Sources matter-N2O production and consumption in grasslands

The use of organic fertilisers such as manures in grasslands is a viable strategy to recycle nutrients, reduce mineral N fertiliser inputs and increase soil organic carbon, maintaining productivity. However, such inputs may also increase emissions in the form of nitrous oxide - a greenhouse gas - 300 times as powerful as carbon dioxide. The balance between soil organic carbon gains and resulting emissions of nitrous oxide therefore determines the net carbon benefit of organic fertilizer use in grassland systems. Different microbial pathways form nitrous oxide, yet predictions on magnitude and controls of specific pathways remain elusive, and nitrous oxide consumption, i.e., the reduction to atmospheric dinitrogen via denitrification is still a major uncertainty for nitrogen cycling in grasslands. The severe environmental consequences of nitrous oxide emissions demand a mechanistic understanding for nitrogen transformations and nitrous oxide production and consumption pathways in grassland soils. This research will establish shifts in nitrogen cycling and nitrous oxide production and consumption in a long-term grassland experiment in response to mineral and organic N fertiliser application. The project will combine leading edge isotope tracer and natural abundance approaches to demonstrate changes in nitrogen transformations, establishing a quantitative understanding for nitrous oxide production and consumption pathways in grasslands across scales. A unique suite of isotope methods combined with state-of-the-art isotope tracing will allow to constrain magnitude and controls of nitrous oxide production and consumption pathways in soil microcosm and in-situ experiments. This data will then feed into biogeochemical modelling using novel isotope addon models, enabling accurate predictions on nitrous oxide emissions in response to long term fertiliser management practice. This highly novel combination of isotope methods and state-of-the-art modelling is a unique opportunity to inform an improved process understanding for nitrogen cycling across scales, enabling climate smart management of grasslands without environmental trade-offs.