Sphagnum N and P Stoichiometry May Predict the Magnitude of N2 Fixation Rates in Ombrotrophic Bogs

English Abstract:

Biological N₂ fixation is an important nitrogen (N) input in ombrotrophic, nutrient poor, and Sphagnum dominated bogs. Ombrotrophic bogs have accumulated large amounts of N that could not be explained by wet atmospheric N deposition, thus N₂ fixation is the only other possible N input. Until recently, N₂ fixation rates in bogs were considered low or negligible due to methodological constraints and non-systematic measurements. Only handful of studies have shown that N₂ fixation can explain both net primary production within the bog ecosystem and the discrepancy between large N accumulation rates and wet N deposition. However, N₂ fixation is an enzymatic process done by nitrogenase enzyme only found in diazotrophic microbes that have shown to be either free living in bog pore water or to live in association with Sphagnum mosses. As an energetically costly process, N₂ fixation might be phosphorus (P) limited process. In this study we tested whether moss P and N concentrations, and N:P ratios could explain N₂ fixation in photosynthetically active Sphagnum plants in ombrotrophic bogs across south-north geographical gradient in Ontario and Quebec.

We measured N₂ fixation rates in the top 6cm of Sphagnum mosses from eight bogs by using acetylene reduction assays (ARA) and ¹⁵N₂ enriched method under constant environmental conditions (relative humidity 80%, temperature 22⁰C and PAR = 600 µmol m^-2 s^-1. Subsamples were later dried, ground and analyzed for N and P concentrations.

Preliminary results suggest that the increase of P concentration within moss capitula is followed by significant linear increase of ARA rates (R₂=0.18, LOG ARA nmol g(dw)^-1 h^-1 = 0.74 + 1.27*LOG P mg g^-1, p<0.0001, N=150). N:P ratios show significant negative linear relationship with ARA (R₂=0.34, ARA nmol g(dw)^-1 h^-1 = 2.96 - 2.1*LOG N/P), indicating that P limitation in the photosynthetically active part of mosses may control microbial N₂ fixation, and thus N:P ratios could predict the magnitude of nitrogenase activity.