Aggregate stability and carbon characteristics of particle-size fractions in cultivated and forested soils of semiarid Spain

Organo-mineral particle-size fractions play a major role in soil structure. However, our knowledge of these size fractions must increase if we are to optimise the physical regeneration of degraded soils in semiarid areas. Twenty soils (sieved <2 mm) used for two contrasting purposes (agriculture and forestry) in southeastern Spain were fractionated into <2 and 2–20 μm particle-size fractions by ultrasonic dispersion in water followed by sedimentation. The organic matter from these size fractions was separated by extraction with 0.1 M Na4P2O7 (pH 9.8) and by oxidation with H2O2. The different size fractions were analysed for C concentration before and after treatment with pyrophosphate and peroxide. For both cultivated and forested soils, the highest proportion of organic C was associated with 2–20 μm particle-size fraction. The <2 and 2–20 μm particle-size fractions were characterised by a high proportion of organic matter non-extractable with sodium pyrophosphate, which was partly resistant to peroxidation. The aggregate stability of cultivated soils was significantly lower (mean 40%) than that of forested soils (mean 82%). Soil aggregate stability was not correlated with the organic constituents associated with the <2 and 2–20 μm particle-size fractions, either for cultivated or for forested soils. In conclusion, the organic matter associated with the <20 μm particle-size fractions does not appear to contribute greatly to the stability of macroaggregates in semiarid soils.