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Nutritional Management of Microbial Polysaccharide Production and Aggregation in an Agricultural Soil

Abacus Concepts, Inc., 1918 Bonita Ave., Berkeley, CA 94704

Carol Shennan

Dep. of Vegetable Crops, Univ. of California, Davis, CA 95616

Mary K. Firestone

Dep. of Environmental Science, Policy, and Management, 108 Hilgard Hall, Univ. of California, Berkeley, CA 94720

Shlomo Sarig

Dep. of Plant Pathology and Microbiology, Faculty of Agriculture, P.O. Box 12, Rehovot 76100, Israel

*Corresponding author.

ABSTRACT

Microbial extracellular polysaccharides (EPS) contribute to the stability of soil aggregates. Nitrogen supply affects microbial growth and metabolism. The effects of fertilizer and cover crop N supply on EPS production and soil aggregation were examined in an irrigated annual cropping system. Two winter cover crops, wooly pod vetch (Vicia dasycarpa L.), a N₂ fixer, and oat (Avena sativa L.), and three N fertilizer regimes [0, 168, or 280 kg ha^–1 (NH₄)₂SO₄-N] were used to vary soil N supply in a California tomato (Lycopersicon esculentum Mill.) field. Carbohydrate in the soil heavy fraction (HF, density >1.74 g mL^–1) was found to have a monosaccharide composition consistent with microbial EPS and was used as an indicator of EPS. The HF carbohydrate content, aggregate slaking resistance, and saturated hydraulic conductivity (K_sat) were greatest in the vetch and 168 kg N ha^–1 treatments and lower in the 0 and 280 kg N ha^–1 treatments. The HF carbohydrate content was significantly correlated with K_sat and aggregate stability. Organic C and N, microbial biomass, and light-fraction carbohydrate (density <1.74 g ml^–1) were not correlated with either aggregate stability or K_sat. These results demonstrate that EPS can be important factors affecting soil structure in cultivated soils and that EPS production can be managed by N supply. The HF carbohydrate content may be a useful indication of the effects of soil nutrient and organic matter management on microbial EPS production.

This research was supported by the Kearney Foundation of Soil Science and by a Hatch project of the University of California experiment station.

Received for publication April 29, 1994.

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