NUTRITIONAL MANAGEMENT OF MICROBIAL POLYSACCHARIDE PRODUCTION AND AGGREGATION IN AN AGRICULTURAL SOIL

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 N-2 fixer, and oat (Arena sativa L.), and three N fertilizer regimes [0, 168, or 280 kg ha(-1) (NH4)(2)SO4-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.