Mechanisms of macroaggregate stabilisation by carbonates: implications for organic matter protection in semi-arid calcareous soils Oihane Fernández-Ugalde A,B , Iñigo Virto A,D , Pierre Barré B , Marcos Apesteguía C , Alberto Enrique A , María J. Imaz A , and Paloma Bescansa A A Departamento Ciencias del Medio Natural, ETSIA, Universidad Pública de Navarra, 31006 Pamplona, Spain. B Laboratoire de Géologie, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France. C Instituto Navarro de Tecnologías e Infraestructuras Agroalimentarias, SA, 31610 Villava, Spain. D Corresponding author. Email: inigo.virto@unavarra.es Abstract. Carbonates interfere with soil aggregation in semi-arid calcareous soils, promoting the stability of macroaggregates and decelerating the decomposition of the organic matter within them. Our aim was to determine the process through which carbonates participate in aggregation. We hypothesised (i) a tendency to accumulate reactive clay minerals via Ca 2+ bridging, and (ii) a precipitation of carbonates within aggregates due to dissolution/re-precipitation phenomena. The <250-mm fractions of a Typic Calcixerept (CALC) and a decarbonated Calcic Haploxerept (DECALC) were incubated from the same agricultural field in semi-arid Spain with added maize straw during 28 days. A size-based fractionation was used to separate different aggregates in incubated and field-moist samples, and aggregates were analysed for organic C and maize-derived C, clay mineralogy by X-ray diffraction, and micromorphology in digital images of thin sections. Contrary to the first hypothesis, the two soils showed a similar tendency to accumulate smectite in aggregates, probably because the cation exchange capacity was saturated by Ca 2+ in both CALC and DECALC. Macroaggregates showed a less porous structure in CALC than in DECALC due to the accumulation of calcite microcrystals, as formulated in the second hypothesis. We propose that low porosity of macroaggregates is mainly responsible for the slower turnover of organic matter observed in CALC than in DECALC. These results explain the greater concentration of organic C in microaggregates within macroaggregates in field-moist samples in CALC than in DECALC. The different porosity of macroaggregates may also result in differences in physical properties between CALC and DECALC. These observations suggest a different response of calcareous soils in terms of organic matter protection, resistance to erosion, and water storage compared with other soil types in semi-arid lands. Additional keywords: aggregates, calcite microcrystals clay minerals, soil organic C, micromorphology of aggregates. Received 10 August 2013, accepted 1 November 2013, published online 6 March 2014 Introduction The physical quality of soil has a key role in soil functioning. Soil aggregation, in particular, influences water and gas fluxes (Hinsinger et al. 2009), sensitivity to erosion (Le Bissonnais et al. 2007) and soil organic (C) dynamics (Six et al. 1998). Aggregates form due to the interaction of soil minerals with organic matter. The relationship between aggregation and the dynamics of organic matter has been the focus of many studies and several comprehensive reviews (e.g. Six et al. 2004). The concept of aggregate hierarchy developed for temperate soils, in which the mineral fraction is dominated by phyllosilicates, postulates that different binding agents (i.e. fungal hyphae, root and microbial exudates, humified organic matter, polyvalent cations, oxides and disordered aluminosilicates) act at different hierarchical stages of aggregation (Tisdall and Oades 1982), and that microaggregates (50–250 mm) are formed within macroaggregates (>250 mm) (Oades 1984). According to this hierarchy of aggregates, macroaggregates are first formed around decomposing organic matter through the enmeshment of mineral particles, particulate organic residues and old microaggregates by fungal hyphae and microbial and root exudates. The particulate organic matter within macroaggregates is further decomposed and impregnated with microbial exudates produced during decomposition. These coated organic fragments become encrusted with highly disordered aluminosilicates, oxides and humified organic matter and form new microaggregates. Eventually, fungal hyphae and microbial products holding macroaggregates are degraded, macroaggregates are disrupted and newly formed microaggregates are released (Six et al. 2004). Thus, a relation exists between aggregation and the dynamics of organic matter in temperate soils, in which organic matter is the most important agent of macroaggregate formation and stabilisation. Journal compilation Ó CSIRO 2014 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2014, 52, 180–192 http://dx.doi.org/10.1071/SR13234