NITROGEN CYCLING MEDIATED BY BIOLOGICAL SOIL CRUSTS AND ARBUSCULAR MYCORRHIZAL FUNGI

Plants in terrestrial systems obtain nitrogen primarily by root uptake from soil, yet current models of ecosystem nitrogen cycling assume little interaction of plant roots with the soil microbial community. The objective of this study was to determine how soil microorganisms in biological soil crusts interact with herbaceous plants for nitrogen in a pyrogenic ecosystem in Florida (USA), using a combination of natural abundance and tracer isotope methods. I demonstrate that plant nitrogen is strongly affected by interactions with mycorrhizal fungi and biological soil crusts. Measurements of nitrogenase activity indicated a high capacity of crusts to fix nitrogen that dramatically decreased after disturbance. Natural abundance of δ 15 N differed between arbuscular mycorrhizal and nonmycorrhizal herbs. When a small amount of enriched nitrogen was used as a tracer, crusts initially captured a greater amount of the 15 N that was added compared to plants, with some turnover of nitrogen from the crusts to plants observed after 15 days. Observations in this study indicate that crusts have more than one function in nitrogen dynamics: cyanobacteria in crusts fix nitrogen, crusts retain nitrogen in the system that would otherwise be lost to leaching, and though some of this nitrogen is available to plants, they also face competition from heterotrophic bacteria in crusts. Corresponding Editor: P. M. Groffman.