New insights into organic carbon stabilization in soil macroaggregates: An in situ study by optical microscopy and SEM-EDS technique

The purpose of this study was to investigate the in situ characterization of organic matter (OM) within soil macroaggregates, and to assess the relationships between OM characteristics and macroaggregate size indicating different OM stabilization mechanisms. Optical micro-morphological investigations, coupled to SEM-EDS (scanning electron microscopy and energy X-ray spectroscopy) technique, were carried out on thin sections of 1–4 and 0.25–1 mm soil aggregates (coarse and fine macroaggregates, respectively) from 0 to 20 cm soil layer corresponding to A horizon of four different sites in which soil structure were not disturbed by tillage. The intraggregate porosity, measured by image analysis of four different size classes (<50, 50–100, 100–200, >200 µm), showed that fine macroaggregates were significantly less porous (3.70–6.71% of total porosity) and had higher presence of the finest pore class (<50 μm) compared to coarse macroaggregates (5.93–9.08% of total porosity), independently from sites. The percentage of organic matter forms (OMFs) identified by optical investigation was significant higher in fine (13.5–17.7%) than in coarse (4.19–8.27%) macroaggregates. In particular, fine macroaggregates were richer in red and black amorphous organic forms, which were characterized by the highest values of Al:C, Fe:C and Ca:C molar ratios. These findings suggested thus an accumulation of OM in fine macroaggregates than in coarse macroaggregates occurred. It was probably due to a more efficient OM stabilization in fine than in coarse macroggregates related to both physical occlusion (lower porosity and smaller pore size) and organo-minerals interaction (higher presence of OMFs characterized by the highest Al:C, Fe:C and Ca:C ratios), The OM exposure index (EI), a measurement of the OM surface exposed to pores and thus potentially available for microbial activity, was unexpectedly higher in fine than in coarse macroaggregates (EI: 0.48–0.79 and 0.25–0.58 mm−1 in fine and coarse macroaggregates, respectively). However, the accessibility of OM defined by the EI seemed to facilitate neither the oxidative transformation nor the damage of enzyme activities, being the EI positively related to C:N ratio (r = 0.66), negatively to δ13C values (r = -0.74) and positive to the geometric mean of the five assayed enzyme activities related to C-cycle (r = 0.79). Therefore, even more potentially exposed, in fine macroaggregates the OM was not accessible to microorganisms due to the effective physical occlusion, and thus both accumulation of few transformed OM and maintenance of functionality related to C-cycle occurred. The OM stabilization in macroaggregates thus involved both physical occlusion and organo-metals/mineral phase interactions processes. Both these processes are often related to microaggregates rather than macroaggregates. Our findings thus seem to provide a new insight for studying the potentiality of OM stabilization and C sequestration in soil macroaggregates.