THE CONTRIBUTION OF CARBOHYDRATE-C AND EARTHWORM ACTIVITY TO THE WATER-STABLE AGGREGATION OF A SANDY SOIL

An incubation study was conducted to test the effects of decomposing c lover tops (added at 0, 6.2 or 12.5 mg organic matter/g soil) and eart hworm activity on the contribution of carbohydrate C to the stability of aggregates in a sandy soil. Soils incubated with and without earthw orms were separated into surface-casts and bulk soil, and the amounts of water-stable aggregates >1mm surviving slow and rapid rewetting (wh en air-dry) in these soil separates were determined. Organic C and aci d- and water-extractable carbohydrate C concentrations were determined in the aggregates and bulk soil. The treatments of 6.2 and 12.5 mg or ganic matter/g soil increased the >1 mm aggregation of the bulk soil b y more than 2.2- and 2.8-fold, respectively, compared with the non-ame nded soils. With the addition of earthworms, there were increases from 1.7- to 1.8-fold only in aggregates surviving slow rewetting. The aci d- and water-extractable carbohydrate C contents of aggregates >1mm in the bulk and surface-cast soils were generally not greater than the c arbohydrate C in the bulk soil. Generally, the carbohydrate C fraction s were also not increased in the more stable aggregates (rapidly rewet ) compared with the weaker aggregates (slowly rewet). Carbohydrate C i n bulk soil was generally (P < 0.05) correlated with the amounts of ag gregates surviving each rewetting treatment (r > 0.71, P < 0.01). In c ontrast, greater amounts of carbohydrate in aggregates surviving slow rewetting were not correlated (r < -0.45, P > 0.05), with a greater pr oportion of these aggregates resisting disruption when the soils were rapidly rewet (except for acid-extractable carbohydrate C; r = -0.84, P < 0.05). These results cast doubt on the usefulness of correlations in assessing the contribution of carbohydrate C to aggregation. The am ounts of carbohydrate materials in the soil appeared to have little in fluence on aggregation, probably because the location of bonding compo unds in the soil pore matrix is more critical.