Lemke, R. L., Izaurralde, R. C., Nyborg, M. and Solberg, E. D. 1999. Tillage and N source influence soil-emitted nitrous oxide in the Alberta Parkland region. Can. J. Soil Sci. 79: 15-24. Zero tillage systems are receiving attention as possible strategies for sequestering atmospheric carbon. This benefit may be offset by increased N 2 O emissions, which have been reported for soils under zero tillage (ZT) compared to those under more intensive tillage (IT). Comparisons of N 2 O emissions from the two systems have been restricted to the growing season, but substantial losses of N 2 O have been reported during spring thaw events in many regions. Inorganic and organic additions of nitrogen and fallowing have also been shown to increase levels of soil-emitted N 2 O. The objectives for this study were: (i) to confirm that losses of N 2 O are higher under ZT than under IT in Alberta Parkland agroecosystems; (ii) to compare the relative influence of urea fertilizer (56 or 100 kg N ha -1 ), field pea residue (dry matter at 5 Mg ha -1 ), sheep manure (dry matter at 40 Mg ha -1 ) additions, and fallow on total N 2 O losses; and (iii) to investigate possible interactions between fertility and tillage treatments. Gas samples were collected using vented soil covers at three sites near Edmonton, Alberta during 1993, 1994, and 1995. Gas samples were analyzed using a gas chromatograph equipped with a 63 Ni electron capture detector. Estimated annual N 2 O loss ranged from 0.1 to 4.0 kg N ha -1 . Emissions during summer were slightly higher, similar, or lower on ZT compared to those under IT, but were consistently lower on ZT plots during spring thaw. Combined estimates (spring plus summer) of N 2 O loss under ZT were equal to or lower than those under IT. Highest overall losses were observed on fallow plots, followed by fertilizer, pea residue, and then either manure or control plots. We conclude that ZT management systems have potential for reducing agricultural greenhouse gas emissions in the Alberta Parkland region. Les émissions durant l'été étaient légèrement supérieures, semblables ou inférieures en semis direct qu'en travail classique, mais au dégel du printemps elles étaient régulièrement moins fortes en SD. Les valeurs combinées (printemps et été) des déperditions en SD étaient, soit semblables soit inférieures à celles obtenues en régime TI. C'est la jachère qui entraînait les déperditions totales les plus fortes, suivie de la fumure minérale, des restes de culture de pois et de, soit l'apport de fumier de mouton ou le traitement témoin. Il ressort de ces observations que le semis direct peut réellement permettre de réduire les émissions de gaz agricoles à effet de serre en Alberta dans la région de la tremblaie boréale.

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Previous field research in Alberta has suggested that denitrification occurs mostly when soil thaws in the spring, with associated soil water saturation. Our objective was to determine if denitrification and N2O emission in fact take place in cold, thawing soil in the field. Denitrification and N2O flux were measured in two springs and the intervening summer. Cylinders were placed in soil in November, 1988, and 57 kg N ha−1 of 15Nlabeled KNO3 was added. Soil 15N mass balance technique showed 23 kg N ha−1 of added-N was lost by 15 May 1989. Gas trappings were made (28 March to 29 April) and nearly all of the N2O emission (3.5 kg N2O-N ha−1) occurred during an 11-d period of thaw. The accumulated N2O flux from 20 June to 31 August was small (0.5 kg N2O-N ha−1, or less); during that time there were no rainfall events intense enough to produce water saturated soil. In 1990, 15N-labeled KNO3 (100 kg N ha−1) was applied on 26 March (outset of the thaw) and mass balance showed 32.7 kg N ha−1of added-N was lost by 7 May. A flux of 16.3 kg N2O-N ha−1 occurred largely in a 10-d period during and immediately after soil thaw. The N2O emitted from soil left a considerable fraction of the lost N unaccounted for. This unaccounted N was most likely lost as gaseous N other than N2O (e.g., N2). We conclude that large amounts of soil nitrate may be denitrified, with smaller amounts emitted as N2O, as the soil thaws and soon thereafter. Key words: Denitrification, frozen soil, thawing soil, nitrogen, nitrous oxide

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The temporal variability of soil‐derived N2O emissions presents a major challenge to the accurate quantification of N2O‐N losses from agroecosystems. We characterized the seasonal distribution of N2O emissions from two agricultural sites in the Parkland region of Alberta during 1993 and 1994. Treatments studied were fallow, and spring wheat (Triticum aestivum L.) with and without urea fertilizer, under conventional till management. Gas samples were collected from vented static soil chambers and were analyzed for N2O with a gas chromatograph equipped with a 63Ni electron capture detector. Soil water content and concentrations of NO‐3‐N, NH+4‐N, and water‐soluble organic C (WSOC) were measured several times during the season. A brief burst of N2O emission was recorded at both sites during and immediately following spring snow melt. A second period of activity occurred between mid‐June and mid‐July. Between 16 and 60% of estimated annual N2O‐N loss occurred during spring thaw, while >80% of cumulative annual N2O‐N loss had occurred by mid‐July. Mean soil NO‐3‐N concentration explained up to 65% of the temporal variability in geometric mean N2O emissions. A multiple regression model that included fall soil concentrations of NO‐3‐N, NH+4‐N, and WSOC explained 94% of the variability in estimated cumulative N2O‐N loss during the following spring thaw. Most N2O‐N losses in the Parkland region appear to occur during spring thaw and early summer; therefore, sampling schedules need to focus on these time periods. Management practices that minimize N availability during spring thaw may be an effective mitigation strategy for this region.