Genome size-driven shifts in grassland productivity and species composition under nitrogen enrichment and mowing

Grasslands underpin critical ecosystem services, yet their biodiversity and functioning are increasingly threatened by nitrogen (N) deposition and influenced by biomass removal through mowing to provide fodder. Trait-based approaches have advanced our understanding of plant responses, but many commonly used traits are plastic and context-dependent, limiting their predictive power. Here, we propose that plant genome size (GS), a relatively stable genetic character between individuals of a species, offers predictive power through nucleotypic constraints that are hypothesized to influence plant nutrient use, growth dynamics, and competitive strategies. By utilizing a large dataset of aboveground net primary productivity (ANPP) from an N addition and mowing field experiment over 15 years, we showed that GS–N interactions caused a selective stimulation of large-GS species under N enrichment, but only when N addition treatment exceeded a saturation threshold of 15 g N m⁻² year⁻¹. Conversely, low levels of N addition preferentially enhanced ANPP of small-GS plants. These divergent responses between two genomic groups, pivoting at the N saturation threshold, could be ascribed to a shift from competition for N to competition for light, as N availability increases. Meanwhile, biomass removal by mowing delayed the impact of this threshold by selectively benefiting small-GS species and disadvantaging large-GS species. This response could be due to the increased amount of canopy light penetration following biomass removal favoring small-GS species even under higher N availabilities. Additionally, large-GS species showed greater sensitivity of ANPP to interannual fluctuations in temperature and precipitation compared to small-GS species. The climate sensitivity of large-GS species was further amplified with increasing N treatment, reflecting a pervasive large genome constraint among grassland plants. Our findings pinpoint the co-regulation of GS, nutrients, and climate on plant growth, providing evidence that GS is a predictive character for ANPP change trajectories and community dynamics under the combined influence of global changes and management scheme shifts.