No abstract

Soil is an important source for the production of greenhouse gases such as CO2 and N2O. Significant temporal trends exist in the production and concentration of these gases in temperate climates where soils are subject to freezing. We examined the effect freeze‐thaw cycles had on soil profile CO2 and N2O concentrations. A multilevel sampling probe was designed to allow sampling the atmosphere composition of soil profiles. Changes in CO2 and N2O gas concentrations in the profile of a loamy sand were studied during two winter periods. Fluxes of N2O were strongly influenced by the formation of a frozen layer that segregated the soil profile into two distinct regions. The surface region was characterized by brief, intense events, which contribute to highly variable CO2 and N2O concentrations near the surface. The subsurface region beneath the ice layer allowed N2O accumulation. The highest concentrations of N2O were observed in the subsurface region following ice‐layer formation, indicating that N2O production is not restricted to surface horizons in this soil. Thawing of the frozen layer in the spring resulted in the release of N2O from the subsurface region. The formation of a frozen layer influenced subsurface CO2 accumulation through its influence on water percolation.

No abstract

. 1993. Nitrogen availability for corn in soils amended with urea' cattfe slurry, and solid anh composted manures. Can. J. Soil . This study was conducted to determine whether manu.e N availability for corn (Zea mays L.) was best estimated by a component of rhe manure N or by soil inorganic N in May or June. Liquid dairy cattle manure,-solid beef cattle manure, and composted beef cattle manure were applied in the spring of 1988, 1989 and 1990 at rates of 100, 200 unO IOO kg N ha-r. Urea was applied at rates of 50' 100 and 150 kg N ha-r for comparison. The N recovery Uy the harvested portion of the corn (grain * stover) in 1988 and 1990 averaged 49, 18, and 5% ofthe total N in urea, liquid dairy cattle manure, and solid or composted beefcatt-I. rnunu.", respectively. There was no yield response to any_N^source in 1989 because bttrigtr soit fertility. Relative nitrogenuptake by the corn grain + stov"er in-1988 and 1990 was significantly correlated with inorganic N appiiea as manure or fertilizer 1rz :0.56), but not with total N apptied 1r2 = 0.02). When-the data from all 3 years were analyzed, relative nitroqen uptake was better cbirelated with soil NH4 + NO3 in mid-May and soil NO3 in early-June (r' = 0.83 and 0.76' respectively), than with inorganiiN applied as manure or fertilizer (rz : O.20 1988 + 1990 or from all 3 years, relative N uptake was correlated with the total and inorganic N in the manures, soil NHo * NO, or NO, measured in mid-May, early June, and mid-June. A significance level of P < 0.05 was used throughout the analysis.

Maximizing the environmental and economic benefits of cover crops partially depends on an accurate estimate of the N fertilizer requirement of subsequent crops. Four trials involving cover crop, tillage, and N rate variables were conducted from 1992 to 1995 in southcentral Ontario on well‐drained Typic Hapludalf soils. Rye (Secale cereale L.), oilseed radish [Raphanus sativus (L.) var. oleiferus Metzg (Stokes)], oat (Avena sativa L.), and red clover (Trifolium pratense L.) cover crops were established after winter wheat (Triticum aestivum L.) to evaluate their effects on soil NO3–N levels as well as subsequent corn (Zea mays L.) grain yield response at fertilizer rates of 0 and 150 kg N ha−1. Corn response to cover crops was compared in autumn plow and no‐till tillage systems. Within no‐till, autumn vs. spring chemical kill for red clover and rye was also evaluated. Although red clover biomass N yields were usually at least double those with other cover crops, all cover crops were equally effective at lowering residual soil NO3–N concentrations following wheat harvest. Presidedress NO3–N concentrations after autumn‐killed or plowed red clover were at least 24% higher than after any other cover crop. Grain corn yield responses indicated that red clover substantially enhanced N availability to corn in both autumn plow and no‐till systems, but that oilseed radish, oat, and rye cover crops did not enhance N availability to succeeding corn, compared with the no‐cover treatment, in either tillage system. Furthermore, the presidedress NO3–N test reliably estimated N fertilizer requirements of corn following all cover crop systems except spring‐killed red clover.

Potential benefits associated with establishing cover crops, such as reduced NO3 leaching risk and lower fertilizer N requirements for succeeding crops, will be fully realized only when the cover crop N contribution is accurately accounted for and availability is synchronous with succeeding crop N demands. The objectives of this study were to evaluate spring soil NO3−N accumulation patterns and N availability to corn (Zea mays L.) following annual ryegrass (Lolium multiflorum L.), oilseed radish [Raphanus sativus (L.) var. oleiferus (Stokes) Metzg.], red clover (Trifolium pratense L.), and no cover crop established after either winter wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.). The wheat and barley were produced with 0.5, 1.0, and 2.0 times the amount of recommended N fertilizer. Six field trials were conducted on well‐drained Typic Hapludalf soils in southwestern Ontario intermittently from 1989 to 1995. Corn was produced using a spring mulch‐till system with only 10 kg ha−1 of fertilizer N, which was applied as part of the P starter fertilizer. Applying more fertilizer N to the previous year's small‐grain crop rarely increased spring soil NO3−N concentrations or corn yields. Soil NO3−N concentration increases between the May and June sample dates following annual ryegrass and oilseed radish did not differ substantially from where a cover crop had not been established; following red clover, however, NO3−N increases were always at least 2.8 times greater than after no cover crop. Average aboveground corn biomass N at anthesis following annual ryegrass was 25.6 kg ha−1 less than when no cover crop was grown, whereas following red clover it was 40.4 kg ha−1 greater than with no cover crop. Corn yields were consistently the highest following red clover and often the lowest following annual ryegrass; yield response was positively correlated with June soil NO3−N concentrations (r = 0.61‐0.93). These results suggest that N availability to succeeding corn differs among the cover crop treatments evaluated in the order red clover > oilseed radish ≥ no cover crop > annual ryegrass.

Field studies conducted throughout the calendar year are needed to improve flux estimates for the greenhouse gas nitrous oxide (N2O). In this study, we report monthly N2O emissions measured using micrometeorological techniques and a Tunable Diode Laser Trace Gas Analyzer (TDLTGA). Nitrous oxide fluxes were measured at the Elora Research Station (20 km north of Guelph, Ontario) from July to November 1992, and from March 1993 to February 1995, giving a total of 2445 daily averages obtained during the full length of the experiment. The soil at the experimental site was a Conestogo silt loam (Gleyed melanic brunisol). Several fields were monitored including fallow, manured fallow, Kentucky bluegrass, alfalfa, barley, canola, soybeans and corn plots. Spring thaw emissions from fallow or ploughed plots measured from March to April ranged from 1.5 to 4.3 kg N ha−1, corresponding to approximately 65% of the total annual emission. Similar effects were not observed on the vegetated (alfalfa and grass) plots. The lowest total annual N2O emissions were measured for second year alfalfa (1 kg N ha−1 yr−1) and bluegrass (0 to 0.5 kg N ha−1 yr−1). Higher annual emissions (2.5 to 4.0 kg N ha−1 yr−1) were observed for corn, barley, canola, and fallow plots. Highest annual emissions were measured after addition of nitrogen in the form of animal manure on a fallowed plot (5.7 to 7.4 kg N ha−1 yr−1), and alfalfa residue by fall-ploughing (6.1 kg N ha−1 yr−1). Plot management during the previous year affected N2O emissions, particularly on the soybean plot (5.9 kg N ha−1 yr−1) that followed a manured fallow treatment. The micrometeorological technique used in this study was successful at quasi-continuous monitoring of N2O fluxes from several plots, and therefore, useful for detecting long-term effects of management on emissions. Key words: Nitrous oxide, N2O fluxes, trace gases, agriculture, greenhouse gases

. 1997. Nitrous oxide emission from agricultural soils. Can. J. Soil Sci. 77: [113][114][115][116][117][118][119][120][121][122][123]. A review of the salient features of N 2 O emissions from agricultural soils was done to assess our current understanding and associated problems. Nitrous oxide is an important globe warming gas and a destructive agent of ozone in the stratosphere. A major concern is the increasing contribution of chemical fertilizers to atmospheric N 2 O buildup. There is only a limited understanding of the contributions from manures, biological N 2 fixation and crop residues. A recent estimate suggests that agriculture's share of N 2 O emissions is 80% although such estimates are highly uncertain because of imprecise data and the physical and biological complexities of the production process. As a product of the nitrification and denitrification process in soils, a major problem is our understanding of the proportion of N 2 O produced, i.e. the product ratios, although there is a good general understanding of the processes involved. Measurements of N 2 O emissions from the soil surface fail to take into account N 2 O flux from the bottom of the root zone into the subsoil and aquifers although they are generally considered to be significant. There is a need to apply newly available methodology and for combining this methodology and modelling together to predict N 2 O emissions on the landscape (or field) scale taking climate, soil and cropping variables into account. There is enough information available now to exercise some control of N 2 O emissions from cultivated soils. It is suggested that this be done focusing on factors that directly affect the soil microbes involved with the nitrification (NH 4 + , O 2 ) and denitrification (NO 3 -, C, O 2 ) processes. Cropping practices and some soil characteristic amendments are suggested herein for this purpose.Key words: Denitrification, nitrification, emission control, gas ratios Beauchamp, E. G. 1997. Émission d'oxyde nitreux à partir des sols agricoles. Can. J. Soil Sci. 77: 113-123. L'auteur fait un rappel des caractères saillants des émissions de N 2 O à partir des terres agricoles et évalue l'état de nos connaissances dans ce domaine ainsi que les problèmes qui s'y rattachent. L'oxyde nitreux est un agent important du réchauffement de la planète, en plus de contribuer à la destruction de l'ozone dans la stratosphère. Particulièrement préoccupante est la contribution croissante des engrais chimiques à l'accumulation de ce gaz dans l'atmosphère. On ne comprend que partiellement la part jouée par le fumier, par la fixation biologique de N 2 et par les restes de cultures. Selon une estimation récente, 80 % des émissions de N 2 O seraient imputables à l'agriculture. Ces chiffres sont, cependant, très incertains du fait de l'imprécision des données et de la complexité des mécanismes physiques et biologiques en cause dans les processus de production. Ce qui laisse le plus à désirer, c'est notre compréhension de la proportion de N 2 O, c.-à-d. les ratio...