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What are the impacts of grazing and cutting events on the N₂O dynamics in humid temperate grassland?

Rafique, Rashad, Anex, Robert, Hennessy, Deirdre, Kiely, Gerard
Geoderma 2012 v.181-182 pp. 36-44
agricultural soils, cattle, cutting, denitrification, grassland soils, grasslands, grazing, nitrification, nitrogen, nitrous oxide, silage, soil temperature, spring, summer, temporal variation
Atmospheric concentrations of nitrous oxide (N₂O) have been increasing over the last century with much of this increase from agricultural soils, fertilized with nitrogen. To understand the N₂O emissions from terrestrial ecosystems (e.g. grassland soils) it is necessary to understand the processes leading to N₂O production. From February to August in 2010, we conducted a field study to measure the N₂O fluxes using the chamber technique, at a grazed and cut grassland site in South West Ireland. The objectives of the study were: 1) to understand the temporal variation of the N₂O fluxes during the spring and summer periods; 2) to examine the separate effects of grazing and cutting events on N₂O fluxes; and 3) to examine the relationship of soil ammonium (NH₄+−N) and soil nitrate (NO₃--N) with N₂O fluxes. We found the highest peak of daily N₂O flux occurred at the start of spring; however the total of summer fluxes (June, July and August) of 1.81±0.7kg N₂O–Nha⁻¹ were higher than those of the spring (March, April and May) fluxes of 1.51±0.6kg N₂O–Nha⁻¹. The soil NH₄+−N concentration was higher than the soil NO₃–N concentration over the study period and elevated N₂O fluxes coincided with elevated soil NH₄–N concentrations. There were two short (2day duration) cattle grazing events; one on April 26/27 and the second on June 27/28. There were two grass cutting (for silage) events: on May 30 and on August 4. After the two grazing and two cutting events, the N₂O fluxes increased markedly. After both grazing events, there was an immediate step increase of ~200μg N₂O–Nm⁻²h⁻¹, after which the fluxes decreased over the next few weeks. After both cutting events, there was a gradual increase in N₂O fluxes over the next several weeks. We found that the N₂O flux increases post grazing, were due to grazing only, since the other variables (soil temperature, WFPS, N application) did not change. However, the flux increases post cutting could not ascribed to cutting only, as other flux favouring variables of: changes in soil temperature and WFPS also occurred at this time. The N₂O fluxes correlated better with soil NH₄–N concentration (r²=0.73 (p=<0.05)) than with NO₃--N (r²=0.25 (p=not significant)). The occurrence of elevated NH₄–N in conjunction with elevated WFPS, frequently in the range of 50–60% suggests that nitrification rather than denitrification was likely the dominant process involved in the production of N₂O at this site.