wAbstract
@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}0.7 kg N2O-N
ha-1 were higher than those of the spring (March, April
and May) fluxes of 1.51}0.6 kg N2O-N ha-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 (2 day 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 `200Κg N2O-N
m-2 h-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.
Keywords: nitrous oxide; Grassland; NH4-N;
NO3-N; Grass cutting; Animal grazingx
1. Introduction
2. Materials and methods
@2.1. Site description
@2.2. Soil sampling and laboratory analysis
@2.3. Environmental measurements
@2.4. Nitrous oxide flux measurement techniques
@2.5. Grass cutting and grazing events
@2.6. Statistical analysis
3. Results
@3.1. Climatic characteristics
@3.2. N2O fluxes variability
@3.3. Seasonal dynamics of NH4-N and NO3-N concentration in soil
@3.4. Effect of grass cutting on N2O fluxes
@3.5. Effect of grazing events on N2O fluxes
@3.6. Relationship of NH4-N and NO3-N
with N2O emissions
4. Discussion
@4.1. N2O fluxes
@4.2. The impact of cutting events on N2O
fluxes
@4.3. The impact of grazing events on N2O
fluxes
@4.4. Nitrification or denitrification
5. Conclusion
Acknowledgement
References