The effect of nine-year nitrogen addition on foliar fungal diseases of plant groups in an alpine meadow

Introduction Foliar fungal diseases have become one of the non-negligible drivers of accelerated grassland degradation, and global change processes such as increased nitrogen (N) deposition can significantly regulate the occurrence and development of foliar fungal diseases in meadows. However, little is known about how increased nitrogen deposition affects foliar fungal diseases in alpine meadows, and there is limited understanding of the relative importance of different plant groups in the process of N enrichment affecting plant diseases. Methods Here, based on a 9-year continuous nitrogen addition experiment (2014-2022, gradients: 0, 2, 4, 8, 16, 32 g N m -2 year -1 ) in an alpine meadow on the Qinghai-Tibet Plateau, we investigated the effects of long-term N enrichment on plant group attributes and foliar fungal diseases through a one-time field survey conducted in August 2022. We employed linear mixed-effects models and structural equation modeling to explore the relationships between foliar fungal diseases and plant community variables under N enrichment. Results We found that plant groups identity was the dominant factor influencing foliar fungal diseases, with Gramineae consistently exhibiting the highest disease incidence and disease index across all N addition levels. The SEM revealed that N addition did not exert a significant direct effect on disease index; rather, its effect was fully mediated by disease incidence-N addition significantly increased incidence rate, which in turn drove disease index. In addition, foliar fungal diseases tended to infect plants with greater relative coverage, especially Gramineae and Forbs, potentially because denser canopies promote pathogen spread and infection. Discussion Overall, our results suggest that plant group composition plays a dominant role in determining foliar fungal disease patterns, with Gramineae being the most affected group, and that N addition influences disease development primarily by increasing disease incidence rather than by directly exacerbating lesion severity. These findings highlight the importance of considering foliar fungal disease occurrence in response to global change and the utility of plant groups as predictors of disease risk under N enrichment.