Elsevier

Soil Biology and Biochemistry

Volume 42, Issue 9, September 2010, Pages 1472-1478
Soil Biology and Biochemistry

Charcoal mineralisation potential of microbial inocula from burned and unburned forest soil with and without substrate addition

https://doi.org/10.1016/j.soilbio.2010.05.009Get rights and content

Abstract

Effects of fire on the functioning of the soil microbial community are largely unknown. In this study, we addressed the charcoal mineralisation potential of microbial inocula extracted from burned and unburned soil. The mineralisation of charcoal was analysed during a 1 month incubation experiment under controlled conditions with and without substrate addition. The aim of the study was to elucidate (1) the indirect effect of fire on the functioning of the soil microbial community in terms of charcoal degradation and (2) the possibility to stimulate this degradation by addition of two substrates of increasing complexity. Our conceptual approach included the monitoring of CO2 emission from microcosms containing laboratory-made charcoal and microbial inocula from burned and unburned soil with and without 13C labelled glucose and cellulose.

Our results showed higher charcoal mineralisation without substrate addition in microcosms with the inocula from unburned soil compared to burned soil. Charcoal mineralisation was stimulated by the addition of glucose, whereas cellulose addition did not induce a priming effect. We observed a higher stimulation of charcoal mineralisation induced by glucose for the inoculum from burned soil compared to the inoculum from unburned soil. We concluded that fire did affect the functioning of the soil microbial community in terms of charcoal degradation and that the important priming effect induced by glucose may be explained by an increase of the overall microbial activity, rather than selective stimulation of charcoal degrading microbial communities.

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.

References (44)

  • S. Fontaine et al.

    The priming effect of organic matter: a question of microbial competition

    Soil Biology & Biochemistry

    (2003)
  • U. Hamer et al.

    Interactive priming of black carbon and glucose mineralisation

    Organic Geochemistry

    (2004)
  • J. Kaal et al.

    Can pyrolysis-GC/MS be used to estimate the degree of thermal alteration of black carbon?

    Organic Geochemistry

    (2009)
  • H. Knicker et al.

    N-15 and C-13 CPMAS and solution NMR studies of N-15 enriched plant material during 600 days of microbial degradation

    Organic Geochemistry

    (1995)
  • H. Knicker et al.

    13C- and 15N-NMR spectroscopic examination of the transformation of organic nitrogen in plant biomass during thermal treatment

    Soil Biology & Biochemistry

    (1996)
  • I. Kögel-Knabner

    C-13 and N-15 NMR spectroscopy as a tool in soil organic matter studies

    Geoderma

    (1997)
  • Y. Kuzyakov et al.

    Review of mechanisms and quantification of priming effects

    Soil Biology & Biochemistry

    (2000)
  • V. Lindahl et al.

    Evaluation of methods for extraction of bacteria from soil

    FEMS Microbiology Ecology

    (1995)
  • C. Nocentini et al.

    Nature and reactivity of charcoal produced and added to soil during wildfire are particle-size dependent

    Organic Geochemistry

    (2010)
  • J. Pietikainen et al.

    Does short-term heating of forest humus change its properties as a substrate for microbes?

    Soil Biology & Biochemistry

    (2000)
  • A. Prieto-Fernández et al.

    Short-term effects of a wildfire on the nitrogen status and its mineralization kinetics in an Atlantic forest soil

    Soil Biology & Biochemistry

    (1993)
  • V. Riis et al.

    Extraction of microorganisms from soil: evaluation of the efficiency by counting methods and activity measurements

    Soil Biology & Biochemistry

    (1998)
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