13C natural abundance approach for analysis of steps of organic carbon transformation in soil: application for various ecosystems

Most of the organic carbon (C) in soil is linked to various degrees with the mineral matrix, which includes the formation of aggregates and the association of organic C with clay minerals and Fe and Al hydro(oxides). Both mechanisms ensure the physical stabilization of organics against microbial decomposition and the stability of the soil system. The present work analyzed the processes of C stabilization, and C flows between the aggregate size classes of macro- and macroaggregates and density fractions (free and occluded light (FL, OL), occluded dense (OD) and mineral (MF)) based on the ¹³C natural abundance approach using 73 published works and looked at the two groups of factors: i) internal (edaphic soil properties) and ii) external (type of land use and climate).

The global pattern showed the relative enrichment of ¹³C in silt + clay fraction for 0.4‰ and MF for 0.25-0.6‰ compared to bulk soil. In contrast, organics in micro- and macroaggregates and FL and OL fractions were ¹³C depleted, reflecting the fast decomposition of these pools and input of fresh plant-derived organics. The difference in ¹³C enrichment between the OL and MF fractions was 1.3‰, whereas between macroaggregates and silt+clay fraction 0.6‰, showing that density fractionation could more accurately reflect the intensity of organic C processing by microorganisms and fractions are more homogeneous in composition than aggregates.

The ¹³C enrichment in silt + clay and MF increased with the clay content; the difference between the ¹³C enrichment for OL and both dense fractions and macroaggregates and silt + clay fractions rose. Forests, grasslands, and croplands showed the same trend of increasing ¹³C enrichment with decreasing aggregate size classes; grasslands and croplands showed higher enrichment of silt+clay associated C (~0.3‰ relative to bulk soil) than forests. Under grasslands and forests, ¹³C enrichment in OD and MF was higher than in agriculture, showing deep microbial processing of organic matter without structure disturbance. The differences in ¹³C enrichment of organic matter between the aggregate size classes (0.6-0.9‰) and density fractions (2.5-3‰) were higher under subtropical and tropical climates, compared to temperate and Mediterranean, reflecting more intensive recycling of organic matter by microorganisms.

C flows between the aggregates followed the trend from macroaggregates to silt+clay fraction and from large macro- to microaggregates, reflecting the well-known sequential formation of soil structure and macro aggregates' role in stabilizing plant residues. More intensive C flows were found from FL to MF, compared to the C flow from OD to MF, pointing to the importance of plant-derived organics and microbial metabolites for the formation of MF. Thus, the global pattern of organic C transformation identified that the ¹³C natural abundance approach could be used for a broad range of automorphic soil types and can open a new perspective for the estimation of processes of C recycling and reveal the intensity of microbial processes depending on the land use, soil edaphic factors and climate.