Organic matter cycling in grassland soils of the Swiss Jura mountains: biodiversity and strategies of the living communities

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Abstract

Several characteristics of the soils of three nutrient-poor grasslands, pertaining to their biological activity, have been measured three times during the vegetation cycle of 1994 in the Swiss Jura. Properties measured were: abundances of the faunal groups Collembola (Insecta), Oribatida, Gamasida and Actinedida (Acarina); PLFA richness, abundance and diversity (indicating microbial diversity); ATP content (an index of soil biomass), soil respiration (CO2 production); and alkaline phosphatase, urease, chitinase, xylanase and laminarinase activities, as biochemical tracers of biotic activity. Most of these variables showed significant differences from site to site. In particular, the biochemical descriptors ATP content and phosphatase and urease activities varied in accordance with the amount of the clay-humic complex. The soils showed two strategies of litter degradation: (a) an enzymatic strategy, prevailing at the end of winter, when the edaphic climate is unfavourable; (b) a biotic strategy, based upon the work of the whole living community (fauna and microflora), at its maximum at the end of summer. Diversity of the microbial communities (as estimated by the PLFA richness) was highest in spring, when the carbon sources are complex (young litter), and subsequently decreased in inverse relationship with the specific activity of the biomass (viz. CO2 production-to-ATP ratio). This behaviour is attributed to an increase of the amount of simple organic compounds resulting from enzymatic activity and also to the activity of microphytophagous microarthropods.

Introduction

A meadow can be viewed as an ecosystem made of two compartments: epigeal (stems and leaves as primary producers, and a consumer food web based upon them), and edaphic, devoted to the cycling of organic matter (mineralization, humification). The edaphic compartment harbours a complex and diversified living community (fauna and microflora), the functions and interactions of which play key roles in numerous processes, ranging from organic matter decomposition to rhizosphere functioning (Lussenhop, 1992).

The study of the soil living community can be structured around the notion of the foodweb. This approach bears the advantage of organising functions and interactions in a logical manner within the saprophytic foodweb, yielding modelled and quantified representations of energy and element fluxes in the edaphic compartment (Hunt et al., 1987, Moore et al., 1988)

Such studies are based either upon systems reconstituted in the laboratory (Teuben and Roelofsma, 1990, Siepel and Maaskamp, 1994), or in situ (Teuben, 1991), or, less frequently, upon naturally occurring systems (Hassink et al., 1994). These three approaches are complementary, the first allowing the assessment of the parameters needed by the second, and the third confronting the established models with the real world.

Other works address the question of the living community of the soil in terms of microbial biodiversity, by studying either functional diversity or microbial community structure. The former approach involves an array of rather sophisticated physiological tests (Zak et al., 1994). The latter generally relies upon identification of specific components of the cell membrane (phospholipid fatty acids, PLFA) as signatures of the microbial spectrum (Baath et al., 1992, White, 1995, Zelles et al., 1995, Frostegard et al., 1996, Laczkó et al., 1997).

Faunistic diversity assessment relies upon the classic techniques of counts and identifications, widely used in community ecology (Matthey et al., 1984, Dunger and Fiedler, 1989).

Because of the range of the scales encompassed (from the μm for the bacteria to the cm for the macroarthropods), and the diversity of the means of investigation involved, truly integrated techniques, allowing simultaneous evaluation of biodiversity and functions of microflora and fauna, are currently lacking. Yet, operational methods, capable of evaluating effects of human activities on the soil in an ecologically sound manner, are more and more necessary.

Such an endeavour necessarily implies a simplification of the involved experimental approaches, lest the complexity of the techniques becomes the limiting factor in their practical application. In this perspective, the use of ‘biochemical tracers’ seems to be a promising way. Sinsabaugh and Moorhead (1994), for instance, used several enzyme activities to devise a model of litter degradation. They proposed it as an economical alternative in the study of the cycles of biogenic elements of soils (mainly N and P).

The present study addresses this problem. We start from the idea that the biodiversity of the living community of a soil is strongly related to the multiplicity of the functions necessary to its proper working (Odum, 1969). We seek correspondences between abundances or diversities of selected groups of soil organisms ranging from mesofauna to microflora and an array of extracellular enzyme activities located at the interfaces ‘organic substrate–fauna–microflora’ of the saprophytic foodweb.

The research has been conducted at three sites with samplings at three seasons. The underlying hypotheses are formulated as follows:

  • 1.

    Despite our desire for homogeneity, the mean values of the variables studied are expected to differ among sites, due to varying local conditions and history. If existing, these site effects will have to be taken into account in further analyses.

  • 2.

    The variables studied are expected to vary significantly among seasons within the sites. These variations will be interpreted in terms of seasonal variations of biological processes in the soil.

  • 3.

    Correlations are expected between microbial diversity, abundance and activity, arthropod abundances and diversities, and the activities of the enzymes produced by these organisms. These relationships are expected to vary with the season, thereby reflecting the processes occurring at various times of the year.

This type of integrated approach being quite new, we also expect our data to help us generate new hypotheses regarding the relationships between diversity and functions at different scales.

Section snippets

Study sites

The study was conducted at three sites, located in the Swiss Jura mountains within 20 km of each other. These sites are described in detail by Baur et al. (1996). To summarise, they are nutrient-poor grasslands of the Teucrio-Mesobrometum type, on calcareous substrate, situated near Nenzlingen (10 km S of Basel, altitude 500 m a.s.l., SW-facing slope), Movelier (5 km N of Delémont, 780 m, SSE-facing slope) and Vicques (5 km E of Delémont, 570 m, SE-facing slope). The sites were selected on the

Results

Fig. 1 displays the means and standard deviations of the biochemical variables and PLFA parameters, per site and per season.

Discussion

ATP content, an indicator of soil microbial biomass (Oades and Jenkinson, 1979, Maire, 1983), and SOMM, an indicator of soil global catabolic activity, are biochemical quantities that bear a direct link with the living soil microorganisms. The standardised system adopted for measurements of ATP and SOMM in this study, based on air-dried soil samples, does not allow evaluation of their field values, but rather tests the capacity of the microbial populations to recolonise the habitat and reach a

Conclusions

Largely admitted ecological theory advocates three main biological principles acting on the transformation of the organic matter in the soils:

  • 1.

    In a stable ecosystem (climax), the mean primary production rate (P) is globally equal to the rate of mineralization (R). With Odum (1969) we can admit a mean annual P-to-R quotient equal to 1 in our three sites.

  • 2.

    In the conditions defined in (a), the C-to-N ratio of the microbial communities (M) is at equilibrium with the C-to-N ratio of the soil organic

Acknowledgements

This research is supported by a grant from the Swiss National Science Foundation to NM. We are also indebted to Dr Josef Starý, of the Czech Academy of Sciences in Ceské Budějovice, for counting and identifying the mesofauna. Many thanks also to two anonymous reviewers for their helpful comments and great improvements of the English text.

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