N 2 O Emissions from Agricultural Soils in Canada – Preface

Abstract

Nitrous oxide (N2O) is an atmospheric contaminant that influences the radiation balance of the Earth and the concentration of stratospheric ozone. Its atmospheric concentration has increased considerably over the past century and this has initiated international agreements to address this environmental threat. In Canada, transformations of mineral nitrogen in agricultural soils are the largest source of N2O. Among the factors governing these reactions are the amount of nitrogen added to the soil via mineral and organic fertilizers and the aeration status of the soil. Consequently,the emission of soil N2O is not only influenced by farming practices,but is also governed by climatic conditions and soil properties. The effectiveness of proposed measures to reduce emission of N2O is variable because of the many factors and their interactions involved in N2O emissions. This special issue of Canadian Journal of Soil Science includes twelve scientific papers reporting assessment of N2O production by agricultural soils in Canada. Most studies whose results are presented were part of the research program ‘‘Model Farm’’ of Agriculture and Agri-Food Canada (Janzen et al. 2006). The papers describe the dynamics of N2O fluxes from agricultural soils with contrasting texture and subjected to different nitrogen fertilization rates,tillage practices and types of culture across Canada. Because the ultimate goal was to assemble a data set for testing mathematical models, particular attention was given to the measurement of parameters (soil,climate and governance) governing the production and emission of N2O. In addition,in order to compare the results from different experimental sites among themselves,the regional research teams have agreed to use the same standard method for the soil surface gas flux measurements (Rochette and Bertrand 2007) and other soil parameters. The experimental results presented reflect the wide temporal and spatial variability of soil N2O emissions. This is particularly true for a country as large as Canada characterized by important climatic gradients and contrasting soil types. It is shown that it is possible to identify the influence of factors such as nitrogen fertilizer rate and timing of application,as well as the environmental conditions,on the N2O dynamics. Indeed,a test of one model suggests that it could be successfully used in certain situations in Eastern Canada. It is hoped that the experimental data generated by these projects will allow the identification of models that can (1) assess the effectiveness of various practices for mitigating N2O emissions under Canadian conditions,and (2) be used to produce an accurate inventory of annual N2O emissions from agricultural soils in Canada.