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

Microbial extracellular polysaccharides (EPS) contribute to the stabil ity of soil aggregates. Nitrogen supply affects microbial growth and m etabolism. The effects of fertilizer and cover crop N supply on EPS pr oduction and soil aggregation were examined in an irrigated annual cro pping system. Two winter cover crops, wooly pod vetch (Vicia dasycarpa L.), a N-2 fixer, and oat (Arena sativa L.), and three N fertilizer r egimes [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 microb ial EPS and was used as an indicator of EPS. The HF carbohydrate conte nt, 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 co ntent 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 stab ility or K-sat. These results demonstrate that EPS can be important fa ctors affecting soil structure in cultivated soils and that EPS produc tion can be managed by N supply. The HF carbohydrate content may be a useful indication of the effects of soil nutrient and organic matter m anagement on microbial EPS production.