Lower nitrous oxide emissions intensity from multi-species swards

Recent Teagasc research shows that multispecies grasslands can potentially reduce both nitrous oxide (N₂O) emissions and emissions intensities, and can contribute to more sustainable grassland production. Greenhouse gas measurements carried out by Saoirse Cummins showed that the annual greenhouse gas emissions per unit output (DM yield or biomass N) from the multi-species mixture were lower than those from perennial ryegrass receiving either 150 or 300 kg ha^-1 yr^-1 of inorganic nitrogen (N).

Nitrous oxide (N₂O) is a potent greenhouse gas and is strongly associated with losses from inorganic nitrogen (N) fertiliser applied to grassland. Lower-nitrogen grassland systems can curtail N₂O losses through the reduced application of inorganic N fertiliser; this may be replaced and yields sustained by symbiotically-produced N from clovers. Empirical measurement of the effect of legume-based multi-species mixtures on nitrous oxide is lacking, and the specific role of herbs is unknown.

We conducted a year-long experiment to measure nitrous oxide emissions from a variety of multi-species grassland communities that included the monocultures of 6 species from three functional groups; two grasses (Lolium perenne, Phleum pratense), two legumes (Trifolium pratense, Trifolium repens) and two herbs (Cichorium intybus, Plantago lanceolata), and systematically varying multi-species communities comprising different proportions of the six species. Plots were harvested multiple times per year (and were not grazed). Nitrous oxide emissions from the different plant communities were measured using static chambers over one year. Details are in Cummins et al. (2021). Plots received 150 kg ha⁻¹ year⁻¹ of nitrogen (N) (150N), except L. perenne monocultures which received two N levels: 150N and 300N.

Effects on absolute N₂O emissions

Overall, the effect of plant diversity on N₂O emissions was derived from linear combinations of species performances' in monoculture (species identity); there were no net synergistic or antagonistic effects on N₂O due to mixing the species. Increasing from 150N to 300N in L. perenne resulted in a highly significant increase in cumulative N₂O emissions from 1.39 to 3.18 kg N₂O-N ha⁻¹ year⁻¹. Higher N₂O emissions were also associated with the legume functional group.

Effects on N₂O emissions intensity

We also calculated the N₂O emissions intensity (the total annual N₂O emissions (g ha-1 year-1) produced per kg biomass N or tonnes DM yield):

emissions intensity = N₂O emissions / yield 

Emissions intensities (yield-scaled N₂O emissions) from multi-species communities around the equi-proportional mixture were lowered due to interactions among species; thus, there was an antagonistic effect of mixing species that resulted in lower N₂O emission intensities in mixtures, compared to the expected N₂O emissions intensity based on monoculture performances. This was largely caused by the strong synergistic effects of plant diversity on biomass yield and nitrogen yield (Grange et al. 2021; see related blog post). Looking at N₂O emissions intensity scaled by nitrogen yield, the 6-species mixture was significantly lower than that of L. perenne at both 300N and 150N.

In comparison to 300N L. perenne, the same N yield or DM yield could have been produced with the equi-proportional 6-species mixture (150 N) while reducing N₂O losses by 63% and 58% respectively. Compared to 150N L. perenne, the same N yield or DM yield could have been produced with the 6-species mixture while reducing N₂O losses by 41% and 24% respectively.

Fig. 2. N₂O emissions per unit yield produced calculated (left) per unit yield of forage and (right) per unit yield of N in the forage.

Figure 2 (above) looks at three specific communities from our overall design, and ignores most of the combinations of species that are possible with the three functional groups of grasses, herbs, and legumes. The emissions intensity of N₂O is presented as a continuous response surface across the systematic variation in sown proportions of grasses, herbs, and legumes. This clearly shows the reduction in emissions intensity in the community sown with equal proportions of all three functional groups, and increases in grass- and legume- dominated communities.

Fig. 3. Estimated emission intensity of N₂O (scaled by total N yield in forage) in response to variation in the sown proportion of grass (G), herb (H) and legume (L) functional groups (FG) within grassland communities. The communities represented in this ternary diagram are based on an equal proportional contribution of each of the two species within a FG. Thus, each vertex indicates the average of the two component species in the respective FG; the sides represent communities with varying proportions of two FGs (comprising four species), and the interior points represent varying proportions of three FGs (comprising six species).

Overall, this research shows the potential for multi-species grassland mixtures to contribute to sustainable agriculture.

John Finn and Saoirse Cummins, Teagasc, Johnstown Castle, Environment Research Centre

Reference

Cummins et al. 2021. Beneficial effects of multi-species mixtures on N2O emissions from intensively managed grassland swards. Science of the Total Environment. (with Open Access data and code)