Nitrogen Inputs via Nitrogen Fixation in Northern Plants and Soils

Abstract:

Dominated by cold and often acidic water logged environments, mineralization of organic matter is slow in the majority of northern ecosystems. Measures of extractable ammonium and nitrate are generally low and can be undetectable in peat pore waters. Despite this apparent nitrogen limitation, many of these environments produce deep deposits of soil organic matter. Biological nitrogen fixation carried out by autotrophic and heterotrophic diazotrophs associated with cryptograms provides the majority of known nitrogen inputs in these northern ecosystems. Nitrogen fixation was assessed in a variety of northern soils within rhizospheres of dominant plant communities. We investigated the availability of this newly fixed nitrogen to the vascular plant community in nitrogen limited northern plant communities. We tracked nitrogen flow from ¹⁵N₂ gas fixed in Sphagnum mosses into tissues of two native vascular plant species, boreal cranberry (Vaccinium oxycoccus) and black spruce (Picea mariana). ¹⁵N-labeled Sphagnum microcosms were grown within variable mesh size exclusion/inclusion fabrics in a nitrogen addition experiment in situ in order to investigate the role of mycorrhizal fungi in the uptake of newly fixed nitrogen.  Up to 24% of daily fixed ¹⁵N label was transferred to vascular plant tissues during 2 months. Nitrogen addition resulted in decreased N₂ fixation rates; however, with higher nitrogen availability there was a higher rate of ¹⁵N label uptake into the vascular plants, likely the result of increased production of dissolved organic nitrogen. Reliance on mycorrhizal networks for nitrogen acquisition was indicated by nitrogen isotope fractionation patterns. Moreover, N₂ fixation activities in mosses were stimulated when vascular plants were grown in moss microcosms versus “moss only” treatments. Results indicate that bog vascular plants may derive considerable nitrogen from atmospheric N₂ biologically fixed within Sphagnum mosses. This work demonstrates that diazotroph-mediated ¹⁵N labeling is a viable technique for tracking nitrogen flow without altering form and concentration of native nitrogen pools in a nitrogen limited ecosystem.