Wet aggregate stability of some Botswana soil profiles

During intense and short rainfall events, important losses of fertility are expected as a result of erosion in drylands because the nutrient pools are concentrated it? the topsoil. Therefore, we evaluated the kinetics of aggregate stability in some Haplargids and Torripsamments in Botswana as it represents a resistance factor against losses of materials, measuring also the release of organic matter and phosphorus during wet sieving. We found very low contents of 1-2 mm aggregates in both soil types-although the Haplargids had higher levels of this size aggregate (19%) compared with the Torripsamments (10%)-and no differences between the topsoil and the deeper horizons. The total losses of aggregates were similar in topsoils and deep soil horizons and independent from soil order, but the Entisols had a greater proportion of aggregate breakdown, 38% oil the average, when the sample was water-saturated. Only clay and silt content, and cation exchange capacity showed good correlations with aggregate stability. Organic matter or iron oxides had no effect, suggesting that aggregation is the result of cation bridging between mineral particles. Up to 97% of the organic matter released into water was lost at the beginning of the wet sieving, but the amounts were not related to aggregate breakdown, indicating that organic matter, besides not influencing the stability of macroaggregates, had little effect oil the stabilization of microaggregates. Phosphorus was also released during the wet sieving, and was related to soil available P (r = 0.713 after 5 min). With no effect of organic matter oil aggregate stability, and nutrients concentrated in the topsoil, the vulnerability of these soils to fertility losses is therefore extremely high.