Reactive nitrogen (N) losses, and in particular nitrous oxide losses, from dairy grasslands on peat soils are generally high as a result of relative high soil organic matter contents, potential N mineralisation rates and shallow groundwater levels. Effects of the inclusion of the temperate forage species plantain (Plantago lanceolata L.) (PL), which produces secondary compounds with biological nitrification inhibition capacity, on the fate of soil mineral N were studied in a combined mesocosm and field experiment. The experiments comprised four treatments differing in intentional herbage share of plantain versus perennial ryegrass (Lolium perenne L.) (100%PL, 66%PL, 33%PL and 0%PL). Potential nitrification in the mesocosm experiment was significantly lower at 100%PL versus 0%PL (p = 0.018), but soil nitrate concentrations were not. Nitrous oxide fluxes reduced by 39% (p = 0.021) in the presence of plantain in the field experiment, without an obvious link to the quantity of plantain. N use efficiency of plantain tended to increase with the quantity of plantain in the sward in the mesocosm experiment (p = 0.098), but not in the field experiment. Our results suggest that the presence of plantain can affect the fate of soil mineral N of dairy grasslands on peat soils. ARTICLE HISTORY

The increase in the density of woody plants threatens the integrity of grassy ecosystems. It remains unclear if such encroachment can be explained mostly by direct effects of soil conditions and hydrology on woody plant growth or by indirect effects on fire regime and herbivory imposing tree recruitment limitation. Here, we investigate whether woody plant functional traits provide a mechanistic understanding of the complex relationships between these direct and indirect effects. We first assess the role of rainfall, soil fertility, texture, and geomorphology to explain variation in woody plant encroachment following anthropogenically-induced fire suppression across the Serengeti ecosystem. Second, we explore trait-environment relationships and how these mediate vegetation response to fire suppression. We find that woody plant encroachment is strongest in areas with high soil fertility, high rainfall, intermediate catenae positions, and fine soil textures. These conditions promote woody plant communities associated with small stature, small seed sizes, and high recruit densities (linked to a recruitment-stature trade-off). The positioning of species along this recruitment-stature axis was found to be the most important predictor of recruitment limitation. Areas that support such plant communities - e.g. mid-catena position - were most sensitive to woody plant encroachment. These findings demonstrate the importance of trait-environment relationships in predicting the impact of human alterations on local vegetation change. Understanding how environmental factors directly (resources) and indirectly (legacy effects and plant traits) determine woody plant encroachment supports the development of process-based ecosystem structure and function models.