Carbohydrates in hot water extracts of soil aggregates as influenced by long-term management

Microbial carbohydrates are immediate by-products of microbial metabolism and play an important role in the formation and stabilization of soil structure. The effect of long-term management on soil carbohydrate content and monosaccharide composition was investigated in five Danish sandy loams under organic and conventional management with animal manure and mineral fertilizers. Hot-water (80 degrees C) extraction was used to measure the distribution and composition of carbohydrates in aggregate size. Carbohydrates released to hot water were determined after hydrolysis as reducing sugars equivalent to glucose. The monosaccharide composition in hot-water extracts was analyzed as the corresponding alditol acetates. Sites with a history of long-term continous management practices were used. Three treatments from the >100 year Askov long-term field experiment were used to show results of contrasting fertilization on soil carbohydrate content. These were all grown to a four-course crop rotation. Total carbohydrate content was significantly influenced by long-term management practices, with a significantly higher carbohydrate content in soils fertilized with either mineral fertilizers or animal manure (1200 to 800 mg C kg(-1) DM aggregate) than in an unfertilized soil (600 to 500 mg C kg(-1) DM aggregate). These results were as true for micro-aggregates (<0.25 mm) as for the 0.5-1.0 mm and 4.0-8.0 mm fractions. The organically managed soil (>40 years) was sited at a commercial farm with forage crop rotations, organic manure and no use of crop protection chemicals. These results showed significantly higher levels of carbohydrate both in micro-aggregate and macro-aggregates (1200 to 900 mg C kg(-1) DM aggregate) than an adjacent conventionally managed soil with annual cash crop, mineral fertilizers and use of crop protection chemicals (960 to 760 mg C kg(-1) DM aggregate). Carbohydrate C content generally increased as aggregate size decreased in both soils. Monosaccharide distribution was generally similar among three aggregate size classes studied. In all soils the content of monosaccharide was highest in microaggregates and lowest in macro-aggregates. Mannose and galactose were normally the most common monosaccharides in the hot-water extracts of aggregate fractions, indicating a predominantly microbial origin.