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Contrasting effects of long term versus short-term nitrogen addition on photosynthesis and respiration in the Arctic
- van de Weg, Martine J., Shaver, Gaius R., Salmon, Verity G.
- Plant ecology 2013 v.214 no.10 pp. 1273-1286
- Betula nana, Eriophorum vaginatum, chlorophyll, electron transfer, leaf area index, long term effects, net ecosystem exchange, nitrogen, nutrient availability, nutrients, photosynthesis, ribulose-bisphosphate carboxylase, Arctic region
- We examined the effects of short (<1-4 years) and long-term (22 years) nitrogen (N) and/or phosphorus (P) addition on the foliar CO2 exchange parameters of the Arctic species Betula nana and Eriophorum vaginatum in northern Alaska. Measured variables included: the carboxylation efficiency of Rubisco (Vcmax), electron transport capacity (Jmax), dark respiration (Rd), chlorophyll a and b content (Chl), and total foliar N (N). For both B. nana and E. vaginatum, foliar N increased by 20-50 % as a consequence of 1-22 years of fertilisation, respectively, and for B. nana foliar N increase was consistent throughout the whole canopy. However, despite this large increase in foliar N, no significant changes in Vcmax and Jmax were observed. In contrast, Rd was significantly higher (>25 %) in both species after 22 years of N addition, but not in the shorter-term treatments. Surprisingly, Chl only increased in both species the first year of fertilisation (i.e. the first season of nutrients applied), but not in the longer-term treatments. These results imply that: (1) under current (low) N availability, these Arctic species either already optimize their photosynthetic capacity per leaf area, or are limited by other nutrients; (2) observed increases in Arctic NEE and GPP with increased nutrient availability are caused by structural changes like increased leaf area index, rather than increased foliar photosynthetic capacity and (3) short-term effects (1-4 years) of nutrient addition cannot always be extrapolated to a larger time scale, which emphasizes the importance of long-term ecological experiments. © 2013 Springer Science+Business Media Dordrecht.