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SOIL PHYSICS

Continuous Measurements of Belowground Nitrous Oxide Concentrations

Department of Land Resource Science, Univ. of Guelph, Guelph, ON, Canada N1G 2W1

^* Corresponding author (gdrewitt{at}uoguelph.ca).

Nitrous oxide released from soil is a concern since it can act as a potential atmospheric pollutant and it represents a loss of N from the soil. To better understand the factors controlling N₂O production and transport, we developed a system to obtain continuous measurements from below the soil surface. The sampling system pulls small volumes of soil gas from buried sample probes through a tunable diode laser trace gas analyzer. The advantage of this system is that it measures concentrations spectroscopically, allowing regular, continuous measurements. This provides it with the distinct advantage of being able to capture short-term changes in gas concentrations that may be important for nutrient and greenhouse gas budgeting. Furthermore, the system is relatively simple to install and could be integrated into existing field measurements of trace gas flux. Measurements of belowground N₂O concentrations were obtained during the spring thaw from buried probes in a conventionally tilled field that was planted in soybean [Glycine max (L.) Merr.] the previous summer. Measurements showed that belowground N₂O concentrations at the 25-cm depth varied between 65 and 85 µmol mol^–1 before snowmelt. After melting of the snow and the beginning of the soil thawing, N₂O concentrations decreased to a value generally <1 µmol mol^–1. During this period when belowground N₂O concentrations were near atmospheric values, wind speed influenced concentrations, possibly through a pressure pumping effect.