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 µm) by wet sieving and ultrasound, manually isolated aggregates therefrom, and analyzed whether vegetation-related differences in δ¹³C 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 δ¹³C 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 δ¹³C patterns between exterior and interior parts can develop.