Natural perturbations, drying-wetting and freezing-thawing cycles, and the emission of nitrous oxide, carbon dioxide and methane from farmed organic soils

We investigated the effects of drying-wetting and freezing-thawing cycles on the emission of nitrous oxide, carbon dioxide and methane from intact soil cores from farmed organic soils at sites in Germany, Sweden and Finland. During the first week following wetting or thawing, cores from the German and Swedish sites produced an up to 1000-fold increase in N2O emission rates. The total surplus N2O emission due to the first wetting event ranged between 3 and 140 mg N-N2O m(-2), and between 13 and 340 mg N-N2O m(-2) due to the first thawing event but declined following two successive freeze-thaw events. Wetting and thawing produced a greater surplus emission of N2O from grassland sites compared to arable sites. Following wetting, denitrification was responsible for the majority of N2O emission from the German grassland soil while in the German ploughed soil and the Swedish soils denitrification was responsible for less than 60% of the N2O emission. In contrast, following thawing, denitrification was responsible for < 40% of N2O emission from the German grassland soil while in the remaining German and Swedish soils denitrification was responsible for most of the emission. Wetting or thawing of soil cores from the Finnish sites did not result in any significant increase in N2O emission rates perhaps because a prolonged drought at the time of soil core collection had changed soil properties considerably. The CO2 emission rates increased up to 5-fold following wetting or thawing, The total surplus CO2 emission ranged from 0.0 to 13 9 C-CO2 m(-2), differed between locations and land use, and decreased during successive cycles of freezing and thawing. The total surplus CO2 emission from grassland sites following thawing was generally higher than from arable and forest sites probably due to decomposition of carbon sources liberated from stressed grass roots. Methane emission rates were very small and often below our detection limit (< 1 mug C-CH4 m(-2) h(-1)); no effects of wetting or thawing on CH4 emission rates were observed at any of the sites, A comparison with annual emission rates of N2O and CO2 showed that even a single wetting or thawing event may account for a large proportion of the N2O emission from farmed organic soils. (C) 2001 Elsevier Science Ltd. All rights reserved.