Soil organic carbon transfer in aggregates subjected to afforestation in karst region as indicated by 13C natural abundance

The measurement of 13C natural abundance provides valuable information in studying carbon (C) incorporation in soil aggregates. Exploring the pathways of C between aggregates of different size classes are vital to further understand the stabilization of soil organic matter (SOM) and aggregate formation. However, little is known on the mechanism of C stabilization in soil, especially when subjected to afforestation effects in karst rocky desertification area. Soil in the 0-20 cm layer was collected from two afforestation types [natural secondary forest (NF) and artificial Zanthoxylum bungeanum forest (AF)] in the karst rocky desertification area, with cropland (CL) as the reference. Stable isotope fractionation was used to analyze the dynamic changes in organic carbon (OC) in aggregates, and the Delta 13C (difference in delta 13C between the bulk soil and aggregate fractions) was utilized to quantify the C flow in aggregates under different land use types and soil layers. Results showed that afforestation significantly affected the dynamics of soil aggregate OC. Afforestation on cropland increased the number of large macroaggregates (>2 mm, LMA) and decreased the proportions of small macroaggregates (0.25-2 mm, SMA), microaggregates (0.053-0.25 mm, MI), and silt + clay fraction (<0.053 mm, SC). With the increase in aggregate class size, the OC content in aggregates rose under CL and AF, but no obvious regularity was found in NF. The proportion of new SOC was higher in NF (27.0%) than in AF (20.6%) and in 0-10 cm soil layer (27.3%) than in 10-20 cm soil layer (20.3%) under different afforestation measures. No significant differences in particle size were found among aggregates (P > 0.05). The delta 13C in bulk soil and aggregates was significantly decreased by afforestation and increased with the increment in soil depth. Stable isotope analysis results showed that the flow of OC was mainly from larger aggregates to smaller aggregates in CL and AF, which is consistent with the stabilization mechanism of SOM in aggregate hierarchy theory. However, the C flow occurred from smaller aggregates to larger aggregates in NF topsoil due to that newly input OC is first associated with MI and then incorporated into macroaggregates during soil aggregation process. The different human disturbance frequencies and the replenishment levels of plant biomass through litter input, root biomass, and exudates resulted in variation in aggregate turnover (formation and breakdown) among CL, AF and NF. These conclusions can provide sufficient reference for the formation and stabilization of aggregate-associated OC.