Shrub cover attenuates the effect of high-nitrogen deposition on Sphagnum palustre in a subtropical bog

Nitrogen (N) deposition profoundly alters plant community structure and ecosystem functioning in bogs. High N deposition generally enhances vascular plant expansion and inhibits the growth of Sphagnum. However, the shading effect caused by the increased height of vascular plants, particularly shrubs, not only reduces the availability of light for Sphagnum, but also relieves environmental stress by creating a more humid microenvironment. Therefore, understanding how shrub presence modifies Sphagnum responses to N deposition is critical for predicting bog resilience and informing conservation and restoration strategies. To address this, a long-term N addition experiment with multiple deposition levels was conducted in a subtropical Sphagnum bog in southwestern China. After seven years of continuous N addition treatment, physiological and morphological characteristics of S. palustre were measured under both shrub shelter and open interspaces. Across all treatments, S. palustre tissues beneath shrub canopies exhibited consistently lower total N and ammonium N (NH4+-N) contents than those in open interspaces. Following N addition, S. palustre under shrub shelter had significantly greater shoot height, capitulum area, and biomass. Moreover, shrubs markedly alleviated N-induced physiological stress, as indicated by enhanced photosynthetic pigment content, soluble protein accumulation, and antioxidant enzyme activities. These results indicate that shrub shelter is associated with reduced nitrogen accumulation and more stable physiological and morphological performance of S. palustre under elevated N deposition. Shrub canopies may contribute to moderating nitrogen-related stress and sustaining S. palustre growth in bog ecosystems. These findings underscore the importance of microhabitat heterogeneity in mediating peatland responses to global change and suggest that maintaining natural shrub cover could be a valuable consideration in conservation strategies for bogs facing high N deposition.