Soil aggregate and intra-aggregate carbon fractions associated with vegetation succession in an alpine wetland of Northwest China

Alpine wetlands can function as carbon sinks because of their high soil organic content and low decomposition rates. However, the effects of alpine wetland succession on soil structure and soil organic carbon (SOC) stability have rarely been reported. The objective of this study was to investigate the effects of vegetation succession on soil aggregate distribution, bulk soil carbon, aggregate-associated C, and intra-aggregate C fraction in the Bayinbuluk alpine wetland. Research sites were selected in adjacent swamp (S), swampy meadow (SM), and meadow (M) locations. Five soil aggregate fractions (>5000, 2000–5000, 250–2000, 53–250 and <53 μm) were separated using a modified Yoder method. The light fraction organic carbon (LFOC), coarse/fine intra-aggregate particulate organic matter carbon (iPOM-C), and mineral-associated organic matter carbon (mSOM-C) were isolated from soil aggregates. In the wetland succession sequence, the mean weight diameter of soil aggregates increased (from 4.16 to 4.63 mm) from S to SM, and then decreased (from 4.63 to 3.23 mm) from SM to M. The bulk soil carbon content initially increased (from 93 to 129 mg kg−1) from S to SM, and then decreased (from 129 to 54 mg kg−1) from SM to M. The same trends were found for the aggregate-associated C in different size aggregates. The coarse iPOM-C content (88–133 mg kg−1) was higher than the fine iPOM-C content (77–118 mg kg−1) in different size aggregates in S samples. In SM and M samples, however, the coarse iPOM-C contents (98–124 mg kg−1 (SM); 60–82 mg kg−1 (M)) were lower than the fine iPOM-C contents (129–151 mg kg−1 (SM); 71–103 mg kg−1 (M)). The ratio of coarse iPOM-C to fine iPOM-C content (ROC/F) exceeded 1 in S but was lower than 1 in SM and M. These results indicate that soil aggregation stability and SOC content increased and then decreased across the wetland succession sequence, and that ROC/F can be used as an indicator of changes in SOC content caused by wetland succession.