Management matters: testing a mitigation strategy for nitrous oxide emissions using legumes on intensively managed grassland
oleh: K. Fuchs, L. Hörtnagl, N. Buchmann, W. Eugster, V. Snow, L. Merbold, L. Merbold
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2018-09-01 |
Deskripsi
<p>Replacing fertiliser nitrogen with biologically fixed nitrogen (BFN) through legumes has been suggested as a strategy for nitrous oxide (N<sub>2</sub>O) mitigation from intensively managed grasslands. While current literature provides evidence for an N<sub>2</sub>O 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 N<sub>2</sub>O 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 N<sub>2</sub>O fluxes and biomass yields in two differently managed grass–clover mixtures. We measured N<sub>2</sub>O 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 N<sub>2</sub>O 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 <sup>15</sup>N natural abundance method.</p><p>The amount of BFN was similar in both parcels in 2015 (control: 55±5 kg N ha<sup>−1</sup> yr<sup>−1</sup>; clover parcel: 72±5 kg N ha<sup>−1</sup> yr<sup>−1</sup>) 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<sup>−1</sup> yr<sup>−1</sup> with 4 % clover in DM; clover parcel: 130±8 kg N ha<sup>−1</sup> yr<sup>−1</sup> and 44 % clover). The mitigation management effectively reduced N<sub>2</sub>O emissions by 54 % and 39 % in 2015 and 2016, respectively, corresponding to 1.0 and 1.6 t ha<sup>−1</sup> yr<sup>−1</sup> CO<sub>2</sub> equivalents. These reductions in N<sub>2</sub>O 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 N<sub>2</sub>O emissions, indicating that the decomposition of plant residues and rhizodeposition did not compensate for the effect of fertiliser reduction on N<sub>2</sub>O emissions. Annual biomass yields were similar under mitigation management, resulting in a reduction of N<sub>2</sub>O emission intensities from 0.42 g N<sub>2</sub>O-N kg<sup>−1</sup> DM (control) to 0.28 g N<sub>2</sub>O-N kg<sup>−1</sup> DM (clover parcel) over the 2-year observation period. We conclude that N<sub>2</sub>O emissions from fertilised grasslands can be effectively reduced without losses in yield by increasing the clover proportion and reducing fertilisation.</p>