Poorly-crystalline components in aggregates from soils under different land use and parent material

• Poorly-crystalline Fe and Al were studied in aggregates from 28 soils on granite or amphibolite, under two land uses.
• Aggregates from the amphibolite soil were richer in poorly-crystalline components than granite soils.
• Agricultural soils were impoverished in organic-complexed Al.
• Aggregate stability decreased in agricultural soils with respect to shrubland soils.

Fe and Al compounds of low degree of crystallinity are important components for the development and conservation of soil structure in many soils. In this work, the influence of parent material and land use on the abundance of poorly-crystalline forms of Fe and Al was studied in six macroaggregate fractions from 28 soils. The soils selected are developed on two geological materials with very different composition: granites or amphibolites, and under two land uses: shrubland or agricultural. Poorly crystalline Fe and Al were extracted with oxalic acid–ammonium oxalate (inorganic and organic forms), and with pyrophosphate (organic forms). Soils developed on amphibolite were richer in organic matter and poorly-crystalline Fe and Al compounds than soils developed on granite. Agricultural soils were impoverished in organic carbon and organic-complexed Al with respect to shrubland soils, and enriched in inorganic poorly crystalline forms of Fe. The differences related to land use were more evident in the amphibolite soils. Aggregate stability, assessed by water-dispersible clay quantification, decreased in agricultural soils with respect to shrubland soils, in particular for those developed on amphibolite. This reduction of aggregate stability in agricultural soils is likely due to a combination of several processes: loss of organic matter, higher pH, and an overall loss of poorly-crystalline Fe and Al compounds, in particular Al-organic matter complexes.