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 den itrifiers from disrupted soil aggregates. The objective of this study was t o evaluate the effect of freezing and thawing on N2O production in a clay s oil under contrasting crop rotations and tillage practices. Laboratory expe riments were conducted in soil slurries to favor substrate diffusion, in ma croaggregate functions separated by wet sieving to characterize the biologi cally active soil organic matter (SOM) pool, and in undisturbed soil cores to simulate field conditions. In slurries, a freezing and thawing cycle inc reased denitrification rates by 32%. Soil slurries from no-tillage under ro tation (NT-R) exhibited denitrification rates 92% higher than those from co nventional 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 macroaggreg ates. Higher rates of denitrification (55%) found in both aggregate fractio ns of NT-R system were attributed to the higher mineralizable organic C con tent. Undisturbed soil cores sampled in November showed increased N2O produ ction by 220% after thawing. This thawing effect was also significantly hig her in cores from NT-R than in those from CT-C.