Articles | Volume 14, issue 1
Research article
17 Jan 2017
Research article |  | 17 Jan 2017

Contributions of microbial activity and ash deposition to post-fire nitrogen availability in a pine savanna

Cari D. Ficken and Justin P. Wright

Abstract. Many ecosystems experience drastic changes to soil nutrient availability associated with fire, but the magnitude and duration of these changes are highly variable among vegetation and fire types. In pyrogenic pine savannas across the southeastern United States, pulses of soil inorganic nitrogen (N) occur in tandem with ecosystem-scale nutrient losses from prescribed burns. Despite the importance of this management tool for restoring and maintaining fire-dependent plant communities, the contributions of different mechanisms underlying fire-associated changes to soil N availability remain unclear. Pulses of N availability following fire have been hypothesized to occur through (1) changes to microbial cycling rates and (2) direct ash deposition. Here, we document fire-associated changes to N availability across the growing season in a longleaf pine savanna in North Carolina. To differentiate between possible mechanisms driving soil N pulses, we measured net microbial cycling rates and changes to soil δ15N before and after a burn. Our findings refute both proposed mechanisms: we found no evidence for changes in microbial activity, and limited evidence that ash deposition could account for the increase in ammonium availability to more than 5–25 times background levels. Consequently, we propose a third mechanism to explain post-fire patterns of soil N availability, namely that (3) changes to plant sink strength may contribute to ephemeral increases in soil N availability, and encourage future studies to explicitly test this mechanism.

Short summary
To evaluate different mechanisms underlying nitrogen (N) pulses that occur following fires in pyrogenic US savannas, we coupled field measures of soil N cycling with soil N isotopic signatures. We found that neither ash deposition nor changes to microbial activity could account for observed N pulse. We hypothesize that changes to plant uptake may contribute to the post-fire N pulses. We encourage future work to explore the role of plant activity on temporal nutrient availability dynamics.
Final-revised paper