Effects of sward diversity and nitrogen application on agronomic and environmental responses of grasslands

A significant portion of grassland production systems comprise of perennial ryegrass (Lolium perenne) monoculture relying heavily on synthetic nitrogen (N) inputs. These N inputs drive the greenhouse gas (GHG) emissions from synthetic fertilizer manufacturing process until grass production stage, especially after application to soil, which drives the nitrous oxide (N2O) emissions, where N2O is a potent GHG having global warming potential of 273 times higher than carbon dioxide. However, sward composition plays a crucial role in regulating the nitrogen (N) cycling and associated emissions, making it an important factor in grassland management. Moreover, there is also a policy shift across globe towards renewable energy sources and anaerobic digestion (AD) is gaining importance for biogas/biomethane production. The solid effluent of AD process called digestate is an organic N source for grassland production. The effectiveness of digestate as fertilizer, however, is influcnec by the receiving sward composition, which affects both N use efficiency and emissions. As AD gains importance, it is predicted that grasslands will be a source of feedstock for AD as well as receiver of digestate. This will lead to increase in grass demand which will require to increase the yields of grasslands without increasing GHG emissions. Recently, there has been increased use of grass-legume and multi-species swards, mainly due to benefits of diverse swards regarding drought resistance, lower N requirements, higher yields, and animal performance. Sward composition also influences N losses via N2O and NH3 emissions, as well as ruminal fermentaions process in ruminants. This PhD thesis addresses the knowledge gaps regarding potential of increasing grassland diversity to overcome the effects of decreasing N inputs, digestate effectiveness as fertilizer on diverse swards, N2O emissions, emissions intensities, ammonia (NH3) emissions after land-application of digestate to diverse swards. The first experimental chapter (chapter 2) assess the annual dry-matter yield, nitrogen yield, and carbon-nitrogen ratio of the biomass of systematically varied monocultures and mixtures using simplex experimental design. The first experimental chapter also assess the same for selected grassland communities fertilized with digestate and synthetic fertilizer at different N application rates. The second experimental chapter assess N2O emissions and emissions intensity for selected grassland communities fertilized with digestate and synthetic fertilizer in a two-year experiment. This allows for assessing the role of sward composition in affecting N2O emissions responses to different N sources. The third experimental chapter quantifies the NH3 emissions after land-application of digestate to either perennial ryegrass monoculture or a mixture of six grassland species at two different heights. This highlights how plant diversity influences NH3 volatilizaation following digestate application. And the last experimental chapter assess the in vitro fermentation responses and methane (CH4) production after in vitro incubation of individual forages in six-species mixture and six-species mixture itself. Given that plant functional groups differ in their nutrient composition, diet composition may also shape ruminal CH4 emissions. Key findings of this thesis include a significant effect of increasing grassland diversity on yield and nitrogen yield as mixtures with lower N inputs outperformed perennial ryegrass monoculture at high N inputs. This suggests that sward composition can be manipulated to balance productivity with lower N inputs. The N2O emissions for digestate were significantly lower than synthetic fertilizer and mixtures had significantly lower emissions intensities compared to perennial ryegrass with high N inputs. These results reinforce the role of sward composition in mitigating emissions, with diverse swards reducing N2O emissions per unit of yield. There was a significant reduction in NH3 emissions after land-application of digestate to six-species mixture compared to perennial ryegrass monoculture. The in vitro incubation experiment showed a large variability in CH4 production and fermentation responses for different forages, showing the potential of constituent six-species forages in affecting CH4 production. In conclusion, I show that mixing plant species is a more viable strategy of increasing yields of grassland production systems and reducing N inputs. Sward composition emerges as a key determinant of nitrogen cycling, influencing emissions of N2O and NH3, as well as the efficiency of digestate as a fertilizer. Moreover, my results show that digestate can be used as N source for grassland production systems. However, its effectiveness depends on the sward composition, which determines nitrogen retention and loss pathways. Overall, this thesis shows the role of increasing grassland species diversity in climate-smart grassland production system.

Funder

Teagasc

Publisher

University of Galway

Rights

Attribution-NonCommercial-NoDerivatives 4.0 International

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Collections

University of Galway Theses (PhD Theses)