentes conditions du profil du sol, surtout au moment ou`l'eau change de phase. La sonde aà iguille chauffe´e est un capteur permettant de surveiller la densite´de stockage apparente (C a ) in situ, qui tient compte du transport de calories latent lors du gel et du de´gel. L'e´tude devait associer le moment ou`survient le flux de N 2 O a`celui oul 'eau passe de l'e´tat liquide a`l'e´tat solide selon la C a , dans des parcelles non travaille´es (NT) ou travaille´es de manie`re classique (TC), examine´es de l'automne au printemps. Les flux microme´te´orologiques de N 2 O ont e´te´mesure´s de demiheure en demi-heure avec un analyseur de gaz a`l'e´tat de traces a`diode laser. La densite´de stockage apparente a e´te´e´tablie a`5 cm de profondeur graˆce ...

A multi-layer model, combining Lagrangian dispersion at the canopy level with Ohm's Law analogy at the leaf level, was used in numerical simulations to assess the leaf-to-canopy scale translation of surface resistances. The model produced unique profiles of fluxes and scalar concentrations that satisfied both the dispersion and leaf models. Environmental factors and canopy architecture were varied, and stomatal conductance was simulated using either a simple relationship with net radiation or the Ball and Berry model to account for feedback mechanisms. Results showed that, when the assumptions of the Penman-Monteith equation were met, scaled-up leaf conductance closely matched the bulk canopy conductance. However, as the scenarios modeled departed from the ideal conditions of Penman-Monteith, the agreement decreased. In particular, correct estimation of the aerodynamic resistance, through correct parameterization of the roughness length for sensible heat, was identified as a key issue.

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