Nutrient Management Research Database

Most of the nitrous oxide (N₂O) in the atmosphere, thought to be involved in global warming, is emitted from soil. Although the main factors controlling the production of N₂O in soil are well known, we need more quantitative data on the interactions of soil and the environment in the soil that affect the emission. We therefore studied the effects of irrigation, cropping (fallow, barley with grass undersown) and N fertilization (unfertilized, 103 kg N ha⁻¹) on the composition of soil air and direct N₂O emission from soil (using the closed chamber method) in a factorial field experiment on a well-structured loamy clay soil during 1 June−22 October 1993. The measurements were made weekly during the growing season and three times after harvesting. The composition of the soil air did not indicate severe anoxia in any treatment or combination of treatments, but the accumulation of N₂O in the soil air indicated that hypoxia was common. At the start of the irrigation the emissions were small, even though there was much ammonium and nitrate in the soil and therefore a potential for emission of N₂O produced by both nitrification and denitrification. Larger emissions occurred later. The largest emissions were found when 60–90% of the soil pore space was filled with water. Irrigation and fertilization with N both roughly doubled the cumulative N₂O emission. Growing a crop decreased it by a factor of 3–7. Most N₂O was lost from the irrigated fertilized soil under fallow (3.5 kg N ha⁻¹), and least from the unirrigated unfertilized soil under barley (0.1 kg N ha⁻¹).

This study examined the effects of irrigation, cropping, and nitrogen fertilization on nitrous oxide (N2O) and carbon dioxide (CO₂) emissions. CO₂ emissions reflect the overall activity level of soil microbes.

There were experimental plots with every combination of:

1. Irrigation type:
   1. Sprinkler irrigated
   2. Unirrigated
2. Cropping type
   1. Uncropped, sprayed with glyphosate for weed supression
   2. Barley unseeded with grass mix
3. N fertilization
   1. No fertilizaiton
   2. 90 lbs N/acre as calcium nitrate

In addition to N₂O and CO₂ emissions, the experimenters monitored soil moisture and N levels, along with concentrations of the above gases within the soil pore space. Monitoring the levels of greenhouse gases within the soil offered insight into how these gases behave before they are emitted.

1. N₂O Emissions

A five fold increase in emissions was observed following irrigation events. The effect of irrigation was diminished under the barley crop.

Nitrogen fertilization generally increased N₂O emissions.

The highest rates of N₂O emissions were from the irrigated plots that also received N fertilization.

2. Soil nitrogen

While early summer N levels were higher in the fertilized plots, the barley crop's uptake of soil N resulted in lower soil N levels in the cropped plots.

Irrigation reduced soil nitrate levels, likely reflecting nitrate leaching beyond the depth of measurement.

3. Soil Pore Space N₂O

Levels of N₂O in the soil pore space were markedly higher at 6 and 12 inch depths than those released from the soil surface. This reflects the ability of soil microbes to consume N₂O before it reaches the surface and is emitted into the atmosphere.