Abstract
Understanding how nitrogen (N) and/or phosphorus (P) addition affects plants carbon- and water- related ecophysiological characteristics is essential for predicting the global change impact on the alpine meadow ecosystem structure and function in carbon and water cycling. The Qinghai-Tibetan Plateau (QTP) with the largest alpine meadow in the world is regarded as the third pole in the earth and has been experiencing increased atmospheric N deposition. In this project, we focused on two key species (Elymus dahuricus and Gentiana straminea) of the alpine meadow on the Tibetan Plateau and investigated the variability of photosynthetic and stomatal responses to 8-year N and/or P treatments through field measurements and modeling. We measured photosynthesis- and gs-response curves to generate parameter estimates from individual leaves with two widely used stomatal models (the BWB model and MED model) for validation of growth and ecosystem models and to elucidate the physiological basis for observed differences in productivity and WUE. We assessed WUE by means of gas exchange measurements (WUEi) and stable carbon isotope composition (Δ13C) to get the intrinsic and integrated estimates of WUE of the two species. P and N+P treatments, but not N, improved the photosynthetic capacity (Anet and Vcmax) for both species. Stomatal functions including instaneous measurements of stomatal conductance, intrinsic water-use efficiency and stomatal slope parameters of the two widely used stomatal models were altered by the addition of P or N+P treatment, but the impact varied across years and species. The inconsistent responses across species suggest that an understanding of photosynthetic, stomatal functions and water-use should be evaluated on species separately. WUE estimated by Δ13C values had a positive relationship with Anet and gs and a negative relationship with WUEi. Our findings should be useful for understanding the underlying mechanisms of the response of alpine plants growth and alpine meadow ecosystem to global change.
Highlights
Terrestrial ecosystems worldwide are limited or co-limited by nutrients, especially by nitrogen (N) and phosphorus (P) (Elser et al, 2007; LeBauer and Treseder, 2008; Harpole et al, 2011)
Photosynthetic and leaf traits varied between years and among species, with G. straminea possessing higher as the net photosynthetic rate (Anet), gs, Fv/Fm, photosystem II (PSII), Vcmax, Jmax, Nmass, and 13C and lower WUEi and stomatal slope parameters compared with E. dahuricus (Table 1)
Photosynthetic and leaf traits varied among species and treatment, with G. straminea possessing higher Anet, gs, Fv/Fm, PSII, Vcmax, Jmax, Nmass, and 13C and lower WUEi and stomatal slope parameters compared with E. dahuricus (Table 1)
Summary
Terrestrial ecosystems worldwide are limited or co-limited by nutrients, especially by nitrogen (N) and phosphorus (P) (Elser et al, 2007; LeBauer and Treseder, 2008; Harpole et al, 2011). While it is well-known that N and P addition typically increases plant growth, less is known about how N and P addition affects plants ecophysiological characteristics related to carbon and water acquisition. With the increase of global nitrogen deposition and the relative slow mineralization rate due to the low temperature at the high elevation, it is critical to investigate the ecophysiological responses of the alpine grasslands species to N and P addition. The information will be valuable in predicting the global change impact on the alpine meadow ecosystem structure and function in carbon and water cycling
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