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Highest rates of N2O emissions from fertilized as well as natural ecosystems have often been measured at spring thaw. But, it is not clear if management practices have an effect on winter and spring thaw emissions, or if measurements conducted over several years would reveal different emission patterns depending on winter conditions. In this study, we present N2O fluxes obtained using the flux-gradient approach over four winter and spring thaw periods, spanning from 1993 to 1996, at two locations in Ontario, Canada. Several agricultural fields (bare soil, barley, soybean, canola, grass, corn) subjected to various management practices (manure and nitrogen fertilizer addition, alfalfa ploughing, fallowing) were monitored. Nitrous oxide emissions from these fields from January to April over four years ranged between 0 and 4.8 kg N ha-1. These thaw emissions are substantial and should be considered in the nitrous oxide budgets in regions where thaw periods occur. Our study indicates that agricultural management can play a role in mitigating these emissions. Our data show that fallowing, manure application and alfalfa incorporation in the fall lead to high spring emissions, while the presence of plants (as in the case of alfalfa or grass) can result in negligible emissions during thaw. This presents an opportunity for mitigation of N2O emissions through the use of over-wintering cover crops.

Flux-gradient relationships based upon similarity theory have been reported to severely underestimate scalar fluxes in the roughness sublayer above forests, as compared to independent flux estimates (for example, eddy covariance or energy balance measurements). This paper presents the results of a unique three-month investigation into the validity of similarity theory in the roughness sublayer above forests. Eddy covariance and flux-gradient measurements of carbon dioxide (CO^sup 2^) exchange were compared above a mixed deciduous forest at Camp Borden, Ontario, both before and after leaf senescence. The eddy covariance measurements used a Li-Cor infrared gas analyzer, and the flux-gradient (similarity theory) measurements featured a tunable diode laser Trace Gas Analysis System (TGAS). The TGAS resolved the CO^sup 2^ concentration difference to 300 parts per trillion by volume (ppt) based upon a half-hour sampling period. The measured enhancement factor γ (the ratio of independent flux estimates, in this case eddy covariance, to similarity theory fluxes) was smaller and occurred closer to the canopy than in most previous investigations of similarity theory. Very good agreement between the eddy covariance and similarity theory fluxes was found between 1.9 and 2.2 canopy heights (h^sup c^), and the mean enhancement factors measured before and after leaf senescence were 1.10 plusmn; 0.06 and 1.24 ± 0.07, respectively. Larger discrepancies were measured closer to the canopy (1.2 to 1.4 h^sup c^), and mean enhancement factors of 1.60 ± 0.10 and 1.82 ± 0.11 were measured before and after leaf senescence, respectively. Overall, the Borden results suggest that similarity theory can be used within the roughness sublayer with a greater confidence than previously has been believed.[PUBLICATION ABSTRACT]

A micrometeorological mass balance technique was used to quantify the N2O flux from a solid dairy manure pile under field conditions. Flux was determined using time-averaged measurements of wind speed, and nitrous oxide concentration using a tunable diode laser trace gas analyzer. A total of 66 hourly flux averages were collected and values were never lower than 200 ng N2O-N m−2 s−1. The mean hourly N2O flux was 4865 ng N2O-N m−2 s−1 (0.42 g N m−2 day−1), which is of the same order of magnitude, albeit higher, as previously observed for a similar solid pig manure storage.

A small, inexpensive infrared thermometer is described. This instrument is easily used and is more accurate than thermocouples for leaf temperature measurements. Errors are estimated to be less than 0.2 °C when measuring leaf temperatures in a typical leaf chamber.

Modifications of the design and calibration procedure of a diffusion porometer permit determinations of stomatal resistance which agree well with results obtained by leaf energy balance. The energy balance and the diffusion porometer measurements indicate that the boundary layer resistances of leaves in the field are substantially less than those predicted from heat transport formulas based on wind flow and leaf size.

A three-dimensional pressure-sphere anemometer and fast thermometer system (P.S.A.T.) was used to measure vertical heat flux density in the atmospheric surface layer at 1–4 m above alta fescue and snap beans. Good agreement with independent measurements was obtained, which shows the the P.S.A.T. is sufficiently small and has adequate high-frequency response and accuracy for eddy correlation measurements within 1 m of the surface. Also obtained with the P.S.A.T. were $\\left(\\overline{\\mathrm{u}\\prime \\mathrm{T}\\prime }\\right)/\\left(\\overline{\\mathrm{w}\\prime \\mathrm{T}\\prime }\\right)$, ru,T, rw,T and σT/T*, and their dependence upon stability. When the atmosphere was thermally stable, slow wave motions frequently increased σT even though turbulent mixing was lacking.