Chronic nitrogen (N) additions to soils under agriculture have multiple effects on soil ecosystem functioning. Here we focus on N inputs to grasslands, which are crucial to support livestock-based production systems across the UK. We use data from a long-term grassland experiment established in 1970 in Northern Ireland with eight treatments: control, inorganic fertiliser, cow and pig slurry at three different application rates. This study investigated treatments effects on: (i) total soil N and soil carbon: nitrogen rations (soil C:N), (ii) soil N stocks, and (iii) the net ecosystem N balance (NENB). Total soil N significantly increased over time for all nutrient treatments and there were significant slurry type and application rate effects. Cow slurry treatments had greater total soil N than pig slurry. The calculation of NENB highlighted which nutrient treatments had soil N accumulation and N loss to environment. By addressing changes in NENB and ecosystem indicators we identified a trade-off between plant species richness and plant biomass production, and we found that soil C stocks and soil pH were positively related. Understanding these soil–plant relationships is required to optimise grassland management practices to avoid excessive N losses and maintain soil health.