Effects of short-term freezing on nitrous oxide emissions and enzyme activities in a grazed pasture soil after bovine-urine application.

Nitrous oxide (NO) emissions from soils applied with livestock excreta have been widely reported previously. The highest NO emissions from soils are also often reported during thawing periods in cold regions where soil freezing is common. However, the combined effects of cow urine application and freeze-thaw events on NO emissions and the related enzyme activities are still not clear. Thus, we simulated a freeze-thaw event at -3 °C for 7 days, and then increased to 3 °C for 46 days using intact soil cores with cow urine (392 kg N ha). We compared the factors influencing the magnitudes of NO emissions through soil microbial processes with and without the freeze-thaw event. Dicyandiamide (DCD), an inhibitor of nitrification, was added to investigate the significance of nitrification on NO emissions. The NO emission rates from the urine-applied soils peaked to approximately 1000 μg NO-N m h immediately after the soils thawed. Soil freezing with urine application was significantly higher cumulative NO emissions (537 mg NO-N m), compared to non-frozen soils with urine (247 mg NO-N m) during the incubation period (54 days). The effect of DCD application on NO emissions was not clear during the freeze-thaw event, although nitrate production rates were reduced. After the freezing event, soil moisture (water-filled pore space) was significantly higher in the non-frozen soils compared to the frozen soils, due to a 9% decline in bulk density of frozen soils. Additionally, the impact of thawing on urease and denitrification enzyme activities was influenced by the urine application. Urine application increased the urease activity, while the freezing event decreased the magnitudes. The physical changes in the soils were also important controlling factors of the NO emissions from cow urine-applied soils in cold regions.