Nitrous oxide emissions from a fallow and wheat field as affected by increased soil temperatures

In order to determine the effects of increased soil temperature resulting from global warming on microbiological reactions, a 21-month field experiment was carried out in the Bavarian tertiary hills. The major objective was to focus on N₂O releases as either a positive or negative feedback in response to global warming. The soils of a fallow field and a wheat field were heated 3°C above ambient temperature and N₂O fluxes were measured weekly from June 1994 to March 1996. During the experimental period, measured temperature differences between the control plots and the heated plots were 2.9±0.3°C at a depth of 0.01 m and 1.0-1.8°C at a depth of 1 m. Soil moisture decreased with the elevated soil temperatures of the heated plots. The mean differences in soil moisture between the treatments were 6.4% (fallow field) and 5.2%(DW) (wheat field dry weight, DW), respectively. Overall N₂O releases during the experimental period from the fallow field were 4.8 kg N₂O-N ha^-1 in the control plot against 5.0 kg N₂O-N ha^-1 in the heated plot, and releases from the wheat field were 8.0 N₂O-N ha^-1 in the control plot and 7.6 N₂O-N kg ha^-1 in the heated plot. However, on a seasonal basis, cumulated N₂O emissions differed between the plots. During the summer months (May-October), releases from the heated fallow plot were 3 times the rates from the control plot. In the winter months, N₂O releases increased in both the fallow and wheat fields and were related to the number of freezing and thawing cycles.