Nitrogen fertilization but not soil tillage affects nitrous oxide emissions from a clay loam soil under a maize–soybean rotation

Conversion of agricultural systems from conventional to no tillage practices usually increases soil carbon storage. However, adoption of no tillage may increase emissions of N2O, a very potent greenhouse gas. The objective of the study was to determine the effects of tillage (mouldboard plough [MP] and no-tillage [NT]) and three mineral N application rates (0, 80 and 160 kg N ha−1) on annual and biennial (2004–2005) N2O fluxes. The experiment was organized using a split-plot design using tillage as the main plots and fertilizer rates as the sub-plot. The site was an artificially drained and therefore well structured humic gleysol near Montreal QC, on a maize (Zea mays L.)–soybean (Glycine max L.) rotation. Emissions (April–November) ranged from 1.0 to 2.5 kg N2O–N ha−1. Throughout both years tillage had no effect (P > 0.05) on N2O emissions. During 2004, under maize cultivation, N2O emissions increased with the N fertilization rate (P < 0.001). Between 0.9 and 1.3% of applied fertilizer N was lost as N2O–N, similar to the IPCC estimate of 1%. There was also no significant interaction between tillage and N fertilization rate on N2O emissions during either year (P = 0.79). During the second year, under soybean cultivation, no N fertilizer was applied. As N2O emissions from the different fertilizer treatments were similar (P = 0.99), we concluded that there was no carry-over effect of the fertilization applied to maize in the previous year. Yield-based N2O emissions were similar between the different tillage types (P = 0.63 and 0.12 for 2004 and 2005, respectively) and fertilization rates (P = 0.30 and 0.84 for 2004 and 2005, respectively) and ranged from 17.0 to 22.7 g N2O–N kg−1 N export in 2004 and 6.6 to 12.8 g N2O–N kg−1 N export in 2005. Overall our results indicate that for this well-drained soil: (i) N fertilizer applications increased N2O emissions, supporting the use of IPCC-type emission factors; and (ii) soil tillage and the tillage–fertilization interaction had no measurable effect, consistent with previous reports that tillage of well-aerated agricultural soils has little effect on N2O emissions.

• At a long-term study site, N2O emissions were similar in ploughed and no-tillage soils. • N2O emissions increased linearly with fertilizer N rate up to 160 kg ha−1. • N2O–N emission coefficient ranged from 0.9 to 1.3% of applied N. • The N2O emitted, as a proportion of N exported in grains, was not affected by N rate.