Contrasting Effects of Nitrogen Addition on Leaf Photosynthesis and Respiration in Black Mangrove in North Florida

Nutrient enrichment is a major driver of environmental change in mangrove ecosystems. Yet, nutrient enrichment impacts on physiological processes that regulate CO 2 and water fluxes between mangrove vegetation and the atmosphere remain unclear. We measured peak growing season photosynthesis ( A ) and respiration ( R ) in black mangrove ( Avicennia germinans ) leaves that had been subjected to long-term (8-year) nutrient enrichment (added N, added P, control) in north Florida. Previous results from this site indicated that Avicennia productivity was N-limited, but not P-limited. Thus, we expected that N addition would increase light saturated net photosynthesis at ambient CO 2 ( A net ), intrinsic water-use efficiency (iWUE), maximum rate of Rubisco carboxylation ( V cmax ), and leaf dark respiration ( R ), while P addition would have little effect on any aspect of photosynthesis or respiration. We expected that increased photosynthesis and respiration would be most apparent immediately after N addition and in newly formed leaves. Indeed, A net and V cmax increased just after N addition in the N addition treatment; these increases were limited to leaves formed just after N addition. Nonetheless, over time, photosynthetic parameters and iWUE were similar across treatments. Interestingly, R measured at 25 °C increased with N addition; this effect was consistent across time points. P addition had little effect on R . Across treatments and time points, V cmax,25 ( V cmax standardized to 25 °C) showed no relationship with R at 25 °C, but the maximum rate of electron transport for RuBP regeneration standardized to 25 °C ( J max,25 ) increased with R at 25 °C. We conclude that N addition may have small, short-lived effects on photosynthetic processes, but sustained effects on leaf R in N-limited mangrove ecosystems.