Diffuse agricultural pollution is a major contributor to poor water quality in many
parts of the world. Consequently agri-environment policy promotes the use of
riparian buffer strips and/or denitrifying wetlands to intercept and remove diffuse
NO3--N pollution. However, these methods have the potential to cause ‘pollution
swapping’: the exchange of one form of pollution as a result of measures
implemented to reduce another. Thus the benefits of intercepting NO3--N could be
offset by enhanced emissions of the potent greenhouse gases, nitrous oxide (N2O)
and methane (CH4), from buffer strips and wetlands.
This research aimed to: (1) quantify the direct N2O emissions from an irrigated
buffer strip (IBS), using nitrate-rich agricultural drainage water, compared to a non-irrigated
control (CP); (2) improve the understanding of N2O production and
consumption within soils using controlled soil monolith experiments; (3) assess the
effectiveness of a small (60 m2) instream wetland at intercepting and removing
diffuse NO3--N pollution, and quantify pollution swapping in the form of CH4 and
N2O emissions; (4) assess the production of CH4 and N2O within the sediment, and
their emissions as well as inorganic-N concentrations in the overlying water column
in response to temperature and turbulence, using intact wetland sediment and
membrane inlet mass spectrometry (MIMS). The research focused on mitigating
diffuse NO3--N pollution from grazed pasture at a farm in north-east England.
Annual N2O-N emissions from the IBS and CP were not statistically different (P >
0.05): 509 and 263 g N2O-N ha-1, respectively, in 2007 and 375 and 500 g N2O-N ha-
1 year-1, respectively, in 2008. Irrigation of the IBS increased spatial variability in
flux and generated hotspots of denitrification compared to the CP. However, these
changes were short-lived. Direct N2O emission factors (EF1) calculated using the
available NO3--loading data (September 2007 - December 2008) for the IBS were
lower (c.0.1%) than those calculated for the CP assuming N input from biological N
fixation only (<1.9%). Soil monolith experiments under a variety of irrigation and
NO3--N loading regimes confirmed low direct and indirect (of dissolved N2O-N in
leachate) emissions (<3.1 and <2.3% of applied NO3--N emitted as N2O-N,
respectively), similar to the IPCC default emission factors. However, N loss in
leachate was high, up to 82% of added NO3--N with concentrations reaching 24 mg
NO3--N L-1. Therefore even though no pollution swapping occurred the high leachate
losses indicate irrigation of buffer strips are not effective mitigation methods.
Monitoring for 2 years of the instream wetland that received median NO3--N
concentrations of c. 6 mg N L-1, but up to c. 20 mg N L-1, showed it to be ineffective
at intercepting diffuse NO3- pollution: likely a result of the relatively high discharge
and short water residence time, as well as the direct input of NO3--N to the wetland
from secondary sources: field drains and/or overland flow. The wetland was a net
source of NH4+-N in both 2007 and 2008, and a net sink of NO3--N in 2007 only.
Annual wetland CH4 and N2O emissions were 713 and 237 mg CH4 m-2 year-1, and
3.5 and 1.9 mg N2O-N m-2 year-1, for 2007 and 2008, respectively and were highly
variable between seasons. N pollution swapping was minimal from either direct or
indirect emissions, but CH4 emissions were found to be of greater importance at a net
cost of ~ £600 ha-1 over the study period (2007 to 2008), compared to N2O emissions
(~ £60 ha-1) and low NO3--N interception savings (~ £24 ha-1). Incubation
experiments suggest that spatially variable microsites of nitrifying, denitrifying or
methanogenic activity and CH4 oxidation occur within the wetland sediment.
Therefore off-line, larger wetland systems offer the best prospects of enhanced NO3--N interception and potentially reduced CH4 emissions by maintaining shallow water
depths (increased CH4 oxidation) and long residence times (increased opportunity for
denitrification), within the wetland or wetland cells