COVER CROP MANAGEMENT OF POLYSACCHARIDE-MEDIATED AGGREGATION IN AN ORCHARD SOIL

Soil carbohydrates, including microbial extracellular polysaccharides, stabilize soil aggregates and improve soil structure. This study examined whether short-term management of C inputs by cover crops and tillage affected polysaccharide-mediated macroaggregation. Soil was sampled from a California 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-extractable 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 extracellular polysaccharides produced in response to cover-crop C inputs. Heavy-fraction carbohydrates were significantly correlated with aggregate stability and saturated hydraulic conductivity, while total organic C and light-fraction 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.