Community Assembly Regulates Ecosystem Carbon Exchange of Alpine Meadow in Response to Nitrogen Addition on the Tibetan Plateau

Nitrogen (N) deposition impacts profoundly ecosystem carbon balance across the globe. However, how dominant plant species control ecosystem carbon exchange in response to N deposition remains largely unclear, especially in alpine meadows at high elevations. Here, we conducted a manipulated experiment to examine the impacts of community assembly on the response of ecosystem CO 2 exchange to N addition at an alpine meadow site from 2014 to 2018 on the Qinghai-Tibetan Plateau (QTP). Net ecosystem CO 2 exchange (NEE), gross ecosystem productivity (GEP), and ecosystem respiration (ER) were measured. N-addition treatments were 0 (control), 7, 20, and 40 kg N ha -1 yr -1 during plant growing seasons. Our results showed that NEE, ER and GEP increased gradually with increasing N-addition rate over time ( P < 0.01). N addition significantly increased the aboveground biomass (AGB) over time. AGB and soil microbial carbon (MBC) all showed a significant positive correlation with NEE, ER and GEP ( P <0.05). N addition levels showed positive correlations between NEE and sedge dominance ( P <0.05), but a significant negative correlation between ER and forbs dominance ( P <0.01). Structural equation models revealed that changes of AGB, soil microbial biomass and dominance of functional groups (sedge and forbs) mainly contributed to the increase of NEE and ER. Our results highlight that community composition is critical in regulating ecosystem carbon exchange to N deposition for alpine ecosystems