Biologically Fixed N2 as a Source for N2O Production in a Grass–clover Mixture, Measured by 15N2

The contribution of biologically fixed dinitrogen (N2) to the nitrous oxide (N2O) production in grasslands is unknown. To assess the contribution of recently fixed N2 as a source of N2O and the transfer of fixed N from clover to companion grass, mixtures of white clover and perennial ryegrass were incubated for 14 days in a growth cabinet with a 15N2-enriched atmosphere (0.4 atom% excess). Immediately after labelling, half of the grass–clover pots were sampled for N2 fixation determination, whereas the remaining half were examined for emission of 15N labelled N2O for another 8 days using a static chamber method. Biological N2 fixation measured in grass–clover shoots and roots as well as in soil constituted 342, 38 and 67 mg N m−2 d−1 at 16, 26 and 36 weeks after emergence, respectively. The drop in N2 fixation was most likely due to a severe aphid attack on the clover component. Transfer of recently fixed N from clover to companion grass was detected at 26 and 36 weeks after emergence and amounted to 0.7 ± 0.1 mg N m−2 d−1, which represented 1.7 ± 0.3% of the N accumulated in grass shoots during the labelling period. Total N2O emission was 91, 416 and 259 μg N2O–N m−2 d−1 at 16, 26 and 36 weeks after emergence, respectively. Only 3.2 ± 0.5 ppm of the recently fixed N2 was emitted as N2O on a daily basis, which accounted for 2.1 ± 0.5% of the total N2O–N emission. Thus, recently fixed N released via easily degradable clover residues appears to be a minor source of N2O.