The relationship between desiccation and the production of extracellular polysaccharides (EPS) by soil bacteria was investigated by using a Pseudomonas species isolated from soil. Cultures subjected to desiccation while growing in a sand matrix contained more EPS and less protein than those growing at high water potential, suggesting that resources were allocated to EPS production in response to desiccation. Desiccation did not have a significant effect on activity as measured by reduction of iodonitrotetrazolium. Purified EPS produced by the Pseudomonas culture contained several times its weight in water at low water potential. Sand amended with EPS held significantly more water and dried significantly more slowly than unamended sand, implying that an EPS matrix may buffer bacterial colonies from some effects of desiccation. We conclude that bacteria may use EPS production to alter their microenvironment to enhance survival of desiccation.

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Soil carbohydrates, including microbial extracellular polysacchrides, stabilize soil aggregates and improve soil structure. This study examined whether short‐term managment of C inputs by cover crops and tillage affected polysaccharide‐mediated macroaggregation. Soil was sampled from a Californa prune (Prunus domestica L.) orchard where an experiment comparing four management techniques, permanent grass cover crop, mowed cover crop, no‐till herbicide, and conventional tillage, had been in place for two seasons. Cover crops significantly increased saturated hydraulic conductivity, acid‐ex‐tractable heavy‐ fraction carbohydrates (those in soil denser than 1.7 g/mL), and macroaggregate slaking resistance over clean‐cultivated or herbicide treatments. Heavy‐fraction carbohydrates are probably mainly composed of microbial extracellualr polysaccharides produced in response to cover‐crop C inputs. Heavy‐fraction carbohydrates were significantly correlated with aggregate stability and satureated hydraulic conductivity, while total organic C and lightfraction carbohydrates were not. There were no differences between soil under herbicide and clean‐cultivation treatments, showing that tillage alone did not measurably affect carbohydrate or soil structure. Heavy‐fraction carbohydrates were shown to be important in the initial improvement of soil structure by cover crops.

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 (Ksat) 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 Ksat 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 Ksat. 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.

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