Nitrous oxide and methane emissions from storage and land application of organic fertilisers : with the focus on sewage sludge

Sammanfattning: Organic fertiliser handling contributes to greenhouse gas emissions. Through storage and field experiments, this thesis examined strategies to reduce emissions of nitrous oxide (N2O) and methane (CH4) from storage and after land application of cattle slurry (CS) and sewage sludge (SS). Non-digested CS without a roof (1) and digested CS without (2) or with a roof (3) were stored during three months in summer and winter. Mesophilically digested SS without cover (1), with cover (2) or treated with ammonia (NH3) and with cover (3), and thermophilically digested SS with cover (4), were stored during one year. CS treatments (1) and (3) were applied to soil in spring or in autumn. SS treatments (3) and (2) were applied in spring and autumn, respectively, and were either incorporated into the soil immediately or after four hours. A life cycle assessment was conducted to assess the impact on global warming potential, acidification potential, eutrophication potential and primary energy use of different management strategies for SS. Digested CS had significantly higher CH4 emissions than non-digested CS during summer storage. Using a roof in summer decreased CH4 and increased N2O emissions significantly, but these cancelled each other out on a global warming basis. Emissions of N2O and CH4 were small during Winter storage and after land application. Treatment with NH3 significantly reduced N2O emissions from SS during storage and tended to lower CH4 emissions. Thermophilically digested SS had more air-filled pores during storage and emitted significantly more N2O than other treatments, but had the lowest CH4 emissions. Emissions of N2O after SS application to soil were low, but stimulated by wet soil and precipitation, while CH4 emissions were negligible, with no differences between immediate and delayed incorporation. The LCA revealed that shorter storage time and covered storage could mitigate the environmental impact from SS management. NH3 treatment generally reduced negative impacts on environment categories except for primary energy use, which was highest for this treatment. A combination of autumn and spring application was preferable to autumn-only application for most treatments through lowering total storage time.