Soil aggregate-associated organic carbon dynamics subjected to different types of land use: Evidence from C-13 natural abundance

The technique of C-13 natural abundance provides new information that can be applied to study carbon (C) incorporation into soil aggregates and to improve our understanding of the aggregate hierarchy theory. The organic matter (OM) in soil undergoes microbial decomposition, which preferentially removes the lighter C-12 isotope, enriching the remaining soil in the heavier C-13 isotope. We hypothesized that the soil aggregate turnover would gradually lead to C isotope fractionation, and changes in the C-13 natural abundance are closely linked to C turnover that, in turn, is influenced by aggregates turnover. We examined how land use affects soil organic carbon (SOC) dynamics in soil aggregate with the following objectives: (1) evaluate the influence of land uses (woodland, orchard, paddy and upland) on aggregate-associated C isotope compositions and the distribution of new and old SOC in the Danjiangkou Reservoir area of central China, and (2) propose an extended scheme of C transfers between the aggregate size classes. The results showed that SOC content in aggregates generally increased with increasing aggregate size but the delta C-13 values of aggregates decreased. The SOC contents in bulk soil decreased in the order of paddy > woodland > upland > orchard. The most negative delta C-13 values in bulk soil and aggregates were observed in the paddy and woodland soils, whereas maximum delta C-13 values were obtained in the orchard and upland soils. The stable C isotope results suggest that SOC sequestration of fresh OM generally starts in macro-aggregates (> 0.25 mm), and, after disaggregation processes and microbiological consumption, the resulting degraded OM is sequestered in micro-aggregates (< 0.25 mm). This general trend is consistent with the concept of SOM stabilization by association with aggregate hierarchy theory. We conclude that different land uses and management practices significantly affect C incorporation in the aggregate system and C transfer in soil aggregates was considerably greater in the orchard and upland soils than in the woodland and paddy soils. These findings help improve the theory of soil aggregates.