Elsevier

Applied Soil Ecology

Volume 16, Issue 3, March 2001, Pages 195-208
Applied Soil Ecology

Influence of microbial populations and residue quality on aggregate stability

https://doi.org/10.1016/S0929-1393(00)00116-5Get rights and content

Abstract

Soil structure mediates many biological and physical soil processes and is therefore an important soil property. Physical soil processes, such as aggregation, can be markedly influenced by both residue quality and soil microbial community structure. Three experiments were conducted to examine (i) the temporal dynamics of aggregate formation and the water stability of the obtained aggregates, (ii) the effect of residue quality on aggregation and microbial respiration, and (iii) the effect of fungi and bacteria on aggregation.

In the first experiment, 250 μm sieved air-dried soil, mixed with wheat straw, was incubated for 14 days to allow formation of water-stable macroaggregates (>250 μm). Aggregate stability was measured by wet sieving after four different disruptive treatments: (i) soil at field capacity; (ii) soil air-dried and slowly wetted; (iii) soil air-dried and quickly wetted; (iv) 8 mm sieved soil, air-dried and immersed in water (slaking). After 14 days of incubation, maximum aggregation for soil sieved at field capacity was reached; however, these newly formed aggregates were not yet resistant to slaking.

During the second experiment, the effect of low-quality residue (C/N: 108) (with or without extra mineral nitrogen) and high-quality residue (C/N: 19.7) (without extra mineral nitrogen) on macroaggregate formation and fungal and bacterial populations was tested. After 14 days, aggregation, microbial respiration, and total microbial biomass were not significantly different between the low-quality (minus mineral nitrogen) and high-quality residue treatment. However, fungal biomass was higher for the low-quality residue treatment compared to the high-quality residue treatment. In contrast, bacterial populations were favored by the high-quality residue treatment. Addition of mineral N in the low-quality residue treatment resulted in reduced macroaggregate formation and fungal biomass, but had no effect on bacterial biomass. These observations are not conclusive for the function of fungal and/or bacterial biomass in relation to macroaggregate formation. In order to directly discern the influence of soil microflora on aggregation, a third experiment was conducted in which a fungicide (captan) or bactericide (oxytetracycline) was applied to selectively suppress fungal or bacterial populations. The direct suppression of fungal growth by addition of fungicide led to reduced macroaggregate formation. However, suppression of bacterial growth by addition of bactericide did not lead to reduced macroaggregate formation. In conclusion, macroaggregate formation was positively influenced by fungal activity but was not significantly influenced by residue quality or bacterial activity.

Introduction

Maintenance of soil structure is an important feature of sustainable agroecosystems because of its role in many biological and physical soil processes. Good soil structure promotes favorable water relations, root environment, the buildup of organic matter and reduces susceptibility to erosion. Therefore, an understanding of the processes of aggregate formation and degradation will aid in making management decisions to maintain a good soil structure.

Aggregate formation and stabilization are affected by several factors, including the type and amount of organic material, clay content, and iron- and aluminum oxides (Lynch and Bragg, 1985). The main agencies of aggregate stabilization are organic materials, including: (i) decomposition products of plant, animal, and microbial remains; (ii) the microorganisms themselves; (iii) the products of microbial synthesis (e.g. polysaccharides and gums) formed during decomposition of organic residues (Lynch and Bragg, 1985; Martens and Frankenberger, 1992; Schlecht-Pietsch et al., 1994). Residue quality and residue amount influence aggregate formation and stabilization (Lynch and Bragg, 1985). Several researchers have reported that the addition of readily available substrate causes a rapid stimulation of the soil microflora and this is accompanied by an increase in aggregate stability (Schlecht-Pietsch et al., 1994; Roldan et al., 1994). It has been postulated that fungi are involved in binding together larger soil particles (Tisdall and Oades, 1982; Oades and Waters, 1991), whereas bacteria mainly influence stabilization of clay and silt-sized particles (Lynch and Bragg, 1985; Dorioz et al., 1993; Tisdall, 1994). Fungi initiate aggregate formation by enmeshing fine particles into macroaggregates (Molope et al., 1987; Oades, 1993; Tisdall et al., 1997) and producing organic substrates that bind soil particles together into micro- and macroaggregates (Tisdall and Oades, 1982; Chenu, 1989; Beare et al., 1997). Fungi are possibly effective stabilizers because the spread of hyphae between aggregates and into pores distributes fungal binding agents through the soil (Lynch and Bragg, 1985; Oades and Waters, 1991).

Stability of soil structure is usually related to two sources of stress: disruption through the action of water or through the action of external mechanical stresses (Dexter, 1988). Water is considered to be the main agent for aggregate breakdown (Lynch and Bragg, 1985); therefore, in discussing stable soil aggregation, most workers refer to water-stable aggregation. The application of stress to a clod of soil will result in fracture along failure zones. With increasing stress, macroaggregates (>250 μm) can be broken down into microaggregates (53–250 μm) and ultimately into individual soil particles (Kay, 1990). The rate of breakdown of a soil into finer fractions is greater with rapid wetting (Panabokke and Quirk, 1957; Utomo and Dexter, 1982). When soil is wetted quickly, the buildup of air pressure as water rapidly enters soil pores causes a disruption of the soil structure (slaking). Therefore, the initial moisture content of the soil has a great influence on the water stability of aggregates since soil water content determines the amount of air-filled pores (Kemper and Rosenau, 1986).

Our objectives were to study: (i) aggregate formation over time and water stability of newly formed aggregates; (ii) the effect of residue quality of incorporated plant material on aggregation and microbial activity; (iii) the effect of fungi and bacteria on aggregation.

Section snippets

Soil description and preparation

Surface soil (0–20 cm) was collected with a shovel in July 1998 from an experimental site 6.4 km east of Akron, CO. This site is located at the Central Great Plains Research Station (40°9′N, 103°9′W). The soil is a Weld silt loam (Aridic Paleustoll) with a texture of 41% sand, 36% silt and 23% clay. The field has been cultivated (conventional tillage) for at least 90 years (Halvorson et al., 1997). After collection, the soil was air-dried (moisture content after air-drying was 1–2%) and pushed

Aggregation over time

We hypothesized that maximum aggregation would occur some time later than the maximum in respiration, because microbial synthesis products that serve as binding agents for aggregate formation would only be produced in sufficient quantity after the peak in respiration. Maximum respiration rate occurred after 2 days (Fig. 1), being around 9.5 μg CO2–C g−1 soil h−1. However, maximum aggregation for the large macroaggregate fraction (>2000 μm) occurred after 12 days (Fig. 2), confirming our hypothesis.

Conclusion

Several conclusions can be made regarding the influence of residue quality and soil microflora on aggregate stability. During a 14-day incubation, we observed an earlier maximum in respiration rate than in aggregation and the formation of large macroaggregates out of microaggregates. When maximum aggregation was reached (after 14 days), the formed macroaggregates were not yet slaking resistant and fell apart in microaggregates. The microaggregates, however, were stable enough to resist the

Acknowledgements

Thanks to Irene Hesse, Ted Elliott and Katrien Kimpe for earlier methods development. Thanks to Holly Stein for help during the laboratory work. We also gratefully appreciate all the advice from Dan Reuss during the laboratory work. Image analysis was conducted at the Imaging Center in the Department of Anatomy and Neurobiology at Colorado State University. This research was supported by grants from the National Science Foundation (DEB-9419854) and the United States Department of Agriculture

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    Present address: School of Natural Resources, Ohio State University, 2021 Coffey Road, Columbus, OH 43210, USA.

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