The impact of wildfire on microbial C:N:P stoichiometry and the fungal-to-bacterial ratio in permafrost soil

Xuan Zhou,Frank Berninger,Hui Sun,Jukka Pumpanen,Kajar Köster,Jussi Heinonsalo,Outi-Maaria Sietiö

doi:10.1007/s10533-018-0510-6

Abstract

Wildfires thaw near-surface permafrost soils in the boreal forest, making previously frozen organic matter available to microbes. The short-term microbial stoichiometric dynamics following a wildfire are critical to understanding the soil element variations in thawing permafrost. Thus, we selected a boreal wildfire chronosequence in a region of continuous permafrost, where the last wildfire occurred 3, 25, 46, and > 100 years ago (set as the control) to explore the impact of wildfire on the soil chemistry, soil microbial stoichiometry, and the fungal-to-bacterial gene ratio (F:B ratio). We observed the microbial biomass C:N:P ratio remained constant in distinct age classes indicating that microbes are homeostatic in relation to stoichiometric ratios. The microbial C:N ratios were independent of the shifts in the fungal-to-bacterial ratio when C:N exceeded 12. Wildfire-induced reduction in vegetation biomass positively affected the fungal, but not the bacterial, gene copy number. The decline in microbial biomass C, N, and P following a fire, primarily resulted from a lack of soil available C and nutrients. Wildfire affected neither the microbial biomass nor the F:B ratios at a soil depth of 30 cm. We conclude that microbial stoichiometry does not always respond to changes in the fungal-to-bacterial ratio and that wildfire-induced permafrost thawing does not accelerate microbial respiration.

Highlights

Soil microbes play a crucial role in carbon (C), nitrogen (N), and phosphorus (P) cycling in terrestrial ecosystems by mineralizing organic material to inorganic forms (Singh et al 2010; Waring et al 2013; Xu et al 2013)
The soil temperatures in the topsoil were similar across the age classes, but were decreased in 10 and 30 cm layers across the time elapsed since the wildfire (Fig. 1e)
We argue that soil microbial communities exhibit a strict homeostasis in both the short- and long-term following wildfires

Summary

Introduction

Soil microbes play a crucial role in carbon (C), nitrogen (N), and phosphorus (P) cycling in terrestrial ecosystems by mineralizing organic material to inorganic forms (Singh et al 2010; Waring et al 2013; Xu et al 2013). In contrast to marine ecosystems, a meta-analysis of the soil microbial stoichiometry on a global scale indicated that soil microbes might maintain their internal C:N:P ratio regardless of their environment (Cleveland and Liptzin 2007; Xu et al 2015). Microbes tend to maintain their chemical elements in ‘‘optimal ratios’’ for growth and development (Sterner and Elser 2002). They maintain these ratios by releasing excess elements through respiration or excretion (Tempest and Neijssel 1992) or by obtaining deficient elements through excretion of extracellular enzymes (Mooshammer et al 2014). Homeostatic organisms maintain internal nutrient concentration ratios (e.g., C:N:P ratios) independent of their resources stoichiometry, whereas non-homeostatic organisms depend upon it

Methods

Results

Discussion

Conclusion