SOIL MICROSTRUCTURE AND SOIL ERODIBILITY AT IBSRAM SLOPING LANDS MANAGEMENT SITES IN SOUTH-EAST ASIA (THAILAND, LAOS, VIETNAM, PHILIPPINES, MALAYSIA)

Associations among soil microstructural type and behaviour, soil erodibility, and soil erosion were investigated at seven research sites of the International Board for Soil Research and Management (IBSRAM) Asialand Sloping Lands Network in Southeast Asia (Chiang Mai and Chiang Rai, Thailand; Vientiane, Laos; Hoa Son, Vietnam; Los Banos and Mabini, Philippines; and Kuala Dal, Malaysia). Soil microstructure was investigated on thin sections prepared from undisturbed samples and microstructural behaviour and other soil properties were determined on bulk samples taken from the surface immediately adjacent to undisturbed samples. The sites were grouped into three clusters according to their microstructural behaviours: stable microaggregates composed of stable domains; unstable microaggregates composed of stable domains; and unstable microaggregates composed of unstable domains. Erosion and rainfall data indicate that structural stability affects the susceptibility to detachment by raindrop impact and that microaggregate or domain size influence the susceptibility to transport by runoff. The nature of the influence of microstructure on the severity of soil loss was not always the same at the various sites. The Chiang Mai and Vietnam sites have unstable microaggregates which disperse into stable domains with low transportability and experience the least soil loss per unit rainfall. At the Laos site a stable soil structure with low detachability and transportability experienced moderate soil loss per unit rainfall under high erosivity conditions. The unstable soil structures at the Los Banos and Mabini sites are easily dispersed and transported causing moderate soil loss per unit rainfall. At the Chiang Rai site stable microaggregates, which are easily detached and transported, experienced very high soil loss per unit rainfall.