Study Finds That No-Till Can Increase N2O Emissions in Certain Soils, Offsetting CO2 Sink

Jose Michael

Researchers at Agriculture and Agri-Food Canada (Québec City) investigating the short-term impacts of a no-tillage practice in a heavy clay soil found that within the first five years of the practice, nitrous oxide (N2O) emissions could offset the soil carbon dioxide sink. N2O is a greenhouse gas with a global warming potential (GWP) of 280 on a 20-year time horizon, compared to the baseline CO2 GWP of 1. The N2O GWP increases to 310 on a 100-year time horizon, according to data from the UN Framework Convention on Climate Change (UNFCC).

The practice of no-till has increased considerably during the past 20 years. The absence of tillage coupled with the accumulation of crop residues at the soil surface modifies several soil properties but also influences nitrogen dynamics. Soils under no-till usually host a more abundant and diverse biota and are less prone to erosion, water loss, and structural breakdown than tilled soils. Their organic matter content is also often increased. In addition, no-till is proposed as a measure to mitigate the increase in atmospheric carbon dioxide concentration.

To assess the net effect of no-till on greenhouse gas emissions, the researchers compared emissions of nitrous oxide as well as nitrogen contents and physical properties between moldboard plowed (early fall) and no-till soils near Québec City, Canada. Measurements were made during three growing seasons in a poorly drained clay and a well-drained loamy soil cropped to barley. The results of the study were reported in the 2008 September-October issue of the Soil Science Society of America Journal.

Differences in the response of nitrous oxide emissions when converting to a no-till practice between the clay and loam soils were striking. While emissions were similar in both tillage treatments in the well-aerated loam, they more than doubled under no-till in the clay soil.

Differences in emissions between tillage practices in the clay soil were observed in spring and summer but were greater and more consistent in the fall after plowing operations. The influence of plowing on nitrous oxide flux in the heavy clay soil was likely the result of increased soil porosity that maintained soil aeration and water content at levels restricting denitrification and nitrous oxide production. Accordingly, denitrification rates are usually increased in denser and wetter no-till soils and the anticipated benefits of the adoption of soil conservation practices on net soil-surface greenhouse gas emissions could be offset by increases in nitrous oxide emissions.

Our results indicate, however, that NT [no-till] can result in incremental N2O emissions that can more than offset the soil CO2 sink during the first 5 yr after adoption of this soil conservation practice in a heavy clay soil. Differences in N2O emissions between NT and MP [moldboard plowing] were greatest when differences in WFPS [water-filled pore space] between the two tillage treatments were high, suggesting that increased N2O emissions in NT were the result of enhanced denitrification. Consequently, the potential of NT for decreasing net GHG emissions may be limited in fine-textured soils that are prone to high water content and reduced aeration.

—Rochette et al. (2008)

Predicting the impacts of no-till on nitrous oxide emissions is required for a full assessment of the influence of this practice on net greenhouse gas emissions. Researchers at Agriculture and Agri-Food Canada are pursuing their investigations to understand the factors that control the mechanisms leading to nitrous oxide emissions under contrasting soil tillage practices.

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