Charcoal mineralisation potential of microbial inocula from burned and unburned forest soil with and without substrate addition
Introduction
Charcoal is produced during forest wildfires as result of the incomplete combustion of plant material. The many chemical transformations that occur in plant biomass during charring, including the removal of functional groups and cyclization as well as dehydration reactions, increase the recalcitrance of the charred material (Knicker et al., 1996, Czimczik et al., 2002, Baldock and Smernik, 2002). Generally, charcoal has been considered to be part of the stable carbon pools of soils, with mean residence times of up to several centuries (Skjemstad et al., 1996). However, the quality of the charred material that is left on soil following wildfires is heterogeneous. Some fractions of charcoal were found to be enriched in nitrogen and reactive towards oxidising chemicals (Rumpel et al., 2007, Nocentini et al., 2010). The fate of this material in the environment has yet to be elucidated (Czimczik and Masiello, 2007).
The effects of forest wildfires on the soil microbial community depend on fire intensity, and duration, soil moisture and fire history (Certini, 2005). Generally, a marked decrease of soil microbial biomass is observed immediately after wildfire (Prieto-Fernández et al., 1993), mainly due to heat, since the temperatures developed during fires are generally sufficient to kill most living microorganisms in the first few centimeters of soil (Debano and Conrad, 1978). In the months or years after wildfires, soil microorganisms are influenced by large availability of nutrients. Burned soils often have higher pH values, and are enriched in mineral nutrients due to ash addition, representing a favourable habitat for bacteria (Baath and Arnebrant, 1994). Contemporarily, fire destroys most of the organic matter in the first few centimeters of soil and adds recalcitrant charcoal (Fernández et al., 1999). Fire is therefore driving the availability of carbon to the soil microorganisms. These profound alterations of environmental conditions influence the microbial community physiology and structure (Acea and Carballas, 1996, Pietikainen et al., 2000). Nevertheless, few analyses have been performed on the fire-induced changes of the functional structure of soil microbial populations. One of the issues that need to be addressed is, if the microbial community of burned soil is able to use charred organic matter.
Several short term incubation studies showed that the mineralisation of charcoal by soil microorganisms is possible. Nevertheless, during these incubations with charcoal as the sole C source, great decrease of microbial activity was noticed (Baldock and Smernik, 2002). Mineralisation of charcoal in the presence of soil microbial inoculates could be enhanced by addition of labile substrates (Hamer et al., 2004, Kuzyakov et al., 2009). These experiments evidenced that positive as well as negative priming effects may be possible for charcoal. Priming effect was defined by Jenkinson (1966) as a change of mineralisation rate of stable carbon compounds induced by the addition of labile organic matter. The mineralisation rate of stable compounds can increase (i.e. positive priming effect) or decrease (i.e; negative priming effect). By the addition of an easy degradable substrate, like glucose, degradation of charcoal may be enhanced due to an increase in global microbial activity. Recently, it has been suggested, that the intensity of priming might be driven by competition for labile and energy-rich substrates between two microbial communities: the decomposers of simple compounds and the decomposers of more complex material (Fontaine et al., 2003). The decomposers of more complex material have lower specific growth rates as compared with the decomposers of easily available C. Therefore, the decomposers of low available substrate fail in the competition with the decomposers of easily available substrate which do not have such limitation (Andrews and Harris, 1986). As a result they would not access to energy, and not induce a priming effect. Thus, the addition of low available substrates may be more adapted to stimulate the degradation of complex substrates such as charcoal. No study has been carried out so far, which used different substrates for priming of charcoal by stimulation of different microbial communities. Moreover, none of the studies about charcoal mineralisation took into account the changes that might have occurred in microbial community composition during fire.
In this study we assessed the possibility of stimulating the microbial degradation of laboratory produced and chemically well-characterised charcoal by the addition of two different substrates. We used glucose and cellulose to provide the microbial inocula with substrates of different complexity. Moreover, we performed the experiment with two different microbial inocula: one was extracted from burned and the other from unburned soil. The objectives of the study were: 1) to evaluate if fire alters the functioning of the soil microbial community in terms of charcoal mineralisation and 2) to study the possibility of stimulating charcoal mineralisation by the addition of 13C labelled glucose and cellulose.
Section snippets
Soil sampling and inoculum production
The sampling site was located in Migliarino in Tuscany, Central Italy, on a flat area covered by a 14 years old Pinus pinea forest partly affected by wildfire that occurred two weeks before. The fire was an intensive wildfire that lasted several days and burned all of the plant biomass (a forest of about 8 m height and 4000 trees ha−1). Twenty soil samples were collected from the top 0–5 cm in two areas of 300 m2 in the burned and unburned forest respectively. Soil microorganisms were extracted
Characteristics of the charred material
The charcoal produced in the laboratory had a carbon content of 656.4 ± 12.7 g kg−1 (Fig. 1). It contained some nitrogen resulting in a C/N ratio of 82 ± 7. For this material an atomic H/C ratio of 0.65 ± 0.01 was recorded. Solid-state 13C NMR spectroscopy showed, that the charcoal used in the incubation experiment consisted mainly of aromatic carbon with a dominant signal at 130 ppm (Fig. 1). However, additional signals (a shoulder at 150 ppm and a peak at 53 ppm) could be due to the
Discussion
The charred material produced in the laboratory consisted mainly of aromatic C, reflecting the general trends reported in experiments on charcoal production from wood and other ligninocellulosic materials (Baldock and Smernik, 2002, Czimczik et al., 2002). However, other functional groups and molecules related to the presence of polysaccharides, lignin and aliphatic compounds were still detectable with 13C CPMAS NMR and analytical pyrolysis (Fig. 1, Fig. 2). The presence of these compounds was
Conclusion
This experiment aimed at analysing the effects of fire on microbial charcoal mineralisation. Microbial communities extracted from burned and unburned soil mineralized charcoal and we observed an acceleration of charcoal mineralisation after 8 days for the inoculum of unburned soil. We conclude that fire did not eliminate the charcoal mineralisation capabilities of the microbial community in the soil of the fire prone Mediterranean ecosystem, but fire could decrease the rate of charcoal
Acknowledgements
C. Nocentini received a PhD grant from the Italian government. Additional financial support was provided by INSU (CNRS, France) under the framework of the EC2CO program ‘QUANTICHAR’. We also thank the Lehrstuhl für Bodenkunde, TU München for providing the NMR spectrometer.
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2018, Science of the Total EnvironmentCitation Excerpt :On the other hand, recent laboratory incubation studies have reported mean resident times of PyOM in soils in the range of decades to centuries (Hilscher et al., 2009; Santos et al., 2012; Bird et al., 2015) but not millennia as previously claimed (Saldarriaga and West, 1986; Kuzyakov et al., 2009). However such long MRTs were observed for biochars, produced via pyrolysis at temperatures above 550 °C (Zimmerman, 2010) Incubation experiments by Nocentini et al. (2010) and De la Rosa et al. (2018) showed that charcoal can be mineralized by microorganisms. Further indications for biochemical degradation of PyOM were given by Hilscher and Knicker (2011) who performed laboratory incubation experiments with grass-derived charcoal and found evidence of microbial attack of the aromatic network, with a concomitant increase in carboxyl C.