Elucidating the 3D structure of soil microaggregate and the fate of organic matter

Soil microaggregates play an important role for the long-term sequestration of soil organic matter (SOM) and are the nucleus of larger soil structures. However, it is still not well understood how the interplay of microaggregate structure and composition determine their functionality. This is especially due to analytical limitations that lead to the fact that the 3D architecture of microaggregates in conjunction with the fate of OM at the sub-micron scale is still hard to analyze. Combining microscopic and spectroscopic techniques with high sensitivity and high spatial resolution enables the correlation of microscale topographic information together with elemental and isotopic composition. In the present study we use novel imaging techniques at a previously unresolved spatial resolution to analyze and reconstruct the 3D architecture of microaggregates and the associated SOM.

In this study, isotope labelling was used to understand the fate of fresh C and N within newly developed soil microstructures. To explore the role of the inherited carbon content, soils from three different management practices, namely bare fallow, three-field and direct drilling, with different OM content, from an agricultural cropland research farm were used. To investigate the impact of fresh OM differing in C/N ratio on the 3D architecture of new formed soil microstructures, we performed an amendment with a substrate lacking N (highly labelled glucose (>99% ¹³C) ) and a substrate containing N (amino acid mixture (>98% ¹³C, >98% ¹⁵N) . After the incubation, all bulk soils and fractions were measured by Isotope-ratio mass spectrometry (IRMS) for ¹³C and ¹⁵N abundances. Microaggregates, clay and fine silt after 24 hours incubation were analyzed using Helium Ion Microscope (HIM) coupled with Secondary Ion Mass Spectrometer (SIMS) system and Nano Secondary Ion Mass Spectrometry (NanoSIMS) instrument for topography and elements/isotopes distribution, respectively. HIM-SIMS produced secondary electron images with a high resolution down to 0.5 nm and SIMS images of ²⁴Mg, ²⁷Al, ³⁹K and ⁵⁶Fe with a sub 20 nm resolution. NanoSIMS was capable to locate ¹²C₂, ¹²C¹³C, ¹²C¹⁴N and ¹²C¹⁵N for organic matter and ¹⁶O, ²⁷Al¹⁶O and ⁵⁶Fe¹⁶O as mineral phases at a submicron scale.

Significant (p < 0.01) difference found between clay and fine silt fractions in amino acid treatment shows that the clay plays a more important role in fresh OM sequestration than fine silt, as more ¹³C and ¹⁵N were detected on mineral surfaces in the clay sized fraction. Interestingly, no difference between fractions were observed for the glucose (C added only) treatments, here N can be assumed to act as a limiting factor. By correlative image registration of HIM-SIMS with NanoSIMS data, the 3D architectural buildup soil microaggregates was reconstructed. The combination of HIM-SIMS and NanoSIMS analyses allows a considerable step forward in the capability to investigate soil microaggregate and their organo-mineral associations at a previously unresolved sub-micron scale. The ongoing work points to the chemical composition of the mineral microaggregate constituents as being decisive for the spatial arrangement of the organo-mineral associated OM.