Turnover of organic matter in differently textured soils: I. Physical characteristics of structurally disturbed and intact soils
Introduction
Models simulating organic matter turnover in soils usually incorporate functions representing the influence of abiotic factors (e.g., temperature, moisture and soil clay content) on the rate constants ascribed to the various pools of organic matter. The moisture function represents not only the effect of soil water availability, but also the aeration status, both parameters being of vital importance in regulating the activity of decomposer microorganisms. Functions based on soil clay content are adopted as an approximation for the overall effect of soil structure on biological activity and on the stabilization of decomposition products.
Abiotic functions in models are typically based on experimental evidence from laboratory incubations of different soil types (van Veen et al., 1984; Clay et al., 1985; Hansen et al., 1991; Grant and Rochette, 1994), most incubation studies employing homogenized and sieved (often finely) soil samples in which aggregates are broken down before incubation (Cassman and Munns, 1980; Myers et al., 1982; Stott et al., 1986; Gregorich et al., 1991; Amato and Ladd, 1992; Hassink et al., 1993; Killham et al., 1993). This may limit extrapolation of results to field conditions because soil structure and organic matter turnover interacts (Elliott, 1986; Cabrera and Kissel, 1988; Gregorich et al., 1989; van Veen and Kuikman, 1990; Christensen and Christensen, 1991).
Many reports have highlighted the influence of clay on organic matter turnover in soil. Studies based on different soil types and on textural gradients established by the addition of pure minerals (e.g., Sørensen, 1972) may be biased by differences in soil mineralogy, cropping history and other factors related to soil origin. Thus, effects which are ascribed directly to clay may well be due to effects of other soil parameters. The effects of texture on soil water-holding capacity and soil structure should be considered when interpreting results from incubations of differently textured soils.
Greaves and Carter (1920)documented a consistent relationship between soil water content and microbial activity. For 22 differently textured soils, biological activity peaked at approximately 60% of the water holding capacity. More recent research (Linn and Doran, 1984; Doran et al., 1990; Scott et al., 1996), reporting the water content on a volumetric basis, confirmed a strong correlation between soil respiration and the relative volumetric water content. The soil water potential is considered a decisive factor in organic matter turnover (Stanford and Epstein, 1974; Bridge and Rixon, 1976; Orchard and Cook, 1983). Direct effects of water potential upon soil microbial activity may, however, be important only at rather low potentials (Griffin, 1981), and indirect effects (e.g., on soil aeration) derived from differences in pore system of soils may be equally important.
This study examines physical characteristics of undisturbed soils from a textural gradient, of structurally disturbed samples from the same locations, and of texturally corresponding samples established by additions of clay and silt separates isolated from the same soil. Disturbed samples were exposed to a 17-month period with freeze/thaw and wet/dry cycles and tillage operations in order to allow regeneration of soil structure. This report elucidates soil structural conditions of importance to the function of soil microbial decomposer organisms. A subsequent report deals with the turnover of added to these soils in plant residues (Thomsen et al., 1999).
Section snippets
Soil sampling
Bulk samples of soil were retrieved from the 0–20 cm depth at six locations along a naturally occurring texture gradient from an arable field near Lerbjerg, Denmark (56°22'N, 9°59'E). At each location, soil was collected from an area of 1 m2. The gradient is on Weichselian morainic deposits and for several years, the field has been in winter wheat dressed with mineral fertilizers and animal manure. These soil samples (termed NA1 to NA6) ranged in clay content from 11 to 45% and in silt from 7
Results and discussion
Using soils with similar mineralogy and land use history and adjusted to well-defined soil water matric potentials, this study examined the effects of soil texture and previous structural disturbance on a number of physical soil properties. Three sets of samples representing different levels of structural disturbance were employed. One set of samples (RE) taken as undisturbed cores along a textural gradient across the field site represented structurally intact soil. A second set (NA) was from
Conclusions
Previously disturbed soils regained some of their secondary structure during a 17-month period of storage under field-like conditions, but they were still structurally different from intact soils of similar texture. Although partly aggregated, disturbed clayey soils showed less continuous/more tortuous pore systems than the texturally corresponding and structurally intact soils. Generally, the disturbed soil had a pore system with relatively small pores `enmeshed' in the soil matrix, while the
Acknowledgements
The technical assistance of Karin Dyrberg and Michael Koppelgaard is gratefully acknowledged. We also thank Dr. Bendt Jensen for valuable contributions to the manuscript. This work was financially supported by the Danish Environmental Research Programme and by the Ministry of Food, Agriculture and Fisheries.
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