Elsevier

Pedobiologia

Volume 50, Issue 1, 26 January 2006, Pages 1-10
Pedobiologia

Effects of environmental factors and leaf chemistry on leaf litter colonization by fungi in a Mediterranean shrubland

https://doi.org/10.1016/j.pedobi.2005.07.005Get rights and content

Summary

Estimation of litter colonization by fungi, using ergosterol, an indicator of fungal biomass, is a reliable way to describe the process of leaf litter decomposition. This litter colonization by fungi is regulated both by exogenous or environmental factors, and endogenous factors, i.e. litter chemistry. In this work, we have examined the effects of some of these factors on litter fungal colonization in a Mediterranean ecosystem, by determining ergosterol content of Quercus coccifera leaf litter.

Environmental factors have been studied through the fertility of the soil, by comparing plots amended with two rates of compost and plots without amendment. Results indicated that (i) compost had a significant effect on soil fertility but did not increase ergosterol content of leaf litter and (ii) soil humidity improved leaf litter colonization by fungi.

Endogenous factors have been studied through measurements of total phenolic and ergosterol concentrations of seven shrub species leaf litter. We have shown (i) a negative significant correlation between total phenolic compounds and ergosterol concentrations of leaf litter and (ii) a positive significant correlation between total phenolic compound concentrations in green leaves and in leaf litter. We conclude that, in this Mediterranean shrub ecosystem, leaf litter colonization by fungi is controlled by soil moisture and plant leaf litter quality.

Introduction

Litter is an important source of dead organic matter in terrestrial ecosystems, with inputs of 50×109 tons of litter per year (Isidorov and Jdanova, 2002). Litter decomposition contributes directly to nutrient availability both for plant growth and ecosystem productivity (Koukoura et al., 2003). The studies that have taken microorganisms into account agree in giving the fungi as the main contributors to leaf litter decomposition (Toutain, 1987; Isidorov and Jdanova, 2002).

These eukaryotes are able to hydrolyse and assimilate refractory compounds such as lignin (Criquet et al., 1999) or tannins (Iacazio et al., 2000), although bacteria are not thought to degrade the leaf material before it has become partially broken down by microarthropods and partially decomposed by fungi (Périssol et al., 1993; Dilly et al., 2001).

Litter fungal colonization is regulated both by exogenous or environmental factors and endogenous factors. Environmental factors include climate and soil nutrient availability (Cortez et al., 1996), endogenous factors are leaf litter nutrient content (e.g. C, N, P) and secondary metabolites (e.g. phenolic compounds) content (Melin, 1930; Koukoura et al., 2003).

With regard to environmental factors, Mediterranean shrublands are characterized by low water availability and low soil organic matter content, the latter parameter becoming worse with recurrent fires (Borghetti et al., 2004). One of the methods employed for improving nutrient budgets in these low productive ecosystems is to spread composted sewage sludges as organic amendments. Compost may effectively reactivate the biogeochemical cycles since it brings nutrients to microorganisms, and enhances the water retention into the soil (Borken et al., 2002).

With regard to endogenous factors, the vegetation of these shrublands is dominated by evergreen sclerophyllous species which produce high amounts of leaf secondary metabolites, including phenolic compounds (Gershenzon, 1984). Plants produce these compounds in response to different stress factors, such as interspecific competition (Ferrat et al., 2001), animal overconsumption of leaves (Van Hoven, 1984) and atmospheric pollution (Pasqualini et al., 2003). Plant secondary metabolite content has been suggested to be a major inhibiting factor of the activity of microorganisms (Anderson, 1973; Hättenschwiler and Vitousek, 2000; Isidorov and Jdanova, 2002), particularly in nutrient-poor soils (Northup et al., 1998). Consequently, soil organic matter is easily humified instead of mineralized (Shindo and Kuwastsuka, 1976).

In view of the characteristic features of Mediterranean ecosystems, we assumed that the influence of factors controlling leaf litter colonization by fungi and then decomposition, could be of major importance in the matter cycle in these ecosystems.

The aim of this study is to provide comprehensive data on leaf litter colonization by fungi in a Mediterranean shrubland by determining the effects of (i) an organic amendment by biosolids and (ii) phenolic content on litter colonization by fungi. As fungi associated with decomposed leaves are the main actors of leaf litter breakdown (Toutain, 1981), these eukaryotes offer a reliable way to describe the process (Baldy et al., 1995; Gessner et al., 1999). Consequently, the impact of factors controlling litter breakdown could be studied by monitoring changes in fungal biomass dynamics (Gessner and Chauvet, 1994; Isidorov and Jdanova, 2002) and relating them to factors controlling the process.

Section snippets

Study site and experimental design

The experiment was carried out on 6000 m2 in the plateau of Arbois (Southern Province, France; 5°18′6′′E–43°29′10′′N in WSG-84 Geodetic system), at an altitude of 240 m above sea level under Mediterranean climatic conditions (Fig. 1). The soil was a silty-clayey chalky rendzina, with a high percentage of stones (77%) and low average depth (24 cm). The last fire occurred in June 1995 and the site was colonized by a Mediterranean sclerophyllous vegetation, with a 70% total cover, Quercus coccifera

Effects of compost amendment and season on the fine soil organic fraction

Temperature and rainfall, between January 2002 and April 2003, showed marked seasonal changes (Fig. 1), with maximum rainfall in May, September, November 2002 and in January and April 2003. Maximum temperature occurred in June, July and August 2002.

Soil cationic exchange capacity (Fig. 2D) and moisture content (Fig. 2G) varied according to the season (Table 2), while organic matter (Fig. 2A), total nitrogen (Fig. 2B), C/N ratio (Fig. 2C) and total phosphorus (Fig. 2E) varied significantly with

Discussion

Improving knowledge on litter degradation under Mediterranean climate is necessary for understanding the functioning of Mediterranean ecosystems. Litter constitutes an important source of carbon and energy supply for microbial communities (Pascual et al., 2000). In extensive areas of the Mediterranean regions, the natural vegetation is exposed to the harsh climatic conditions (Pascual et al., 2000). Therefore, humidity and soil nutrients are limiting factors in these ecosystems (Rapp et al.,

Conclusion

In conclusion, these data on ergosterol dynamics associated with decomposed Quercus coccifera leaves in a Mediterranean shrub ecosystem show that leaf litter colonization by fungi is not affected by compost amendment but is closely linked to soil humidity and total phenolic concentrations of leaf litter. These findings suggest that nutrient release from decomposing litter should vary according to climatic conditions and plant species. Therefore, it would be of great interest to study leaf

Acknowledgements

This research was supported by the Conseil Général des Bouches-du-Rhône (France), the ADEME (Agence De l’Environnement et de la Maîtrise de l’Energie), the Conseil Régional Provence-Alpes-Côte-d’Azur and the Rhône-Méditerranée-Corse French Water Agency. We also thank Mr. Michael Paul for revision of English.

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