Sphagnum N and P Stoichiometry Indicates P-limitation on N2 Fixation in Ombrotrophic Bogs

Abstract:

Biological N₂ fixation is an important N input in ombrotrophic, nutrient poor and Sphagnum dominated bogs. As an energetically costly process, by which each N₂ molecule is fixed to a cost of 16ATP molecules, N₂ fixation might be P limited process. In this study we tested whether moss P and N concentrations, and N:P ratios could explain N₂ fixation in the top 6cm photosynthetically active Sphagnum moss across eight ombrotrophic bogs along south-north geographical gradient in Ontario and Quebec. Under constant environmental conditions, we incubated subsamples of the surface Sphagnum mosses by using both, acetylene reduction assays (ARA) and ¹⁵N₂ enriched method to measure N₂ fixation rates. Same subsamples were later analyzed for N and P concentrations. Our preliminary data show that the increase of P concentration within moss capitula is related to a significant linear increase of ARA rates (R²=0.18, p<0.0001, N=150). N:P ratios showed a significant negative linear relationship with ARA (R²=0.34, p<0.0001, N=150) indicating that P limitation in the photosynthetically active part of mosses in bogs may also indicate P limitation on microbial N₂ fixation