Effect of carbonates on the hierarchical model of aggregation in calcareous semi-arid Mediterranean soils

Abstract

Lithogenic and/or secondary carbonates are present in many semi-arid Mediterranean soils. Their role in aggregation dynamics is not completely known, and can determine the sensitivity of these soils to land-use changes and soil management. The objective of this work was to test the hierarchical model of aggregation, and to determine the relationship between aggregates formation and organic matter decomposition in calcareous soils of semi-arid Mediterranean regions, by comparing macroaggregation in the upper horizon of a calcareous soil ( Typic Calcixerept) with a non-calcareous soil ( Typic Haploxerept) and a decarbonated terra rosa ( Calcic Haploxerept) following fresh straw addition under controlled laboratory conditions during 105 days. We hypothesized that aggregation would work according to the hierarchical model in the non-calcareous soil (NONCALC) and in the decarbonated terra rosa (DECALC), but not in the calcareous soil (CALC). The three soils had similar organic C concentrations and C/N ratio, but differed in their carbonates and clay minerals content: NONCALC had a similar proportion of clay-size silicates than CALC (~ 10%), and the DECALC had a similar proportion of total clay-sized particles than CALC (including carbonates, ~ 20%). An increase in the amount of macroaggregates (> 250 μm) and microaggregates (50–250 μm) held within macroaggregates associated to an increase in the fungal activity when maize straw was added, a decline of macroaggregates with time associated to organic matter decomposition, a higher concentration of organic C and maize-derived C in microaggregates within macroaggregates than in free microaggregates, and a transfer of maize-derived C from macroaggregates to free microaggregates with time confirmed that aggregation worked according to the hierarchical model in NONCALC and DECALC. In CALC, however, the hierarchical model worked only partially. Macroaggregates formation was also stimulated by organic residues addition according to the model, but stable macroaggregates showed longer turnover rates and no relationship between straw decomposition and their decline was observed. From this, we postulate that carbonates can help stabilizing macroaggregates formed from fresh organic matter decomposition through abiotic processes, such as dissolution and re-precipitation. These results help to understand field data obtained in semi-arid calcareous agricultural soils, where it has been observed that aggregation is not always directly correlated to organic matter concentration, but that increases in soil organic matter can promote aggregation. They also indicate the need for considering the composition of the mineral fraction when modelling soil structure in semi-arid Mediterranean soils.