Carbon pathways in soil: unraveled by 13C natural abundance

Abstract

Soil organic matter (SOM) escapes microbial and enzymatic decomposition by stabilizing on clays and iron- and aluminum (oxyhydro)oxides, or by being trapped within aggregates. We analyzed the importance of these stabilization mechanisms based on ¹³C natural discrimination within SOM pools and generalized the transformations by evaluating 1190 δ¹³C data for aggregates and 365 for density fractions. The key approach is based on the increase of the δ¹³C values of the pools with their transformation intensity and their stability. The main directions of C flow across the land use types and climates are from macro- towards microaggregates and to silt + clay, as well as from free light through occluded light to occluded dense fractions and finally to mineral-associated organic matter (MAOM). The strongest ¹³C enrichment (+1.1 ‰) between occluded light and dense fractions shows that more than one transformation step occurred between these pools during the SOM formation from litter. Considering the heaviest Δ¹³C signature (as difference between δ¹³C_fraction and δ¹³C_{bulk soil}), we conclude that the MAOM is the most stable pool in all soils globally, highlighting the leading role of fixation of organic compounds on minerals as long-term C stabilization. Clay content, pH, and climate shape microbial activities, regulating SOM transformation and ¹³C discrimination. Land use regulates organic inputs and soil disturbance, altering microbial recycling and the δ¹³C signature of the transformation products. Concluding, the pathways of litter-derived C to stable SOM within aggregates and density fractions are similar in a broad range of soils globally. These processes, however, are modified by the edaphic factors, land use, and climate, affecting long-term C stabilization.