The use of no-till (NT) and improved fertilizer management are proposed ways to sequester atmospheric CO2 in agricultural soils. Several studies have shown that the potential to sequester C under NT can be limited in poorly drained fine-textured soils. For a complete analysis of the potential of soils to act as a C sink, emissions of other greenhouse gases (GHG) must be evaluated. Previous work has shown that the effects of NT on N2O emissions are variable, often showing greater N2O emissions under NT in humid temperate climate and the reverse under semi-arid conditions. The objective of this study was to perform an assessment of the net GHG emissions from a clay soil subjected to NT and variable N fertilizer rates.

Soil C content and GHG emissions were measured in a 13-year old research site comparing the effects of NT and rates of N fertilizer in a corn/soybean system on a Gleysolic clay loam at L'Acadie, Québec, Canada. Fertilizer N was applied at rates of 0, 80, 160 kg N ha-1 only during the corn year only. Soil samples were taken in 72 plots at 7 depth increments down to 60 cm during 2 consecutive cropping seasons (2003 under soybeans, 2004 under corn). In addition, N2O and CH4 soil-surface fluxes were measured weekly during the two growing seasons.

The effects of N fertilizer rates on soil C content were not significant at any soil depth despite significant effects of N fertilizer on crop yields and consequently on C inputs. In the 0-5 cm soil layer only, C content was significantly greater in the NT than in the mouldboard plow (MP) treatment. Conversely, in the 20-30 cm layer, C content was significantly greater in the MP than in the NT soil. Overall, when assessed for the whole soil profile, there were no statistically significant effect of tillage on soil C storage; the greater C content in the top soil under NT being off-set by greater C content at depth under MP.

The effects of tillage and N fertilizer on N2O and CH4 emissions varied in both years. During the corn year (2004), emissions were low except for a period of about three weeks following application of N fertilizers. Cumulative annual N2O emissions during the corn year were proportional to N fertilizer rate. N2O emissions during the soybean year were low and similar to soil background values, thereby suggesting that the biological N fixation per se was not a measurable source of N2O. Compared to conventional tillage, no-till decreased N2O emissions by 20% during the corn year but increased them by 50% during the soybean year. Soil under both crops was a weak sink for atmospheric CH4, and CH4 uptake was not significantly affected by tillage practices.

These results confirm that the impact of tillage practices on C storage in soils of eastern Canada can be limited and emphasize the importance of taking the whole soil profile into account when analyzing soil C storage. Despite significant effects on corn yields, increasing N rates did not result in measurable impact on soil C in this corn/soybean system but did increase N2O emissions during the corn year. For both growing seasons, a complete accounting of net GHG emissions from soils suggests that NT had no significant impact on net GHG emissions in this soil but that increased N fertilization resulted in higher emissions. This study illustrates the importance of considering both C storage and N2O emissions in estimating the potential for management practices to mitigate GHG emissions.