Forest soil charcoal colonization by bacteria: Results from a fire chronoseqence

Background/Question/Methods

Wildland fires convert conifer forest into forms of black carbon such as charcoal. Charcoal decomposition in wildlands is poorly known.  Soil charcoal C has been assumed to be resistant to microbial attack and this material does persist in the soil for thousands of years.  We conducted a straightforward set of analyses on soils and charcoal collected from a fire chronsequence in Northern Arizona.  Stand clearing fires in Pinus ponderosa forests occurred in 1977, 1990, 2000 and 2014.  Soil and adjacent surface charcoal samples were collected in July –Augsut 2015.  Subsequently, total genomic DNA was extracted from 0.5 g of soil and of charcoal using PowerLyzer PowerSoil DNA Isolation Kit according to manufacturer’s instructions. DNA concentrations in each sample were quantified using Qubitds DNA NS assay kit. The 16S rRNA gene was amplified using primers 515f and 806r, quantified by qPCR then sequenced using Illumina MiSeq. Subsequent analysis was performed using QIIME. Open reference operational taxonomic unit (OTU) picking was performed at 97% identity using Uclust. The most abundant sequence for each OTU was aligned with Pynast against the Greengenes v13_5 database, and taxonomy was assigned using Ribosomal Data Project (RDP) classifier.  Samples were rarefied at an even sampling depth of 4000 sequences.

Results/Conclusions

DNA was observed in all samples from the fire chronsequence sites, with similar concentrations in both soils and charcoal.  All samples contained bacterial DNA as revealed by 16sRNA amplification.  Initial apha-diversity analysis showed that there is greater bacterial diversity in the soil samples than in the charcoal samples.  Bacterial community composition (beta-diversity) varied not only between sites but also between soil and charcoal samples within the sites.  Charcoal from the site with the most recent fire (2014) is abundantly rich in Cytophagales, Burkholderia and Achtinomycetales compared with charcoal from the older sites.  Overall, charcoal bacterial communities from the different sites are less similar to each other and distinct from the soil bacterial communities. Soil charcoal decomposition is assumed to be an exponential decay curve with ~90% decomposed in the first 100yr with up to 40,000yr total depending on the climate, soil physical and chemical conditions.  We propose that microbial decomposition is dominant in the first 100 yr and after that, soil charcoal loss is mainly via physical and chemical processes. Understanding the processes that control soil charcoal decomposition is crucial for unraveling the potential of soil charcoal and a long-term sink.