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

Abstract

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.