Correlation of physicochemical properties and sub-erosional landforms withaggregate stability variations in a tropical Ultisol disturbed by forestryoperations

The stability of soil aggregates against water erosion is a largely unstudi ed factor in research on the spatially-distributed impacts of tropical fore stry. Soil strata with particularly weak aggregates are likely to be the fo cus for erosional activity whether buried or exposed by either natural proc esses or forestry activities. Understanding the location and likely cause o f such intrinsic instability would, therefore, allow better spatially-distr ibuted parameterisation of erosion models. This study has aimed to identify the range in aggregate stability within a region of Bornean Ultisol distur bed by forestry operations, and to identify the bulk soil properties associ ated with such stability variations. The sites sampled were subject to a ra nge of denudational processes including piping, rilling and landslide-trigg ered erosion. Soil profiles with rates of erosion in excess of 10 mm a(-1) were shown to have less aggregate stability (as characterised by the rainfa ll simulation survival index (RSSI)) than those with no visible signs of er osion. Further, large differences in aggregate stability between soil horiz ons of the same profile were observed and seen to be statistically correlat ed with the bulk soil properties of organic carbon and clay content (i.e. s tabilising agents) and the dispersing agent of exchangeable sodium percenta ge (ESP) at sites undergoing erosion. Organic carbon appeared to be the mos t important governing factor, accounting for 56% of the variance in the agg regate stability. The presence of strongly and weakly expanding 2 : 1 clays within the soils may have been an additional de-stabilising factor. Reduce d porosity was also observed at eroding sites with weak soil aggregates. Th e properties of EC25 and sodium adsorption ratio (SAR) were not correlated with the RSSI or the presence of soil erosion, Identification of the role o f organic carbon, clay and ESP in the stability of these tropical forest ag gregates is important in focusing future, more intensive research on the sp atial parameterisation of models to simulate forestry impacts on erosion. ( C) 1999 Elsevier Science B.V. All rights reserved.