Nitrous oxide and methane fluxes from organic soils under agriculture

Trace gas fluxes of N 2 O and CH 4 were measured weekly over 12 months on cultivated peaty soils in southern Germany using a closed chamber technique. The aim was to quantify the effects of management intensity and of soil and climatic factors on the seasonal variation and the total annual exchange rates of these gases between the soil and the atmosphere. The four experimental sites had been drained for many decades and used as meadows (fertilized and unfertilized) and arable land (fertilized and unfertilized), respectively. Total annual N 2 O‐N losses amounted to 4.2, 15.6, 19.8 and 56.4 kg ha –1 year –1 for the fertilized meadow, the fertilized field, the unfertilized meadow and the unfertilized field, respectively. Emission of N 2 O occurred mainly in the winter when the groundwater level was high. At all sites maximum emission rates were induced by frost. The largest annual N 2 O emission by far occurred from the unfertilized field where the soil pH was low (4.0). At this site 71% of the seasonal variation of N 2 O emission rates could be explained by changes in the groundwater level and soil nitrate content. A significant relationship between N 2 O emission rates and these factors was also obtained for the other sites, which had a soil pH between 5.1 and 5.8, though the relation was weak ( R 2 = 15–27%). All sites were net sinks for atmospheric methane. Up to 78% of the seasonal variation in CH 4 flux rates could be explained by changes in the groundwater level. The total annual CH 4 ‐C uptake was significantly affected by agricultural land use with greater CH 4 consumption occurring on the meadows (1043 and 833 g ha –1 ) and less on the cultivated fields (209 and 213 g ha –1 ).