Modelling soil compaction impacts on nitrous oxide emissions in arable fields

Nitrous oxide (N2O) emissions comprise the major share of agriculture's contribution to greenhouse gases; however, our understanding of what is actually happening in the field remains incomplete, especially concerning the multiple interactions between agricultural practices and N2O emissions. Soil compaction induces major changes in the soil structure and the key variables controlling N2O emissions. Our objective was to analyse the ability of a process-based model (Nitrous Oxide Emissions (NOE)) to simulate the impact of soil compaction on N2O emission kinetics obtained from field experiments. We used automatic chambers to continuously monitor N2O and CO2 emissions on uncompacted and compacted areas in sugar beet fields during 2 years. Soil compaction led to smaller CO2 emissions and larger N2O emissions by inducing anoxic conditions favourable for denitrification. Cumulative N2O emissions during the crop cycles were 944 and 977 g N ha?1 in uncompacted plots and 1448 and 1382 g N ha?1 in compacted plots in 2007 and 2008, respectively. The NOE model (H´enault et al., 2005) simulated 106 and 138 g N2O-N ha?1 in uncompacted plots and 1550 and 650 g N2O-N ha?1 in compacted plots in 2007 and 2008, respectively, markedly under-estimating the nitrification rates and associated N2O emissions. We modified the model on the basis of published results in order to better simulate nitrification and account for varying N2O fractions of total end-products in response to varying soil water and nitrate contents. The modified model (NOE2) better predicted nitrification rates and N2O emissions following fertilizer addition. Using a fine vertical separation of soil layers of configurable, but constant, thickness (1 cm) also improved the simulations. NOE2 predicted 428 and 416 g N-N2O ha?1 in uncompacted plots and 1559 and 1032 g N- N2O ha?1 in compacted plots in 2007 and 2008, respectively. These results show that a simple process-based model can be used to predict successfully the post-fertilizer addition kinetics of N2O emissions and the impact of soil compaction on these emissions. However, large emissions later on during the cropping cycle were not captured by the model, emphasizing the need for further research. (Résumé d'auteur