Turnover of soil microaggregate-protected carbon and the challenge of microscale analyses

Background: Microaggregates are suspected to protect soil organic carbon (SOC) from microbial decay, but its residence time is not well understood.Aims: We aimed at unraveling the relevance of microaggregates for C storage and testing the hypothesis that C in the interior of aggregates is older, compared to the exterior.Methods: We sampled soil under C3 vegetation and at a site where cropping shifted to C4 vegetation 36 years ago. We isolated free and macroaggregate-occluded size fractions (250-53 mu m) by wet sieving and ultrasound, manually isolated aggregates therefrom, and analyzed whether vegetation-related differences in delta C-13 could be traced at the interior and exterior of microaggregate cross-sections using elemental and laser ablation-isotope ratio mass spectrometry.Results: Size fraction weights comprised <5% of microaggregates. Based on a source partitioning approach including C3- and C4-derived C, we found mean residence times of SOC in occluded and free microaggregates of 62 and 105 years, respectively. Thus, C storage was longer than that in size fractions (35 years) and bulk soil (58 years). The small-scale variability of delta C-13 within aggregate cross-sections was considerable, both in C3 and C4 soil, yet without significant (p = 0.46) differences between interior and exterior locations.Conclusions: We conclude that microaggregates do not persist in an intact form in such a long-term that systematic differences in delta 13C patterns between exterior and interior parts can develop.