Abstract. Replacing fertiliser nitrogen with biologically fixed
nitrogen (BFN) through legumes has been suggested as a strategy for nitrous
oxide (N2O) mitigation from intensively managed grasslands. While
current literature provides evidence for an N2O emission reduction
effect due to reduced fertiliser input, little is known about the effect of
increased legume proportions potentially offsetting these reductions, i.e.
by increased N2O emissions from plant residues and root exudates. In
order to assess the overall effect of this mitigation strategy on permanent
grassland, we performed an in situ experiment and quantified net N2O fluxes
and biomass yields in two differently managed grass–clover mixtures. We
measured N2O fluxes in an unfertilised parcel with high clover
proportions vs. an organically fertilised control parcel with low clover
proportions using the eddy covariance (EC) technique over 2 years.
Furthermore, we related the measured N2O fluxes to management and
environmental drivers. To assess the effect of the mitigation strategy, we
measured biomass yields and quantified biologically fixed nitrogen using the
15N natural abundance method. The amount of BFN was similar in both parcels in 2015 (control: 55±5 kg N ha−1 yr−1; clover parcel: 72±5 kg N ha−1 yr−1) due to similar clover proportions (control:
15 % and clover parcel: 21 %), whereas in 2016 BFN was substantially
higher in the clover parcel compared to the much lower control (control: 14±2 kg N ha−1 yr−1 with 4 % clover in DM; clover
parcel: 130±8 kg N ha−1 yr−1 and 44 % clover). The
mitigation management effectively reduced N2O emissions by 54 %
and 39 % in 2015 and 2016, respectively, corresponding to 1.0 and
1.6 t ha−1 yr−1 CO2 equivalents. These reductions in
N2O emissions can be attributed to the absence of fertilisation on
the clover parcel. Differences in clover proportions during periods with no
recent management showed no measurable effect on N2O emissions,
indicating that the decomposition of plant residues and rhizodeposition did not
compensate for the effect of fertiliser reduction on N2O emissions.
Annual biomass yields were similar under mitigation management, resulting in
a reduction of N2O emission intensities from
0.42 g N2O-N kg−1 DM (control) to
0.28 g N2O-N kg−1 DM (clover parcel) over the 2-year
observation period. We conclude that N2O emissions from fertilised
grasslands can be effectively reduced without losses in yield by increasing
the clover proportion and reducing fertilisation.