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

Abstract

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 δ 15 N 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.

Effects of fire frequency on litter decomposition as mediated by changes to litter chemistry and soil environmental conditions

“…Because we did not analyze the community composition of decomposing microbes [43], we cannot exclude the possibility that differences in decomposer abundance and/or composition between destination fire regimes influenced decomposition. We also cannot discount the possibility that differences in microbial activity influenced decomposition, although previous work has found that net mineralization and net nitrification rates were not influenced by time since fire [27]. While we observed fast decomposition of understory species in triennially burned destination environments, and fast decomposition of triennial understory litter, we did not detect significant interactions between litter source and destination environment showing enhanced decomposition of triennial litter decomposing in triennially burned sites, or “home” litter [44–46].…”

“…Fire can also impact litter decomposition if decomposer microbe populations shift [25] or if the local microbial community is best-adapted to decomposing local litter [26]. Finally, post-fire changes to nutrient availability in mineral soil [19, 27] may alleviate nutrient-limitation and increase activity of decomposing microbes. Fires that incompletely burn woody biomass may produce black carbon or biochar [28], which may increase soil microbial biomass and alter microbial and invertebrate composition by influencing soil pH and improving localized soil fertility [29–31].…”

Better lucky than good: How savanna trees escape the fire trap in a variable world

“…Thus, we expect individuals that escape the fire trap by growing quickly to be widely distributed across the landscape. If local resource availability is responsible for the variability in the growth rate of resprouts, as has been suggested (Grady andHoffmann 2012, Freeman et al 2017), it must be acting primarily at spatial scales smaller than the 50-m transects sampled here, or at temporal scales finer than the fire return interval (e.g., Wilson et al 2002, Schafer and Mack 2010, Ficken and Wright 2017. This pattern in growth rates may reduce spatial aggregation of mature stems somewhat, but the effect is likely small owing to the relatively small influence that variability in growth rate has on escape success.…”

Intra-Annual Variation in Soil C, N and Nutrients Pools after Prescribed Fire in a Mississippi Longleaf Pine (Pinus palustris Mill.) Plantation

“…The pulse of rain could have affected to movement of nutrients beyond the depths studied, the degree to which this affected the results is not known. A more radical hypothesis that fires temporarily depress plant uptake of N for and thereby enhance soil nutrient pools from reduced demand was proposed by Ficken and Wright [29], in their study of N mineralization rates in longleaf pine stands in eastern North Carolina. A period of depressed plant demand followed by a resumption or accelerated demand could help explain our intra-annual observation of peak nutrient pools after burning followed by depression below pre-burn conditions.…”