Time series of the nitrogen dynamics after fertiliser application in the topsoil

Soil organic matter (SOM) serves as a crucial reservoir for nitrogen (N), with particulate organic matter (POM) and mineral-associated organic matter (MAOM) representing distinct pools characterised by varying turnover times and stabilisation mechanisms. POM acts as a rapid cycling pool, whereas MAOM ensures long-term stabilisation through interactions with minerals that protect organic N from degradation. In this thesis, a density and size fractionation of topsoil SOM was performed to examine ammonium-derived N from fertilisers over a fiveyear period, providing insights into long-term N stabilisation across SOM fractions. The contrasting behaviour in SOM formation of NH4+-N from cattle slurry (SLU), anaerobically digested slurry (SLA), and mineral fertiliser (MIN) are also analysed. The fertilisers were added in 2018, and the NH4-N fraction was 15N labelled. The thesis highlights the reliability of a combined SOM density and size fractionation. Results reveal that MAOM fractions, particularly the fSilt-c fraction, dominate N storage, whereas POM fractions contribute minimally. In the first year after application, the majority of 15NH4+-N was poorly stabilised and therefore, the abundance in the fractions decreased rapidly between 2018 and 2019. This leads to the assumption that the ammonium was adsorbed onto minerals and was easily desorbed in the first year (from 2018 to 2019). Recovery and stabilisation of applied N vary with treatments, with SLU showing the highest residual N effect in the soil fractions, emphasising its suitability for long-term retention compared to SLA and MIN. Across all treatments, poorly stabilised N was released rapidly in the first year (from 2018 to 2019), but after 2019 the 15NH4+-N abundance stabilised.