Long-term animal impact modifies potential production of N2O from pasture soil

At cattle overwintering areas, inputs of nutrients in animal excrements create conditions favourable for intensive microbial activity in soil. During nitrogen transformations, significant amounts of N2O are released, which makes overwintering areas important sources of N2O emission. In previous studies, however, increasing intensity of long-term cattle impact did not always increase emissions of N2O from the soil: in some cases, N2O emissions from the soil were lower at the most impacted area than at the moderately impacted one. Thus, the relationships between the level of long-term animal impact and potential production of N2O from soil by denitrification were investigated in field and laboratory experiments. Field measurements indicated that the production of N2O after glucose and nitrate amendments was greater in severely and moderately impacted locations than in an unimpacted location, while differences between the severely and moderately impacted locations were not significant. In laboratory experiments, the potential production of N2O (measured as anaerobic production of N2O after addition of glucose and nitrate) was highest in the moderately impacted soil. Surprisingly, potential N2O production was lower in the most impacted than in the moderately impacted soil, and the net N2O production in the highly impacted soil was further decreased by a significant reduction of N2O to N-2. The expected stimulating effect of an increasing ratio of glucose C to nitrate N on the reduction of N2O to N-2 during denitrification was not confirmed. The results show that cattle increase the denitrification potential of the soil but suggest that the denitrification potential does not increase indefinitely with increasing cattle impact.