Effects of freeze-thaw and soil structure on nitrous oxide produced in a clay soil

Freezing and thawing have been shown to cause significant soil physical and biological changes. The increase in denitrification following thawing may be attributed to the diffusion of organic substrates newly available to denitrifiers from disrupted soil aggregates. The objective of this study was to evaluate the effect of freezing and thawing on N2O production in a clay soil under contrasting crop rotations and tillage practices. Laboratory experiments were conducted in soil slurries to favor substrate diffusion, in macroaggregate functions separated by wet sieving to characterize the biologically active soil organic matter (SOM) pool, and in undisturbed soil cores to simulate field conditions. In slurries, a freezing and thawing cycle increased denitrification rates by 32%. Soil slurries from no-tillage under rotation (NT-R) exhibited denitrification rates 92% higher than those from conventional till under continuous cereal (CT-C). Macroaggregates fractions (0.25-2 and 2-5 mm) from both management systems increased their rates of C mineralization and denitrification activity by 95% following freezing, but the increases tended to be greater (57%) in small than in large macroaggregates. Higher rates of denitrification (55%) found in both aggregate fractions of NT-R system were attributed to the higher mineralizable organic C content. Undisturbed soil cores sampled in November showed increased N2O production by 220% after thawing. This thawing effect was also significantly higher in cores from NT-R than in those from CT-C.