Chapter 9 - Role of Carbonates in the Physical Stabilization of Soil Organic Matter in Agricultural Mediterranean Soils

Abstract

Calcareous soils where lithogenic and secondary carbonates are important constituents of the soil mineral matrix abound in many arid and semiarid areas. Soil carbonates have been described as an organic matter stabilization agent, mainly due to chemical stabilization mechanisms. In this chapter, we focus on the role of carbonates as indirect agents of the physical stabilization of organic C through their impact on soil structure. We describe a series of studies conducted in northern Spain to examine the particularities of carbonate-rich soil horizons in terms of structure development and stabilization. The first studies determined the role of carbonates as macroaggregate stabilization agents in several agricultural soils in the Ebro Basin that differ mostly in their carbonate content. They also showed that carbonates can act as rate-modifiers in soil organic C cycling. The mechanisms responsible for this behavior were subsequently studied by means of image analyses and physical tests examining the architecture of aggregates formed during an incubation experiment using contrasting soils. Carbonate-rich soils developed a less porous structure than those formed from carbonate-depleted soil horizons. The possible influence of aggregates on structure stabilization through favored accumulation of reactive clays was also studied, but this hypothesis was not confirmed. Through the synthesis of these studies, we propose a series of hypotheses that can explain the relationship between soil mineralogy, soil architecture, and organic matter protection in carbonate-rich soils, as well as discuss the interactions of the soil mineral matrix and organic C cycling in these soils.